DEBUG INTERFACE INCLUDING A COMPACT TRACE RECORD STORAGE

Abstract: In-circuit emulation (ICE) and software debug facilities are included in a processor via a debug interface that interfaces a target processor to a host system. The debug interface, located on the target processor, includes a trace controller that monitors signals produced by the target processor to detect specified conditions and to produce a trace record of the specified conditions including a notification of the conditions and selected information relating to the conditions. The trace controller formats a trace information record and stores the trace information record in a trace buffer in a plurality of trace data storage elements. The trace data storage elements have a format that includes a trace code (TCODE) field indicative of a type of trace information and a trace data (TDATA) field indicative of a type of trace information data.

Description: DEBUG INTERFACE INCLUDING A COMPACT TRACE RECORD STORAGE RELATED APPLICATIONS This application claims priority to United States application Serial No. 60/043,070, filed April 8, 1997, which is hereby incorporated by reference. TECHNICAL FIELD The present invention relates to computer systems such as microprocessors. More specifically, the present invention relates to a software debug support system and operating method in processors. BACKGROUND ART The growth in software complexity, in combination with increasing processor clock speeds, has placed an increasing burden on application software developers. The cost of developing and debugging new software products is now a significant factor in processor selection. A failure to include adequate software debug functionality in a processor results in longer customer development times and reduces attractiveness of the processor for use within industry. Software debug support is particularly useful in the embedded products industry, where specialized on-chip circuitry is often combined with a processor core. The software debug tool configuration of a processor addresses the needs of several parties in addition to the software engineer who develops program code for execution on the processor. A "trace" algorithm developer searches through captured software trace data that reflects instruction execution flow in a processor. An in-circuit emulator developer deals with problems of signal synchronization, clock frequency and trace bandwidth. A processor manufacturer seeks a software debug tool solution that avoids an increased processor cost or design and development complexity. In the desktop systems, complex multitasking operating systems are currently available to support software debugging. However, the initial task of getting the operating systems running reliably often calls for special development equipment. While not standard in the desktop environment, development equipment such as logic analyzers, read-only memory (ROM) emulators, and in-circuit emulators (ICE) are sometimes used in the embedded industry. In-circuit emulators have some advantages over other debug environments including complete control and visibility over memory and register contents, and supplying overlay and trace memory if system memory is insufficient. Traditional in-circuit emulators are used by interfacing a custom emulator back-end with a processor socket to allow communication between emulation equipment and the target system. The custom design of emulator interfaces in increasingly unrealistic and expensive as product life cycles are reduced and nonstandard and exotic integrated circuit packages are predominant in present day processor design. Few known processor manufacturing techniques are available that support a suitable full-function m- circuit emulation functionality Most processors m personal computer (PC) systems implement emulation functionality using a multiplexed approach in which existing pins are multiplexed for alternative use in a software debug application Multiplexing of pins is not desirable in embedded controllers, which inherently suffer from overload of pin functionality Some advanced processors mulUplex debug pms in time, for example by using the address bus to report software trace information duπng a Branch Target Address (BTA) cycle The BTA-cycle is stolen from regular bus operation cycles However in debug environments with high branch activity and low cache hit rates, BTA-cycles are often fully occupied handling branches, resulting m a conflict over access to the address bus that necessitates processor "throttle back" to prevent a loss of instruction trace information For example, software in the communications industry is branch-intensive and suffers poor cache utilization often resulting in 20% or more throttle back, an unacceptable amount for embedded products which are subject to real-time constraints In another approach, a second "trace" or "slave" processor is combined with a mam processor, with the two processors operating m-step Only the main processor fetches instructions The second, slave processor momtors fetched instructions on the data bus and maintains an internal state in synchronization with the main processor The address bus of the slave processor supplies trace information After power-up, via a JTAG (Joint Test Action Group) input, the second processor is switched into a slave mode of operation The slave processor, freed from instruction fetch duties, uses the slave processor address bus and other pins to supply trace information Another existing debug strategy utilizes implementation of debug support into every processor m a system, but only bonding-out signal pms in a limited number of packages The bond-out versions of the processor are used duπng debug and replaced with the smaller package for final production The bond-out approach suffers from the need to support additional bond pad sites in all fabπcated devices, a burden m small packages and pad limited designs, particularly if a substantial number of extra pins are required by the debug support variant Furthermore, the debug capability of specially-packaged processors is unavailable in typical processor-based production systems In yet another approach, specifically the Background Debug Mode (BDM) implemented by Motorola, Inc , limited on-chip debug circuitry is implemented for basic run control The BDM approach utilizes a dedicated seπal link having additional pins and allows a debugger to start and stop the target system and apply basic code breakpoints by inserting special instructions in system memory Once halted, special commands are used to inspect memory vaπables and register contents The BDM system includes trace support, but not convementlv using the seπal link Instead the BDM system supplies trace support through additional dedicated pins and expensive external trace capture hardware that transfer instruction trace data Accordingly, present day techniques for software debugging suffer from a variety of limitations including increased packaging and development costs, circuit complexity, processor throttling, and bandwidth matching difficulties. Furthermore, no adequate low-cost procedure for providing trace information is currently available. The limitations of the existing solutions are likely to be exacerbated in the future as internal processor clock frequencies continue to increase. What is needed is a software debug system and operating procedure that includes an improved ttace capability. DISCLOSURE OF INVENTION In-circuit emulation (ICE) and software debug facilities are included in a processor via a debug interface that interfaces a target processor to a host system. The debug interface includes a ttace conttoller that monitors signals produced by the target processor to detect specified conditions and produce a ttace record of the specified conditions including a notification of the conditions are selected information relating to the conditions. The ttace controller formats a ttace information record and stores the trace information record in a trace buffer in a plurality of ttace data storage elements. The ttace data storage elements have a format that includes a ttace code (TCODE) field indicative of a type of ttace information and a ttace data (TDATA) field indicative of a type of trace information data. In accordance with an embodiment of the present invention, a debug interface in a target processor interfaces the target processor to a host system. The debug interface includes a ttace conttoller for connecting to the target processor and receiving trace information from the target processor. The ttace conttoller detects selected ttace information and formats samples of the selected ttace information. The debug interface also includes a ttace buffer connected to the ttace controller. The ttace buffer includes a plurality of ttace data storage elements including a ttace code (TCODE) field indicative of a type of ttace information and a ttace data (TDATA) field indicative of a type of trace information data. The trace buffer is highly useful in applications controlling field equipment. If an unexpected system crash occurs, the ttace buffer is examined to observe the execution history leading up to the crash event. When used in portable systems or other environments in which power consumption is a concern, the trace buffer is disabled by power management circuitry. A ttace record is read from the ttace buffer that includes a record of ttace events. A trace event is an action that causes ttace records to be generated. Trace events are caused, for example, by x86 instructions, instructions causing an exception, hardware interrupts, trace synchronization events, activation or deactivation of trace capture, and events that change privilege level. The described debug interface and associated operation method has many advantages. The compact TCODE and TDATA formats efficiently store important operational information in a limited storage space. The TCODE and TDATA formats are defined to produce a highly compressed record but still record a substantial amount of useful diagnostic information. BRIEF DESCRIPTION OF DRAWINGS The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. FIGURE 1 is a schematic block diagram showing a software debug environment utilizing a software debug port according to an embodiment of the present invention. FIGURE 2 is a schematic block diagram illustrating details of an exemplary embedded processor product incorporating a software debug port according to the present invention. FIGURE 3 is a simplified block diagram depicting the relationship between an exemplary instruction trace buffer and other components of an embedded processor product according to the present invention; FIGURE 4 is a flowchart illustrating software debug command passing according to one embodiment of the invention; FIGURE 5 is a flowchart illustrating enhanced software port command passing according to a second embodiment of the invention; and FIGURES 6A - 6G illustrate the general format of a variety of ttace buffer entries for reporting instruction execution according to the invention. FIGURE 7 is a state diagram illustrating states of a standard JTAG interface access operation. FIGURE 8 is a state diagram illustrating states of an enhanced JTAG interface access operation. FIGURE 9 is a timing diagram showing signal behavior for acquisition and release of a parallel debug bus. FIGURE 10 is a flow chart depicting a parallel debug port interface protocol followed by the host system during communications via the parallel port. The use of the same reference symbols in different drawings indicates similar or identical items. MODES FOR CARRYING OUT THE INVENTION Referring to FIGURE 1, a schematic block diagram illustrates a software debug environment utilizing a debug port 100. A target system 101 is shown containing an embedded processor device 102 coupled to system memory 106. The embedded processor device 102 is an integrated debug interface for flexible, high-performance in an embedded hardware/software debug system. The embedded processor device 102 includes a processor core 104 and the debug port 100. In some embodiments, the embedded processor device 102 may incorporate additional circuitry (not shown) for performing application specific functions, or may take the form of a stand-alone processor or digital signal processor. In the illustrative embodiment, the debug port 100 includes an IEEE-1149.1-1990 compliant JTAG interface or other similar standardized serial port interface. A host system 111 is used to execute debug control software 112 for transferring high-level commands and controlling the extraction and analysis of debug information generated by the target system 101. The host system 111 and target system 101 of the disclosed embodiment of the invention communicate via a serial link 110. Most computers are equipped with a serial or parallel interface which can be inexpensively connected to the debug port 100 by means of a serial connector 108, allowing most computers to function as a host system 111. In some embodiments, the serial connector 108 is replaced with higher speed JTAG-to-network conversion equipment. Referring to FIGURE 2, a schematic block diagram illustrates the embedded processor device 102 in more detail, including the processor core 104 and various elements of the debug port 100 in an enhanced form. The embedded processor device 102 includes a plurality of state machines that communicate messages and data between a serial port of a JTAG TAP conttoller 204, a trace control circuit 218, and the processor core 104. In some embodiments, the embedded processor device 102 includes a parallel port 214 and the state machines similarly establish communication between the parallel port 214, the trace control circuit 218, and the processor core 104. The state machines include a debug port state machine 206, a parallel port state machine 226, a processor interface state machine 202, and a pending/finished state machine 224. The state machines read the commands and data from the serial/parallel ports and direct decoding of the commands by a command decode and processing block 208 logic. Some commands, such as a command for reading data from memory, utilize processor core 104 intervention and are appropriately sent to the processor core 104. The state machines do not accept further commands until execution of a previous command is complete. Once the command is completed, a flag in a Debug Registers 210 block is asserted or an output pin is asserted to indicate command completion to the host system 111. A minimal embodiment of the debug port 100 supports only conventional JTAG pins in a fast JTAG compliant interface that advantageously attains controllability and observability. The JTAG pins are a transportation mechanism that use existing pins to enter commands for processing by the processor core 104. Conventional JTAG pins carry conventional JTAG support signals that are well known in the art including a test clock signal TCK, a test mode select signal TMS, a test data input signal TDI, and a test data output signal TDO. The conventional JTAG pins are driven by a JTAG Test Access Port (TAP) conttoller 204. The JTAG interface is enhanced to improve software debug capabilities and to transfer high-level commands into the processor core 104, rather than to scan processor pins. The JTAG-compliant serial interface is augmented for higher-speed access via supplemental sideband signals and a bond-out parallel interface with a 16-bit data path. Specifically, four pins are added to an embedded processor device 102 that supports JTAG functionality in a non-bondout package to fully support the enhanced 10-pin debug port 100 format. The enhanced embodiment of the debug port 100 supports the four additional pins carrying "sideband" signals including a command acknowledge signal CMDACK, a break request trace capture signal BRTC, a stop transmit signal STOPTX, and a trigger signal TRIG to the standard JTAG interface. The additional sideband signals advantageously enhance performance and functionality of the debug port 100 by attaining highly precise external breakpoint assertion and monitoring, by triggering external devices in response to internal breakpoints, and by eliminating status polling of the JTAG serial interface. The sideband signals offer extra functionality and improve communications speeds for the debug port 100, but are optional and not utilized in the simplest embodiments of debug port 100 which uses conventional JTAG support signals. In the illustrative embodiment the sideband signals are used with an optional parallel port 214 provided on special bond-out versions of the embedded processor device 102. Using conventional JTAG signals, the JTAG TAP conttoller 204 accepts standard JTAG serial data and control signals. When public TJAG DEBUG instruction is written to a JTAG instruction register, the serial debug shifter 212 is connected to a serial interface formed by the JTAG TAP conttoller 204. A JTAG test data input signal TDI and a test data output signal TDO are received by the serial debug shifter 212 via the JTAG TAP controller 204 so that commands and data are loaded into debug registers 210 and read from the debug registers 210. In the disclosed embodiment, the debug registers 210 include two debug registers for transmitting (TX DATA register) and receiving (RX DATA register) data, an instruction ttace configuration register (ITCR), and a debug conttol status register (DCSR). Data are typically read from the JTAG TAP conttoller 204 using a Capture DR - Update DR sequence in the JTAG TAP controller state machine. The Instruction Trace Configuration Register (ITCR) is written to conttol enabling, disabling, and configuration of Instruction Trace debug functions. The ITCR is accessed through the serial/parallel debug register write/read commands or by using a reserved instruction LDTRC. The Debug Control/Status Register (DCSR) indicates when the processor core 104 enters debug mode and allows the processor core 104 to be forced into debug mode using the enhanced JTAG interface. DCSR also includes miscellaneous conttol feature enables bits. DCSR is accessible only through the serial parallel debug register write/read commands. A debug port state machine 206 coordinates loading and reading of data between the serial debug shifter 212 and the debug registers 210. The debug port state machine 206 interfaces to the JTAG TAP conttoller 204 via a parallel port state machine 226. A command decode and processing block 208, and the processor interface state machine 202, in combination, decode commands and data and dispatch the commands and data to a data steering circuit 222 which, in turn, communicates with the processor core 104 and ttace conttol circuit 218. The processor interface state machine 202 communicates directly with the processor core 104 and a trace conttol circuit 218. A parallel port 214 communicates with the debug port state machine 206 and the data steering circuit 222 to perform parallel data read and write operations in optional bond-out versions of the embedded processor device 102. The ttace conttol circuit 218 generates trace record information for reconstructing instruction execution flow in the processor core 104. The ttace control circuitry 218 interfaces to the processor core 104 and supports tracing either to a ttace pad interface port 220, which is also called a DEBUG ttace port 220, or to an internal ttace buffer 200 and implements user control for selectively activating and deactivating trace functionality. The ttace control circuitry 218 controls a trace pad interface port 220 which, in the illustrative embodiment, shares communication lines of the parallel port 214. When utilized, the ttace pad interface port 220 supplies trace data while the processor 104 is executing instructions, although clock synchronization and other difficulties may arise. The ttace conttol circuitry 218 enables other features including programmability of synchronization address generation and user specified ttace records. The processor core 104 supplies the tracing information that is used to generate ttace records. The debug port interface 100 supplies the commands to enable and disable the ttace function, and to turn trace capture on and off via the ITCR- Commands to read the ttace buffer come from the processor core 104. At reset, ttacing is disabled and ttace capture is turned off. To begin generating trace records, tracing is enabled and trace capture turned on. When the processor core 104 enters the debug mode, trace capture is turned off. When the processor core 104 exits debug mode, trace capture status returns to the state prior to entering debug mode. Tracing is enabled by setting the GTEN bit in the ITCR. Tracing is disabled by resetting the GTEN bit. When ttacing is disabled, all ttace records are discarded from the trace buffer 200. Trace capture is turned on by setting the TRON bit in the ITCR. Trace capture is turned off by resetting the TRON bit. The TRON bit is modified in one of several ways including directly writing to the TRON register, applying the break request/trace capture signal BRTC if the B TRC is enabled in the DCSR to conttol trace capture, and via DR0- DR3 register usage to turn ttace capture on/off " by enabling the option in the ITCR. The disabling of ttace gathering is advantageously a software option, reducing processor power consumption and eliminating natural throttle-back tendencies. Trace gathering is enabled both from the host system 111 and the target system 101. Trace information is not generated when the processor is operating in Debug mode. All other modes may be "traced" if the appropriate switches are set. When tracing is disabled, contents of the ttace buffer 200 are lost. However, when commands or trace/breakpoint conttol registers are used to temporarily stop/start trace accumulation, the ttace buffer 200 is not flushed. The ttace conttol circuit 218 operates in one of two modes, external trace mode and internal ttace mode, that differ in how the controller handles the ttace records associated with trace events. The external trace mode is available on a bondout chip. In external ttace mode, trace records are sent to the ttace port 220. External trace mode is enabled by setting I TM=1 in the ITCR. The internal trace buffer temporarily stores trace records that cannot be sent to the trace port at the same rate as the records are generated by the processor core 104. The internal ttace mode is available on both a bondout and a non-bondout integrated circuit. In the internal trace mode, ttace records are stored in the internal ttace buffer 200. The internal ttace mode is enabled by setting ITM=0 in the ITCR. The ttace buffer 200 contents are accessed either through debug port commands or through the enhanced x86 instruction STBUF. The ttace buffer 200 improves bandwidth matching and reduces the need to incorporate throttle-back circuitry in the processor core 104. In one embodiment, the ttace buffer 200 is a 256 entry first-in, first-out (FIFO) circular cache. Increasing the size of the ttace buffer 200 increases the amount of software ttace information that is captured. When the processor core 104 takes a debug trap, the trace buffer might contain ttace records. The processor core 104 will continue to send ttace records to the ttace port 220 until the buffer is empty. If the TBUS signals are shared with the parallel debug port 214, the ttace port 220 does not switch to the parallel port 214 until the ttace buffer 220 is empty. The ttace buffer 220 is functionally used as a circular buffer for the internal trace mode of operation. Trace records overwrite existing data after 256 records are written into the ttace buffer 200. The JTAG TAP controller 204 functions as an IEEE-1149.1-1990-compliant serial command port that serves as a primary mode of communication between a host processor 111 and target processor to a maximum TCK rate of 25MHz. Before debug information is communicated via the debug port 100 using only conventional JTAG signals, the port 100 is enabled by writing the public JTAG instruction DEBUG into a JTAG instruction register contained within the JTAG TAP conttoller 204. The JTAG instruction register is a 38-bit register including a 32-bit data field debug_data[31:0], a four-bit command field cmd[3:0] to point to various internal registers and functions provided by the debug port 100, a command pending flag (P), and a command finished flag (F). Some commands use bits of the debug data field as a sub-field to extend the number of available commands. The pending and finished flags are controlled by a pending/finished state machine 224 that is connected to the JTAG TAP conttoller 204. JTAG Instruction Register This JTAG instruction register is selected by toggling the test mode select signal TMS. The test mode select signal TMS allows the JTAG clocking path to be changed in the scan path, enabling usage of multiple paths of varying lengths. Preferably, the JTAG instruction register is accessible via a short path. This register is configured to include a soft register for holding values to be loading or receiving from specified system registers. Referring now to FIGURE 3, a simplified block diagram depicts the relationship between an exemplary instruction trace buffer 200 and other components of an embedded processor device 102. In one embodiment, the ttace buffer 200 is a 256 entry first-in, first-out (FIFO) circular cache that records the most recent ttace entries. Increasing the size of the ttace buffer 200 increases the amount of instruction ttace information that is captured, although the amount of required silicon area may increase. The trace buffer 200 stores a plurality of 20-bit (or more) ttace entries indicative of the order in which instructions are executed by the processor core 104. Other information, such as task identifiers and ttace capture stop/start information, is also placed in the ttace buffer 200. The contents of the ttace buffer 200 are supplied to external hardware, such as the host system 111, via either serial or parallel ttace pins 230. Alternatively, the target system 101 can be configured to examine the contents of the ttace buffer 200 internally. Referring to FIGURE 4, a high-level flow chart illustrates a technique for passing a command when using a standard JTAG interface. Upon entering debug mode in step 400 the DEBUG instruction is written to the TAP conttoller 204 in step 402. In step 404, the 38-bit serial value is shifted in as a whole, with the command pending flag set and desired data (if applicable, otherwise zero) in the data field. Conttol proceeds to step 406 where the pending command is loaded/unloaded and the command finished flag checked. Completion of a command typically involves transferring a value between a data register and a processor register or memory/IO location. After the command has been completed, the processor 104 clears the command pending flag and sets the command finished flag, at the same time storing a value in the data field if applicable. The entire 38-bit register is scanned to monitor the command finished and command pending flags. If the pending flag is reset to zero and the finished flag is set to one, the previous command has finished. The status of the flags is captured by the debug port state machine 206. A slave copy of the flag status is saved internally to determine if the next instruction should be loaded. The slave copy is maintained due to the possibility of a change in flag status between TAP conttoller 204 states, allowing the processor 104 to determine if the previous instruction has finished before loading the next instruction. If the finished flag is not set as determined in step 408, conttol proceeds to step 410 and the loading unloading of the 38-bit command is repeated. The command finished flag is also checked. Conttol then returns to step 408. If the finished flag is set as determined in step 408, control returns to step 406 for processing of the next command. DEBUG mode is exited via a typical JTAG process. Returmng to FIGURE 2, the optional sideband signals are utilized in the enhanced debug port 100 to provide extra functionality The optional sideband signals include a break request ttace capture signal BRTC that functions as a break request signal or a trace capture enable signal depending on the status of bit set in the debug control/status register If set to function as a break request signal, the break request/trace capture signal BRTC is asserted to cause the processor 104 to enter debug mode The processor 100 is also stopped by scanning in a halt command via the convention JTAG signals If set to function as a trace capture enable signal, asserting the break request ttace capture signal BRTC enables ttace capture Deasserting the signal turns trace capture off The signal takes effect on the next instruction boundary after the signal is detected and is synchronized with the internal processor clock The break request/trace capture signal BRTC is selectively asserted at any time The tπgger signal TRIG is configured to pulse whenever an internal processor breakpoint has been asserted The tπgger signal TRIG may be used to tπgger an external captuπng device such as a logic analyzer, and is synchronized with the ttace record capture clock signal TRACECLK When a breakpoint is generated, the event is synchronized with the ttace capture clock signal TRACECLK, after which the tπgger signal TRIG is held active for the duration of ttace capture The stop transmit signal STOPTX is asserted when the processor 104 has entered DEBUG mode and is ready for register interrogation/modification, memory or I/O reads and writes through the debug port 100 In the disclosed embodiment of the invention, the stop transmit signal STOPTX reflects the state of a bit m the debug conttol status register (DCSR) The stop transmit signal STOPTX is synchronous with the trace capture clock signal TRACECLK The command acknowledge signal CMD ACK is descπbed in conjunction with FIGURE 5, which shows a simplified command passing operation in the enhanced debug port 100 of FIGURE 2 A DEBUG instruction is wπtten to the TAP conttoller 204 in step 502 to place the target system 111 into DEBUG mode Conttol proceeds to step 504 and the command acknowledge signal CMD ACK is momtored by the host system 111 to determine command completion status The CMD ACK signal is asserted high by the target system 111 simultaneously with the command finished flag and remains high until the next shift cycle begins The command fimshed flag status is accessible when using the command acknowledge signal CMD ACK without shifting out the JTAG instruction register The command acknowledge signal CMD ACK transitions high on the next πsing edge of the test clock signal TCK after the command fimshed flag has changed from zero to one When using the enhanced JTAG signals, a new shift sequence m step 506 is not started by the host system 111 until the command acknowledge signal CMD ACK pin is asserted high The command acknowledge signal CMD ACK is synchronous with the test clock signal TCK The test clock signal TCK is not necessaπly clocked at all times, but is ideally clocked continuously when waiting for a command acknowledge signal CMD ACK response OPExRATING SYSTEM/APPLICATION COMMUNICATION VIA THEDEBUGPORT Also included in debug register block 210 is an instruction ttace configuration register (ITCR), a 32- bit register for enabling/disabling and configuring instruction trace debug functions. Numerous functions are contemplated including various levels of ttacing, trace synchronization force counts, ttace initialization, instruction tracing modes, clock divider ratio information, as well as additional functions shown in the following table. The ITCR is accessed through a JTAG instruction register write/read command as is the case with the other registers of the debug register block 210, or via a reserved instruction. Instruction Trace Configuration Register (ITCR) Another debug register is the debug control status register (DCSR) that designates when the processor 104 has entered debug mode and allows the processor 104 to be forced into DEBUG mode through the enhanced JTAG interface As shown in the following table, the DCSR also enables miscellaneous conttol features including forcing a ready signal to the processor 104, conttolling memory access space for accesses imtiated through the debug port, disabling cache flush on entry to the DEBUG mode, supplying ttansmit and received bits TX and RX, enabling the parallel port 214, forced breaks, forcing a global reset, and other functions The ordeπng or inclusion of the vaπous bits m either the ITCR or DCSR is a design choice that typically depends on the application and processor implementing the debug system FRESET Forces global reset Debug Control/Status Register (DCSR) When operating in a cross debug environment such as the environment shown in FIGURE 1, a parent task running on the target system 111 sends information to the host platform 101 that controls the target system 101. The information may include, for example, a character stream from a ρrintf() call or register information from a Task Conttol Block (TCB). In one technique for transferring the information, an operating system places the information in a known region then causes DEBUG mode to be entered via a trap instruction. The host system 111 uses debug port 100 commands to determine the reason for entering DEBUG mode and responds by retrieving the information from the reserved region. However, normal processor execution is stopped while the processor 104 is in DEBUG mode, an undesirable condition for real-time systems. The undesirable condition is addressed in the illustrative system by supplying two debug registers in the debug port 100 including registers for transmitting data (TX DATA register) and receiving (RX DATA register) data. The TX DATA and RX_DATA registers are accessed using the soft address and JTAG instruction register commands. The soft address is auto-incremented to assist data ttansfer. After the host system 111 writes a debug instruction to the JTAG instruction register, the serial debug shifter 212 is connected to the test data input signal TDI line and test data output signal TDO line. The processor 104 executes code that transmits data by first testing a TX bit in the ITCR. If the TX bit is set to zero then the processor 104 executes either a memory or I/O write instruction to transfer the data to the TX_DATA register. The debug port 100 sets the TX bit in the DCSR and ITCR, indicating to the host system Ili a readiness to transmit data, and asserts the STOPTX pin high. After the host system 111 completes reading the ttansmit data from the TX DATA register, the TX bit is set to zero. A TXINTEN bit in the ITCR is then set to generate a signal to interrupt the processor 104. The interrupt is generated only when the TX bit in the ITCR transitions to zero. When the TXINTEN bit is not set, the processor 104 polls the ITCR to determine the status of the TX bit to further ttansmit data. The host system 111 transmits data by first testing a RX bit in the ITCR. If the RX bit is set to zero, the host system 111 writes the data to the RX_DATA register and the RX bit is set to one in both the DCSR and ITCR. A RXINT bit is then set in the ITCR to generate a signal to interrupt the processor 104. This interrupt is only generated when the RX in the ITCR ttansitions to one. When the RXINTEN bit is not set, the processor 104 polls the ITCR to verify the status of the RX bit. If the RX bit is set to one, the processor instruction is executed to read data from the RX DATA register. After the data is read by the processor 104 from the RX DATA register the RX bit is set to zero. The host system 111 continuously reads the ITCR to determine the status of the RX bit to further send data. The information transfer technique using the RX_DATA and TX DATA registers enables an operating system or application to communicate with the host system 111 without stopping processor 104 execution Commumcation is conveniently achieved via the debug port 100 with minimal impact to on-chip application resources In some cases it is necessary to disable system interrupts This requires that the RX and TX bits be examined by the processor 100 In this situation, the commumcation link is dπven in a polled mode PARALLEL INTERFACE TO DEBUG PORT 100 The seπal debug port (SDP) is accessed either with the JTAG-based seπal link (JTAG based) or a somewhat more parallel interface The parallel port 214 interface supports higher code down-load speeds, but is included in a bond-out part Full-function ICE developers are typical users of the bond-out parts A full-function In Circuit Emulator (ICE) uses a dual-ported pod-space memory to gain high speed access to the target system 101 to gain faster down-load speeds than are available with the serial debug port alone To avoid a relatively complex dual-port design that is different for vaπous target processors, an alternative the T/P input pm is implemented in bond-out versions only and used to switch the bond-out pins from a ttace mode to parallel port mode Some embedded systems specify that instruction ttace is to be examined while maintaining I O and data processing operations Without the use of a multi-tasking operating system, a bond-out version of the embedded processor device 102 is preferable to supply trace data since, otherwise, examination of the ttace buffer 200 via the debug port 100 requires the processor 104 to be stopped In the disclosed embodiment, a parallel port 214 is implemented m an optional bond-out version of the embedded processor device 102 to support parallel command and data access to the debug port 100 The parallel port 214 interface forms a 16-bit data path that is multiplexed with the ttace pad interface port 220 More specifically, the parallel port 214 supplies a 16-bit wide bi-directional data bus (PDATA[15 0]), a 3-bit address bus (PADR[2 0]), a parallel debug port read/wnte select signal (PRW), a ttace valid signal TV and an instruction ttace record output clock TRACECLOCK (TC) Although not shared with the ttace pad interface port 220, a parallel bus request grant signal pair PBREQ/PBGNT (not shown) are also implemented The parallel port 214 is enabled by setting a bit in the DCSR Seπal communications via the debug port 100 are not disabled when the parallel port 214 is enabled The bond-out TC pin is a trace capture clock, a clock signal that is also used to capture system memory accesses performed on other busses Captuπng system bus activity is used to trace bus activity in conjunction with program ttace The TC clock is preferentially accessed last in a sequence of clock signals since system bus data can be acquired at the time of the TC clock signal Thus other bus signals are to remain active at the time of the TC clock, or be latched m the ICE preprocessor and held until the TC clock is active When the parallel port 214 is selected, rather than the ttace port 220, the TC clock is used to read and write parameters in to the Debug Registers 210, a faster process than serially clocking data into the 38-bit JTAG instruction command register. 22 21 20 19 16 0 Bond-Out Pins/Parallel Port 214 Format The parallel port 214 is primarily intended for fast downloads/uploads to and from target system 111 memory. However, the parallel port 214 may be used for all debug communications with the target system 111 whenever the processor 104 is stopped. The serial debug signals, either standard or enhanced, are used for debug access to the target system 111 when the processor 104 is executing instructions. In a manner consistent with the JTAG standard, all input signals to the parallel port 214 are sampled on the rising edge of the test clock signal TCK and all output signals are changed on the falling edge of the test clock signal TCK. In the disclosed embodiment, the parallel port 214 shares pins with the ttace pad interface 220 so that commands directed to the parallel port 214 are initiated only while the processor 104 is stopped and the trace pad interface 220 is disconnected from the shared bus. The parallel bus request signal PBREQ and parallel bus grant signal PBGNT are included to expedite multiplexing of the shared bus signals between the ttace buffer 200 and the parallel port 214. When the host interface to the parallel port 214 detects that the parallel bus request signal PBREQ is asserted, the host interface begins driving the parallel port 214 signals and asserts the parallel bus grant signal PBGNT. In some embodiments, when entering or leaving DEBUG mode with the parallel port 214 enabled, the parallel port 214 may be used for the processor state save and restore cycles. Some processors may avoid the usage of a context save. The parallel bus request signal PBREQ is asserted immediately before the beginning of a save state sequence penultimate to entry of DEBUG mode. On the last restore state cycle, the parallel bus request signal PBREQ is deasserted after latching the write data. The parallel port 214 host interface responds to parallel bus request signal PBREQ deassertion by tri-stating its parallel port drivers and deasserting the parallel bus grant signal PBGNT. The parallel port 214 then enables the debug ttace port pin drivers, completes the last restore state cycle, asserts the command acknowledge signal CMD ACK, and returns conttol of the interface to trace control circuit 218. During communication via the parallel port 214, the address pins PADR[2:0] are used for selection of the field of the JTAG instruction register, which is mapped to the 16-bit data bus PDATA[15:0] as shown in the following table: PADR[2:0] Data Selection If only a portion of the bits of the debug data [31:0] register are utilized during a ttansfer, such as on 8-bit I/O cycle data write operations, only the used bits need be updated. The command pending flag is automatically set when performing a write operation to the four-bit command register and is cleared when the command fimshed flag is asserted. The host system 111 monitors the command acknowledge signal CMD ACK to determine when the finished flag has been asserted. Use of the parallel port 214 offers full visibility of execution history without throttling the processor core 104. The trace buffer 200, if needed, is configured for use as a buffer to the parallel port 214 to alleviate bandwidth matching issues. OPERATING SYSTEM AND DEBUGGER INTEGRATION In the illustrative embodiment, the operation of all debug supporting features including the ttace buffer 200 is controlled through the debug port 100 or via processor instructions. The processor instructions are commonly accessed from a monitor program, a target hosted debugger, or conventional pod-wear. The debug port 100 performs data moves that are initiated by serial data port commands rather than processor instructions. Operation of the processor from conventional pod-space is very similar to operations in DEBUG mode from a monitor program. All debug operations are controlled via processor instructions, whether the instructions are accessed from pod-space or regular memory, advantageously extending an operating system to include additional debug capabilities. Operating systems have supported debuggers via privileged system calls such as pttace() call for some time. However, the incorporation of an on-chip trace buffer 200 now enables an operating system to offer instruction trace capability. The ability to ttace is often considered essential in real-time applications. In the illustrative improved debug environment, functionality of an operating system is enhanced to support limited trace without the incorporation of an external logic analyzer or in-circuit emulator. Instructions that support internal loading and retrieving of trace buffer 200 contents include a load instruction ttace buffer record command LITCR and a store instruction trace buffer record command SITCR. The command LITCR loads an indexed record in the trace buffer 200 specified by a trace buffer pointer ITREC.PTR with the contents of the EAX register of the processor core 104. The ttace buffer pointer ITREC.PTR is pre-incremented, so that the general operation of the command LITCR is described, as follows: ITREC.PTR <- ITREC.PTR +1; YTKECflTREC.PTRJ <- EAX. If the instruction trace record, in a trace record format described hereinafter, is smaller that the EAX record, only a portion of the EAX register is used. Similarly, the store instruction ttace buffer record command SITCR is used to retrieve and store into the EAX register an indexed record from the trace buffer 200. The contents of the ECX register of the processor core 104 are used as an offset that is added to the ttace buffer pointer ITREC.PTR to create an index into the trace buffer 200. The ECX register is post-incremented while the ttace buffer pointer ITREC.PTR is unaffected, so that: EAX <- TTRECfECX + ITREC.PTR]; ECX <- ECX + 1. The LITCR and SITCR commands may be configured in numerous variations of formats that are known in the computing and encoding arts. Extension of the operating system to support on-chip ttace has many advantages. In the communications industry, support of on-chip trace maintains system I/O and communication activity while a task is traced. Traditionally, the use of most in-circuit emulators has demanded that the processor be stopped and operating system execution suspended before the processor state and ttace are examined, disrupting continuous support of I/O data processing. In conttast, the pttace() capabilities of the illustrative enhanced system allow the processor and operating system to continue execution while ttace data is available. The ttace buffer 200 is highly useful in applications conttolling field equipment. If an unexpected system crash occurs, the ttace buffer 200 can be examined to observe the execution history leading up to the crash event. When used in portable systems or other environments in which power consumption is a concern, the ttace buffer 200 can be disabled via power management circuitry. A ttace record is read from the ttace buffer 200 that includes a record of ttace events. A ttace event is an action that causes ttace records to be generated. Trace events are caused, for example, by x86 instructions, instructions causing an exception, hardware interrupts, trace synchronization events, activation or deactiv ation of ttace capture, and events that change privilege level. Several x86 instructions generate trace records including software interrupt instructions, BOUND instructions, CALL instructions, interrupt taken and not taken (INT/INTO) instructions, return (RET) and interrupt return (IRET/IRETD) instructions, jump (JMP) and conditional jump (JCC) instructions, LOOP instructions, MOV CR0 instructions, and the like. A REP prefix does not generate a trace record. All CALL and CALLS instructions in which the target address is supplied by a register or memory location produce a TCODE=0010 entry. If the target address arises from the instruction (an immediate address) no TCODE=0010 entry is used. If a segment change occurs (a long address) then a TCODE=0011 entry is generated ahead of a TCODE=0010 entry. A segment change entry may be generated even if no target address entry occurs. An IRET instruction generates a branch target entry, TCODE=0010, which may be preceded by a segment change entry, TCODE=0011. All RET instructions generate branch target entries, TCODE=0010, and may be preceded by a segment change entry, TCODE=0011. Conditional branch instructions J xx produce a one-bit entry in a BFIELD trace element. The bit is set if the branch is taken, and not set if not taken. Looping instructions such as LOOP xx are tteated like conditional branching instructions. The REP instruction prefix is not tteated like a conditional branch and hence not reported, like other instructions which do not disrupt address flow. All unconditional JMP instructions in which the target address is supplied by a register or memory location produce a TCODE=0010 entry and are handled in the same way as CALL instructions. All INT imm8, INT3 and INTO interrupts generate a branch target entry when the interrupt is taken. The entry may be preceded by a segment change entry. The INTO instruction is conditional on the Overflow Flag being set. If this flag is not set, the interrupt and ttace entry are not generated. The BOUND instruction is a conditional interrupt, like INTO, and is similarly only reported if the interrupt is taken. BOUND is used to ensure that an aπay index is within specified limits. A direct move into the CRO register may be used to change the processors operating mode. Although infrequently done, any such move that results in a mode change is reported in the ttace stream. An instruction causing an exception generates trace records. The ttace records generated will report the EIP of the instruction causing the exception, previous segment base address and attributes, the vector number of the exception handler, and the EBP of the target instruction in the interrupt handler. A hardware interrupt generates trace records that report the EIP of the instruction that was most recently executed before the interrupt was serviced, the previous segment base address and attributes, the vector number of the interrupt handler, and the EIP of the target instruction in the interrupt handler. A ttace synchronization register implemented in the ttace controller is used to generate a trace synchronization event. The trace synchronization register is updated from the TSYNC bits of the ITCR register whenever the ITCR is loaded. The trace synchronization register is decremented by 1 when a conditional branch trace record (TCODE =0001) is created. Any other ttace record causes the register to be reloaded from the TSYNC bits of the ITCR register. The register counts down to zero and generates a ttace event. The trace synchronization is then reloaded from the TSYNC bits of the ITCR register. The TSYNC value represents the maximum number of consecutive conditional branch trace records (TCODE =0001) created before a ttace synchronization event occurs. A TSYNC value of 0 disables trace synchronization events. Trace records are generated when trace capture is turned on. The ttace records report the ED? of the instruction at which ttace capture was turned on. Trace records are generated when ttace capture is turned off. The ttace records report the ED? of the instruction at which trace capture was turned off and optionally the base address and segment attributes of the cuπent segment. Generation of trace records with the current segment base is controlled by the DISCSB bit of the ITCR. If the code segments do not change when the ttace capture is turned off, then disabling generation of current segment base ttace records prevents generation of redundant ttace records. Trace records are generated upon entering debug mode. The trace records report the ED? of the last instruction executed before entering debug mode and, optionally, the base address and segment attributes of the cuπent segment. Generation of ttace records with the cuπent segment base is controlled by the DISCSB bit of the ITCR. Trace records are generated when exiting debug mode. The trace records report the ED? of the first instruction executed after exiting debug mode. When entering privilege level ring 0 or 1, a capability to stop ttace capture is advantageous, for example, to prevent ttacing of system calls. Therefore, when entering lower privilege levels 0 and 1, tracing is optionally turned off via the DISL0TR, DISLlTRbits in the ITCR. If lower privilege level ttacing is selected, ttace records are generated before transitioning to lower privilege levels 0 and 1. The ttace records report the ED? of the last instruction executed before transitioning to the lower privilege level and optionally report the base address and segment attributes of the current segment. Generation of ttace records with current segment base is controlled by the DISCSB bit of the ITCR. Trace records for the call, jump, and other instructions that cause the transition to the lower privilege level are also reported preceding the privilege trace records. If an option to turn ttacing off when entering lower privilege levels is set, then when transitioning out of the lower privilege level, ttace records are generated. The ttace records will report the ED 3 of the first instruction executed after transitioning out of the lower privilege level. Note, the trace records for all these trace events can be preceded by a conditional branch ttace record if there was a pending conditional branch ttace record that had not been stored in the trace buffer or reported to the trace port. EXEMPLARY TRACE RECORD FORMAT In the disclosed embodiment of the invention, an instruction trace record is 20 bits wide and includes two fields, TCODE (Trace Code) and TDATA (Trace Data), as well as a valid bit V that indicates if the buffer entry contains a valid trace record. An internal write pointer keeps track of the last location written in the buffer. The write pointer is incremented before a ttace record is written to the buffer. The write pointer is reset by writing to the TINIT bit of the ITCR. The V bit indicates an invalid ttace record for all ttace records greater than the write pointer before the write pointer has wrapped around once. After the write pointer wraps around, all records read back as valid. The TCODE field is a code that identifies the type of data in the TDATA field. The TDATA field contains software ttace information used for debug purposes. 20 19 15 0 Instruction Trace Record Format In one embodiment, the embedded processor device 102 reports eleven different ttace codes as set forth in the following table: TRACE COMPRESSION The ttace buffer 200 has a limited storage capacity so that compression of the captured trace data is desirable. Trace data is acquired as a program executes on the target system 101, trace data is captured so that an image of the executed program is made available to the host system 111. In one example of a compression technique, if an address is otherwise obtainable from an accessible program image such as an Object Module, then the address is not supplied in the ttace data. Preferably, only instructions that disrupt the instruction flow are reported. Only instructions in which the target address is data dependent are reported. For example, disruptive events include call instructions or unconditional branch instructions in which the target address is supplied from a data register or other memory location such as a stack. CALL instructions are disrupting events in which in almost all cases the target address is supplied by the instruction in immediate format. Therefore, the branch is not reported unless the target address is data dependent, such as supplied from a data register or other memory location such as a stack. Unconditional Branch instructions, like a CALL has a target address that is usually in immediate format. Unconditional branches are compressed in the same manner as CALL instructions. Conditional instructions have few data register-dependent target addresses. Therefore the target address is only reported when the address is not in immediate format. Otherwise, a 1-bit ttace is used to indicate if the branch was taken or not taken. Exception events, whether synchronous or asynchronous, are relatively infrequent events. The target address of the trap or interrupt handler is reported in the ttace record. Return instructions pop the target address of the stack before starting a new instruction stteam. The target address is supplied in the ttace record. The traced address values are all logical and not necessarily physical. Address information relates to the address space of an executing program and not to an address ttanslation scheme that is utilized. Offset addresses are translated to physical equivalents, typically by adjusting an Extended Instruction Pointer (ED?) value by code segment and selector information. Therefore segment information is reported separately whenever a change occurs. In a system that uses paging, a virtual or logical address supplied by a process controlling debugging is presented to the target processor including hardware or software support and the coπesponding physical address is determined and then supplied to the debugger. Unfortunately conversion to a physical address is difficult if no coπesponding physical page is cuπently in memory. Therefore, operating system involvement is utilized. Paging is unlikely in an embedded processor environment. Most frequently-recorded trace addresses result from procedure returns. Between the return addresses, a stteam of single bits indicating the outcome from branch decisions is typically reported using the Conditional branch TCODE in which the BFELD is initially cleared except for the left most bit which is set to 1 and the outcome of up to 15 branch events is grouped into a single trace entry. As each new conditional branch is encountered, a new B-bit is added on the left and the entries are all shifted right one-bit. Instructions such as CALLs and unconditional Jumps produce no trace data if the target address is in immediate form. When a branch target address is reported, the current "BF1ELD" entry is marked complete, even if 15 entries are not yet accumulated. The target address is recorded in a trace entry pair. The first entry in the pair supplies the high-order 16 bits (TADDR.H) of the target address. The second entry supplies the low-order 16 bits (TADDR.L) of the target address. When a branch target address is supplied for a conditional jump instruction, no BFIELD entry appears for the reported branch. The compressed BFIELD ttace record that includes single bits designating whether branches are taken or not taken has a potential to cause difficulties in synchronizing trace entries since few trace entries contain address values. When a trace is examined, data is identified with a particular address only to the extent that a known program address is available. For example, starting at the oldest entry in the ttace buffer 200, all entries up to an entry containing a known address have no use and are discarded. Algorithm synchronization starts from a ttace entry supplying a target address. If the ttace buffer 200 contains no address supplying entries, then no ttace analysis is possible. A TSYNC register for the serial debug port, which is discussed in further detail hereinafter, is included for injecting an address reference in to the ttace data stteam. Other trace information includes a the target address of a trap or interrupt handler, a target address of a return instruction, a conditional branch instruction having a target address which is data register dependent (otherwise, all that is needed is a 1-bit ttace indicating if the branch was taken or not), and, most frequently, addresses from procedure returns. Other information, such as task identifiers and ttace capture stop/start information, may also be placed in the ttace buffer 200. Various data and information of many natures are selectively included in the trace records. The Missed Trace (TCODE = 0000) code is used to indicate that the processor missed reporting some ttace records. The TDATA field contains no valid data. A ttace execution algorithm typically responds to the Missed Trace TCODE by resynchronizing at the next trace record that includes address information. The Conditional Branch (TCODE=0001) code is used report the status for conditional branches. The TDATA field contains a bit for each conditional branch. The bit is marked as either taken/not taken. The format of the trace record is shown in FIGURE 6A. Each ttace record reports status information for a maximum of 15 conditional branches. The record is stored either in the trace buffer 200 or reported to the trace port 220 when 15 conditional branches are executed and the record is complete, or when the record is not complete and a trace event occurs, requiring another type of ttace record to be reported. In the disclosed embodiment, the outcome of up to 15 branch events are grouped into a single ttace entry. The 16-bit TDATA field, also called a "BFIELD", contains 1-bit branch outcome ttace entries, and is labeled a TCODE = 0001 entry. The TDATA field is initially cleared except for the left-most bit, which is set to 1. As each new conditional branch is encountered, a new one bit entry is added on the left and any other entries are shifted to the right by one bit. Each conditional branch is marked as either taken (1) or not taken (0). To identify the conditional branch bits, the least significant bit with a 1 value is located and bits to the left are conditional branch bits. A 256-entry trace buffer 200 allows storage of 320 bytes of information. Assuming a branch frequency of one branch every six instructions, the disclosed trace buffer 200 supports an effective trace record of 1,536 instructions, disregarding call, jump and return instructions. The ttace control circuit 218 monitors instruction execution via processor interface logic 202. When a branch target address is reported, information contained within a cuπent conditional branch TDATA field is marked as complete by the ttace conttol circuit 218, even if 15 entries have not accumulated. The Branch Target (TCODE=0010) code is used to report the target address of a data-dependent branch instruction and always occurs in pairs. Referring to FIGURE 6B, the TDATA field of the first record in the pair has the TCODE=0010 and indicates the high-order 16 bits of the Extended Instruction Pointer (EIP) of the target instruction. The TDATA field of the second record in the pair has a Multiple Trace (TCODE=0111) code that indicates the low-order 16 bits of the ED? of the target instruction. The target address, for example in a processor-based device 102 using 32-bit addressing, is recorded in a ttace entry pair, with the first entry (TCODE = 0010) supplying the high-order 16-bits of the target address and the second Multiple Trace entry (TCODE = 0111) supplying the low-order 16-bits of the target address. When a branch target address is supplied in conjunction with a conditional jump instruction, no 1-bit branch outcome ttace entry appears for the reported branch. The Multiple Trace code is used to report records for ttace entry with multiple records. The format of this ttace record is not fixed and depends on the ttace code that report multiple ttace records. STARTING AND STOPPING TRACE CAPTURE Referring to FIGURE 6C, a capability to start and stop ttace gathering during selected sections of program execution is advantageous, for example when a task context switch occurs. The Trace Stop (TCODE=1000) code is used to report the address of the instruction at which ttace capture was stopped and occurs paired with the Multiple Trace (TCODE=0111) code. The TDATA field of the first record with a TCODE=1000 indicates the high-order 16 bits of the ED? of the instruction at which the ttace capture terminated. The TDATA field of the second Multiple Trace record with a TCODE=0111 indicates the low- order 16 bits of the ED? of the instruction at which ttace capture stopped. When ttace capture is stopped, no trace entries are entered into the ttace buffer 200, nor do any entries appear on the bond-out pins of ttace port 220. Various known methods are contemplated for enabling and disabling ttace capture. For example, x86 commands are supplied for enabling and disabling the trace capture function. Alternatively, an existing x86 command is utilized to toggle a bit in an I/O port location. Furthermore, on-chip breakpoint control registers (not shown) are configured to indicate the addresses at which ttace capture is to start and stop. When ttacing is halted, a Cuπent Segment Base ttace entry (TCODE = 0100) and a Multiple Trace entry (TCODE = 0111) that record the last ttace address is placed in the ttace stteam. When ttacing resumes, a trace synchronization entty (TCODE = 0110, TCODE = 0111) that contains the address of the currently executing instruction is generated Because a debug conttoller can change the state of the processor core 104 before beginning execution, a trace synchronization entry (TCODE = 0110) is generated when leaving Debug mode A Sync address is then the first address executed on leaving Debug mode A useful debug function is a capability to account for segment changes that occur while ttacing is stopped The function is performed by selecting an option to report the segment base address and segment attπbutes of the cuπent code segment using a Cuπent Segment Base (TCODE = 0100) entry, shown in FIGURE 6C, which occurs in pairs with a second Multiple Trace (TCODE=0111) entry The TDATA field of the first record with a TCODE=0100 indicates the high-order 16 bits of the cuπent segment base address The TDATA field of the second Multiple Trace record with a TCODE=0111 indicates the low-order bits 15-4 of cuπent segment base address and the cuπent segment attπbutes The segment attπbutes report status for paging (PG), operand sizes (SZ), and addressing modes, either real or protected (R P) The address reported in the records only identify bits 31-4 of the base address The operand size (SZ) bit indicates the operand size and addressing mode and reflects the D bit of the code segment descπptor with 1 indicating a 32-bit operand and addressing mode and 1 indicating a 16-bit operand and addressing mode The paging status (PG) indicates if paging is enabled (1) or disabled (0) and reflects the PG bit of the CR0 register The Read Protected bit (R/P) indicates real mode (0) or protected mode (1) and reflects the PE bit of the CR0 register If the segment is not aligned to a 16-byte boundary, the low-order 4 bits of the base address are determined from the object module file of the program or from the descπptor tables The Cuπent Segment Base records occur whenever trace capture is turned off, and when debug mode is entered Generation of ttace records with the Cuπent Segment Base are controlled by the DISCSB bit of the ITCR The Cuπent Segment Base records function is also performed using a configuration option that enables a cuπent segment base address entry at the end of a ttace pπor to enteπng Debug mode Conversely, when the segment base does not change, such as when an interrupt occurs, supplying segment base information is typically not desirable The processor core 104 can enter Debug mode via an exception or other interrupt or command No trace capture occurs while executing in Debug mode A ttace entry is generated by the exception event, including a SDP command causing entry to Debug mode A trace-stop entry is not generated following the exception-event entry A configuration option is supported that enables a cuπent-segment base address entry (TCODE=0100) to be placed at the end of the trace when enteπng Debug mode The interrupt (TCODE=0101) code is used to report an exception or hardware interrupt and occurs m ttiphcate with two consecutive Multiple Trace codes (TCODE=0111) Referring to FIGURE 6D, following the occuπence of an asynchronous or synchronous event such as an interrupt or trap, a TCODE = 0101 trace entry is generated to supply the address of the target interrupt handler. The TDATA field of the first record with a TCODE=0101 indicates the vector number of the exception/interrupt handler. A System Management Interrupt (SMI) is reported with a TDATA value set to FFFFh. The TDATA field of the second Multiple Trace record with a TCODE=0111 indicates the high-order 16 bits of the Extended Instruction Pointer (ED?) of the target instruction. The TDATA field of the third Multiple Trace record with a TCODE=0111 indicates the low-order 16 bits of the EIP of the target instruction. The target instruction is the first instruction of the exception interrupt handler. When an asynchronous or synchronous event such as an interrupt or trap occurs, merely generating a interrupt (TCODE=0101) code alone is insufficient. The interrupt code supplies the address of the target interrupt handler, but does not reveal the address of the instruction interrupted. The address of the instruction which was interrupted by generating a ttace synchronization (TCODE = 0110) entry immediately prior to the interrupt entry is advantageously recorded along with the previous segment base address (TCODE = 0011). The ttace synchronization entry contains the address of the last instruction retired before the interrupt handler commences. The Trace Synchronization (TCODE=0110) code is used to report the address of the currently executed instruction. The Trace Synchronization code always occurs in pairs with the second Multiple Trace TCODE=0111. The TDATA field of the first record with a TCODE=0110 indicates the high-order 16 bits of ED? of the cuπently executed instruction. The TDATA field of the second Multiple Trace record with a TCODE=0111 indicates the low-order 16 bits of ED? of the cuπently executed instruction. The records occur when an exception or interrupt is taken, a ttace synchronization event occurs, ttace capture is turned on, debug mode is exited, and transitioning out of lower privilege levels if that option has been set in the ITCR register. SEGMENT CHANGES A segment change should not occur while tracing is stopped since the event will not be recorded in the ttace. Segment change difficulties are partially resolved by selecting an option to immediately follow a Trace Stop TCODE=1000 entry with a current-segment base entry. Referring to FIGUFtE 6E, a ttace entry used to report a change in segment parameters is shown. During processing of a trace stteam, ttace address values are combined with a segment base address to determine an instruction's linear address. The base address, as well as the default data operand size (32 or 16-bit mode), are subject to change. As a result, the TCODE = 0011 and TCODE=0111 entries, which always occur in pairs, are configured to report information for accurately reconstructing instruction flow. The Previous Segment Base (TCODE=0011) code is used to report the segment base address and segment attributes of the previous code segment. The TDATA field of the first record with a TCODE = 0011 entry indicates the high-order 16-bits of the previous segment base address. The associated second Multiple Trace record with a TCODE = 0111 entry indicates the low-order 15 - 4 bits of the previous segment base address and the previous segment attributes. The segment attributes report status for paging (PG), operand sizes (SZ), and addressing modes, either real or protected (R P) the same as the segment atttibutes report status for the Cuπent Segment Base (TCODE = 0100) that is discussed in conjunction with FIGURE 6C. The Previous Segments Base records occur whenever a trace event causes instruction execution to begin in another segment or whenever a change in segment attributes occurs. Note that the previous segment refers to the segment from which instruction execution arose. The segment information generally relates to the previous segment, not a cuπent (target) segment. Cuπent segment information is obtained by stopping and examining the state of the processor core 104. USER SPECIFIED TRACE ENTRY Under some circumstances an application program or operating system advantageously adds additional information into a trace stteam. In one example, an x86 instruction is supported that enables a 16- bit data value to be placed in the trace stream at a selected execution position. The instruction is implemented as a move to I/O space with the operand supplied by memory or a register. When the processor core 104 executes the x86 instruction, the user-specified ttace entry is captured by the ttace conttol circuit 218 and placed in the ttace buffer 200. Referring to FIGURE 6F, the User Trace (TCODE = 1001) entry indicates a user-specified ttace record and inserts the selected additional information into the ttace stream such as a previous or cuπent task identifier when a task switch occurs in a multi-tasking operating system. The User Trace (TCODE=1001) entry is also useful with a multitasking operating system. For example all tasks may run with a segment base of zero and paging is used to map the per-task pages into different physical addresses. A task switch is accompanied by a segment change. The ttace entry for the segment change reports little information, merely a zero base address. During task switching, the operating system may generate a User Trace (TCODE=1001) entry indicating more information, the previous task or the cuπent task identifier. SYNCHRONIZA TION OF TRA CE DA TA During execution of typical software on a processor-based device 102, few ttace entries contain address values. Most entries have the TCODE = 0001 format and a single bit indicates the result of a conditional operation. When examining a ttace stream, however, data is only studied in relation to a known program address. For example, starting with the oldest entry in the ttace buffer 200, all entries until an address entry have little use. Algorithm synchronization typically begins from a trace entry supplying a target address. If the trace buffer 200 contains no entries with an address, then ttace analysis cannot occur, a rare but possible condition. For this reason, a synchronization register TSYNC is supplied to conttol the injection of synchronizing address information. If the synchronization register TSYNC is set to zero, then ttace synchronization entries are not generated. TSYNC (Trace Synchronization) Trace Entry Synchronization Entry Conttol Register Referring to FIGURE 6G, an exemplary trace synchronization entry is shown. During execution of the processor core 104, a counter register is set to the value contained in the synchronization register TSYNC whenever a trace entry containing a target address is generated. The counter is decremented by one for all other trace entries including each TCODE=0001 (BFDEsLD-type) ttace entries, ffthe counter reaches zero, a ttace entry is inserted (TCODE = 0110) containing the address of the most recently retired instruction (or, alternatively, the pending instruction). When a synchronizing entry is recorded in the trace buffer 200, the entry also appears on the trace pins 220 to ensure sufficient availability of synchronizing ttace data for full- function ICE equipment. The TSYNC value is optionally programmed to adjust the trade off between ttace buffer 200 efficiency and ensuring an address is available for trace analysis. A synchronizing entry that is recorded in the trace buffer 200, is also output to the ttace pins to ensure sufficient availability of synchronizing ttace data for the full-function in-circuit emulation equipment. Trace entry information is also expanded to include data relating to code coverage or execution performance. The information is useful, for example, for code testing and performance tuning. Even without these enhancements, enabling the processor core 104 to access the ttace buffer 200 is useful. In the case of a microcontroller device, the ttace buffer 200 is accessed by mapping the ttace buffer 200 within a portion of I/O or memory space. A more general approach involves including an instruction that supports moving ttace buffer 200 data into system memory. The foregoing describes a processor-based device providing a flexible, high-performance solution for furnishing instruction ttace information. The processor-based device incorporates an instruction ttace buffer supplying ttace information for reconstructing instruction execution flow on the processor without halting processor operation. Both serial and parallel communication channels are supported for communicating ttace data to external devices. The disclosed on-chip instruction ttace buffer alleviates various of the bandwidth and clock synchronization problems that arise in many existing solutions, and also allows less expensive external capture hardware to be utilized. A signal definition includes two sections, a standard section and a bond-out section. Standard signals are shown in TABLE I and are available for usage on all embedded processor device 102 integrated circuits. Bond-out signals are shown in TABLE H and are available only on specially-packaged parts, typically for usage by in-circuit emulation (ICE) vendors. A TBUS[18:0] is shared between the parallel port 214 and the trace pad interface 220. The remaining bond-out signals are dedicated to either the parallel port or the trace pad interface 220. TABLE 2: Bond-out Signals The ttace port 220 is available only on bondout chips. The TBUS pins are shared with the parallel debug port 214 if the parallel option has been enabled by setting the PARENB bit in the DCSR. Trace records generated by the processor core 104 are sent to the ttace port 220 in external ttace mode. The ttace port signals are shown in TABLE 2. The TRACECLK timing is programmable through bits in the ITCR. The ttace port signals transition synchronous to the TRACECLK signal. The TV signal asserts when valid ttace data is driven on the TBUS. The TBUS signals are to be sampled on the next rising edge of the TRACECLK signal after assertion of TV. Referring to FIGURE 7, a state diagram illustrates a high-level flowchart of the host/serial port interface of the JTAG TAP conttoller 204 when using standard JTAG communication. The host system 111 writes a DEBUG instruction to the JTAG TAP conttoller 204 in operation 702, shifting a 38-bit instruction value into the JTAG instruction register with the Command Pending Flag (P) asserted and data (if applicable, otherwise zero) in the data field in operation 704. Upon completing the command, which typically involves transferring a value between the data field of the JTAG instruction register and a predetermined processor register or memory/10 location in the processor core 104, the processor core 104 clears the Pending flag (P=0) and asserts the Finished flag (F=l), simultaneously storing a value in the data field, if applicable. The entire 38-bit register is scanned out to monitor the Finished (F) and Pending (P) flags. If the Pending flag P is reset to zero and the Finished flag F is set to one, the previous command is finished. The status of the flags is captured during the Capture-DR TAP conttoller state shown in FIGURE 3. A slave copy of the flag status is saved internal to the JTAG TAP conttoller 204 and checked in operation 706 to determine in logic state 708 whether the next instruction is to be loaded in the Update-DR TAP conttoller state. If the Finished flag (F) is set, a new 38-bit command is scanned in operation 706, otherwise the previous data scan is repeated in operation 710. A slave copy is maintained due to the possibility of the status changing between the Capture- DR and Update-DR TAP conttoller states. The processor saves the slave copy to determine the status the user will detect at the time the status capture is performed to prevent the loading of the next instruction if the user sees that the previous instruction has not fimshed Referπng again to FIGURE 2, the processor interface state machine 202 performs asynchronous control operations for adding the signals CMD ACK, BRTC, STOPTX, and TRIG to the standard JTAG interface The CMD ACK, BRTC, STOPTX, and TRIG signals are enabled when the DEBUG instruction is wπtten to the JTAG instruction register m the JTAG TAP conttoller 204, but forced to a logic zero when disabled The BRTC signal pin is supplied with an internal pull-down resistor (not shown) Referπng to FIGURE 8, a flow diagram illustrates state operation of the enhanced JTAG interface The host system 111 wπtes a DEBUG instruction to the JTAG TAP conttoller 204 in operation 802 then optionally momtors the output signal CMD ACK to determine command completion status in a logic operation 804 The CMD ACK signal is asserted high simultaneous with assertion of the Finished flag (F) and remains high until the next shift cycle begins When using the CMD ACK pin, the Fimshed flag (F) status is captured without shifting out the seπal port register of the JTAG TAP conttoller 204 since the CMD ACK pm transitions high on the next πsmg edge of TCK after the real-time status of the Finished flag (F) changes from 0 to 1 The CMD ACK signal is not delayed from the state saved duπng the previous Capture-DR state entry of the TAP conttoller state machine Accordingly, in the enhanced JTAG mode a new shift sequence is not started m operation 806 until the CMD ACK pin is asserted high The CMD ACK pm asserts high when the seπal port is ready to receive instructions after the DEBUG instruction is loaded into the JTAG instruction register The CMD ACK signal is synchronous with the TCK signal TCK is generally not be clocked at all times, but is clocked continuously when waiting for a CMD ACK response The BRTC input signal functions either as a break request signal or a trace capture enable signal depending on the BRKMODE bit in the DCSR The BRTC signal, when set to function as a break request signal, is pulsed to cause the host system 111 to enter debug mode If the BRTC signal is set to function as a trace capture enable signal, asserting the signal high activates the ttace capture Deassertmg the BRTC signal deactivates the ttace capture The BRTC signal takes effect on the next instruction boundary after detection and is internally synchronized with the internal processor clock The BRTC signal is asserted at any time The TRIG output signal is optionally enabled to pulse whenever an internal breakpoint in the processor core 104 is asserted The TRIG signal event is typically used to tπgger an external captuπng device such as a logic analyzer The TRIG signal is synchronized with TRACECLK, the ttace record capture clock When the processor core 104 generates a breakpoint or, more precisely a pulse point, the TRIG output signal event is synchronized with TRACECLK and pulsed for one TRACECLK peπod In the illustrative embodiment, conventional debug breakpoints DR0-DR3 are modified to alternatively generate a pulse without the processor stopping in the manner of a breakpoint event The STOPTX output signal is asserted high when the host system 111 enters debug mode and is ready for register inteπogation and modification, or memory or I/O reads and writes through the serial/parallel command interface. In a normal (nondebug) mode, the STOPTX signal is asserted high when the host system 111 is ready to ttansmit data during Operating System (OS) or Application communication. The STOPTX signal reflects the state of bit 7 or bit 11 in the debug control/status register (DCSR). The STOPTX signal is synchronous with TRACECLK. When external instruction ttacing through the bond-out DEBUG trace port is enabled, assertion of STOPTX is delayed until all data in the instruction ttace buffer 200 is ttansfeπed out. Referring again to FIGURE 2, the parallel port 214 is a high-performance interface that is typically available in the bond-out version of the target system 101. The parallel port 214 supplies a 16-bit data path is intended to perform fast downloads and uploads between the host system 111 and the target system memory. The parallel port 214 is optionally used for all debug communication with the target system 101 whenever the processor core 104 is stopped. The serial port interface, either standard or enhanced, is used for debug access to the target system 101 when the processor core 104 is executing instructions. The parallel port 214 includes a 16-bit wide bidirectional data bus PDATA[15:0], a two-bit address bus PADR[2:0], a read/write sttobe PRW, and a request-grant pair PBREQ - PBGNT. The interface uses TCK (see Table 1) for synchronization of all transactions. TCK is continually clocked while the parallel debug interface is enabled. The parallel port 214 is enabled by setting DCSR:3 to logic 1 via the serial port of the JTAG TAP conttoller 204. The serial port interface is not disabled when the parallel port is enabled and is mandatory for access while the processor core 104 is executing instructions. Any ttansaction started on the parallel port 214 completes on the parallel parallel port 214. Similarly, any ttansaction begun on the serial port of the JTAG TAP conttoller 204 also completes on the serial port. In the illustrative embodiment the parallel parallel port 214, when enabled, is used for all save state and restore state cycles to avoid unpredictable behavior. All input signals to the parallel port 214 are sampled on the rising edge of TCK. All output signals are changed on the falling edge of TCK. Referring to FIGURE 9, a timing diagram illustrates three complete bus cycles of signal behavior for acquisition and release of a parallel debug bus PDATA[15:0] and PADR[2:0]. Because pins are shared between the parallel port 214 and the DEBUG ttace port 220, parallel port transmission commands are initiated only while the processor core 104 is stopped and the DEBUG ttace port 220 is disconnected from the shared bus. A request-grant handshake pair of signals PB REQ / PB GNT are supplied to expedite turnaround of the shared bus signals between the DEBUG ttace port 220 and the parallel port 214. When the host system 111 interface to the parallel parallel port 214 determines that PBREQ is asserted high, the host system 111 is to begin driving the parallel port 214 signals and assert PBGNT. When PBREQ is deasserted, the host system 111 interface to the parallel port 214 responds by tri-stating host system 111 interface signals and deasserting PBGNT to indicate that the host system 111 interface is isolated from the bus. To prevent bus contention, devices driving the parallel port 214 are tri-stated whenever PBGNT is deasserted. The PBREQ signal is asserted immediately before beginning a save state sequence penultimate to debug mode entry and is deasserted after the last restore state sequence data word is ttansfeπed. When the debug mode is either commenced or terminated when the parallel port 214 is enabled, the parallel parallel port 214 is used for processor core 104 state save and restore cycles. On the last restore state cycle, the parallel port 214 conttoller deasserts the PBREQ signal after latching the write data. The CMD ACK instruction is not yet asserted because the processor core 104 is not released to execute code until the DEBUG ttace port is available. The parallel port 214 host interface responds to the PBREQ deassertion by tri-stating the parallel port 214 drivers and deasserting the PBGNT signal. The parallel port 214 conttoller then activates the DEBUG trace port pin drivers in the debug ttace port 220, completes the last restore state cycle, asserts the CMD ACK signal, and returns control of the debug ttace port 220 interface to the ttace conttol circuit 218. Referring to FIGURE 10, a flow chart depicts a parallel debug port interface protocol followed by the host system 111 during communications via the parallel port 214. Address pins PADR[2:0] select the field of the 38-bit internal debug register that is mapped to the 16-bit data bus PDATA[15:0], The field is selected from among a null selection, the right 16 bits of debug data, the left 16 bits of debug data, and a four-bit command register (PDATA[3:0]). The internal pending flag (P) is automatically set when performing a write operation to the 4-bit command register, and is cleared when the Finished flag (F) is asserted. The host system 111 typically monitors the CMD ACK pin to determine when the Finished flag (F) is asserted. On read cycles, PADR is set to 00 for one clock cycle before and after reading data to allow for bus turn-around. The JTAG instruction register, also called a serial/parallel debug command register, uses the 4-bit command field cmd[3:0] to point to the various internal registers and functions in the JTAG TAP conttoller 204 interface. The serial/parallel debug command register is accessible only via the serial parallel debug interface of the JTAG TAP controller 204. Some of the commands use bits from the debug data field as a sub- field to extend the number of available commands. Table 3 is a map of available functions. The Load Soft Address register command places a new 32-bit value for the soft address pointer in the debug data[3 1 :0] field in combination with the command code and the pending bit (P). Debug logic transfers the data to the Soft Address register and asserts the Finished flag (F) and the CMD ACK signal. The Memory or I/O read or write command is issued following a command that sets an address and port designation in the Soft Address register. For a read command, data is ttansfeπed directly to the debug data register when the Finished flag (F) and CMD ACK pin is set. For a write command, data is supplied in the debug data register in combination with the command in the Debug Command register. The address is postincremented by the appropriate size for any read/write transfer to allow block transfers without continually setting up the Soft Address. On memory accesses of a microcontroller with multiple physical memory spaces, the appropriate chipset mapping registers are set prior to issue of the access command. Memory accesses occur as data accesses. A memory access is directed either to normal memory space or SMM space based on the setting of the SMMSP bit in the DCSR. Issue of a General or Serial parallel Debug Register Read command includes designation of a register address in the debug data register in combination with identification of the command in the Debug Command register. The address is ttansfeπed to the Soft Address register as a side effect to simplify read/modify/write setting of individual bits, and the register contents are transfeπed directly to the debug data register when the Finished flag/CMDACK pin is set. For a General or Serial/parallel Debug Register Write command, the Soft Address is used to hold the register address. The soft address is set up with a previous Read Register or Load Soft Address command. The data to be written is supplied in the debug data register in combination with the command in the Command register. In some embodiments, Read/Write and Save/Restore Data commands include state save reads and state restore writes. For state save read commands, the command code is loaded with the Pending bit (P) asserted. When the Finished flag (F) is set or CMD ACK is asserted, the save state data is placed in debug_data[31 :0]. If no save state data is left to read, the command field cmd[3 :0] reads back all zeros. If additional save state data is available to read, the command field cmd[3:0] reads back all ones. On state restore write commands, restore data is loaded into debug_data[31:0] with the command code and the Pending bit (P) set. When the Finished flag (F) and CMD ACK pin are set, and the command field cmd[3:0] reads back all ones, the processor is ready for the next restore ttansaction. When the Finished flag (F) and CMD ACK pin are set and the command field cmd[3:0] reads back all zeros, the state restore is complete. The save/restore commands may be avoided in embodiments that do not utilize context switching. The Read Trace Record command implements read access to the 256-record internal trace buffer 200 when the internal instruction trace configuration is selected. The read ttace record command is not applicable to an external ttace configuration since all trace record data is output to the bond-out DEBUG trace port upon generation. The read trace record command is invoked by setting bit 0 of the Instruction Trace Configuration Register (ITCR) to logic 1 to enable the instruction trace buffer 200, then the internal trace configuration is selected by setting bit 3 of the ITCR register to 0. The Read Trace Record command reads the 21 -bit ttace record at the displacement given in the soft address register, places the record in bits 20:0 of the debug data register and asserts the finished flag and CMD ACK pin. The soft address register is post-incremented so that a subsequent read retrieves the next location in the buffer in reverse order of history. A read operation from displacement 0 retrieves the most recent ttace record. A read operation from displacement 255 retrieves the oldest trace record in the buffer. When the instruction ttace buffer 200 is read, the valid bit on each record is to be checked. If the valid bit of a record is zero, an event that occurs only when one or more of the 256 locations of the buffer is not loaded with ttace record data since last initialized, the record should be discarded. The ttace buffer 200 wraps around and continually overwrites the oldest data when full. Once the buffer has wrapped around, all valid bits are set, and are cleared only when the TDSJTT bit (bit 2) in the Instruction Trace Configuration Register is set. The Peripheral reset command sends a reset signal to all system logic except the processor core 104 to be pulsed active and released. The peripheral reset command allows peripheral logic to be reinitialized without resetting the processor core 104. A command to enter or exit Debug mode enables and disables debug functionality. Debug functions are enabled by writing the DEBUG instruction to the JTAG TAP controller. When a DEBUG instruction is received, the debug serial port register is enabled to receive commands. While the processor is executing instructions only the Debug Control/ Status register, Instruction ttace configuration register, RX_DATA, and TX_DATA registers are accessed through the serial interface. All serial debug registers become accessible when the processor has stopped. The processor may be stopped (forced into debug mode) by one of the following methods: • Setting bit 1 of the Debug Control/Status register (DCSR) • Pulsing the BRTC pm (low-to-high transition) • Via legacy processor core 104 debug registers DRO-DR3 after setting bit 4 of the Debug Control/Status register (DCSR) Single stepping with the TF bit set causes entry to debug mode when DCSR bit 4 is set Moving to and from debug registers with the GD bit of DR7 set causes entry to debug mode when DCSR bit 4 is set • Executing a EDEBUG instruction Inserting a EDEBUG instruction into the code stream enters debug mode at a specific, arbitrary point in source code The processor core 104 is set in debug mode (DEBUG instruction must have been wπtten to the TAP conttoller) or the EDEBUG instruction causes an invalid opcode exception The external pin STOPTX or bit 7 of the Debug Control/Status register (DSCR) is optionally monitored to determine when the processor enters debug mode The flushing of cache on entry to debug mode is controlled by the DISFLUSH bit of DCSR If the bit is reset the cache is flushed (using the SLE486 FLUSH pin) upon entry to debug mode If the bit is set the cache is not flushed on entry to debug mode A partial state save is utilized upon entry to debug mode On receipt of the STOPTX signal, either by sampling of the STOPTX pin or by polling DCSR bit 7, the host system 111 performs a seπes of reads using command code 111 lb before entry to debug mode is complete The restore data is read from the parallel port interface if the interface is enabled, otherwise the restore data is read from the seπal port Debug mode is exited through command 0111 The exit options include • Exit and begin execution at cuπent value of ED? In one example, the processor jumps to a section of patch code A breakpoint is set at the end of the patch code to enable reentry to debug mode In another example, the processor state is restored to the oπginal debug entry state, using the general register wnte commands, before exiting A partial state restore is performed before execution resumes • Exit and perform a single instruction step A partial state restore is performed The processor executes a single instruction, takes a trap, and reenters debug mode, performing a partial state save • Exit via a hard reset No state restore performed A hard CPU reset is asserted immediately • Exit via a soft reset A state restore is performed before the soft CPU reset asserted In embodiments that employ context switching, before completing an exit from debug mode the pro- cessor core 104 performs a partial state restore, except on a hard reset, retπeving the data saved on entry to debug mode The host supplies the data by a seπes of wπtes using command code 111 lb before execution resumes. The host optionally modifies the data saved. The restore data is written to the parallel port interface if enabled, otherwise the restore data is written to the serial port. The X86 Enhanced Software debug mode supplies a ttace and breakpoint interrupt debug functionality on the processor core 104 without inclusion of external debug hardware. The mode is enabled by setting DR7 bit 12. The serial interface does not need to be enabled. When the X86 Enhanced Software debug mode is enabled, access and control of the instruction ttace buffer 200 and the ITCR (Instruction Trace Conttol Register) are supplied through a set of reserved instructions. The instructions cause an illegal opcode exception if executed when DR7 bit 12 is not set. Debug conttol and status is configured by setting the Debug Control Status Register (DCSR) through the Serial/Parallel debug interface, using the serial/ parallel debug registers read/write command, address 00. DCSR conttol and status bits are described, as follows: • Bit 0 (PRESET) is a reset bit allowing the host system 111 to completely reset all devices on the target system 101 other than the Serial/Parallel conttoller. FRESET is useful in the event of a total target system crash. • Bit 1 (FBRK) is a register version of the BRTC pin. The host system 111 writes a 1 to FBRK to force a break into debug mode at the next instruction boundary following synchronization of the received command with the internal processor clock. • Bit 2 (DSPC) is a processor clock disable bit for disabling the stopping of internal processor clocks that normally occurs when entering Halt and Stop Grant states. DSPC allows the processor to continue to respond to break requests while halted or in the Stop Grant state. • Bit 3 (PARENB) is a parallel port enable bit that enables the auxiliary parallel data port. PARENB is not set on non bond-out versions of the target system 101. • Bit 4 (DBTEN) is a debug trap enable bit that causes entry into a hardware debug mode for all debug traps/faults of the processor core 104 that otherwise cause a software INT 1. The trace mode on/ off toggling control in the ITCR has priority over DBTEN in that breakpoints mapped for ttace mode on/off toggling are not affected by setting of DBTEN. • Bit 5 (BRKMODE) is a bit for conttolling entry into debug mode. BRKMODE allows a developer to change the functionality of the BRTC pin to become an external control for enabling and disabling the trace capture operation. When the JTAG TAP conttoller 204 is programmed with a DEBUG instruction, the BRTC pin causes the processor core 104 to stop executing an instruction sequence and enter debug mode. Setting of the BRKMODE pin causes the BRTC pin to conttol activation of the trace capture operation The ttace capture status is designated by the TRON bit of the ITCR • Bit 6 (FRCRDY) is a bit that controls forcing the RDY signal to the processor that is active to enable the processor to move on to the next bus cycle in cases where a bus operation to an undefined memory or I/O space has occuπed in the absence of a subtractive decode agent Use of FRCRDY is to be coordinated with chipset logic • Bit 7 (STOP) is a stop bit that supports a software technique for checking the state of the STOPTX pin When STOP reads back high, the processor is in debug mode and all debug commands are enabled • Bit 8 (SMMSP) is an SMM conttol bit that allows memory accesses imtiated through the debug port 100 to take place in the SMM space When SMMSP is reset, memory accesses imtiated through the debug port 100 apply to the normal memory address space Setting Of SMMSP causes memory accesses imtiated through the debug port 100 to apply to the SMM address space • Bit 9 (DISFLUSH) is a conttol bit for conttolling flushing of a cache on entry to debug mode With DISFLUSH reset the cache is flushed on entry to debug mode Setting DISFLUSH prevents the cache from flushing on entry to debug mode • Bit 10 (RX) is a data received bit that indicates whether data has been received from the host system 111 so that the processor core 104 can read the data from the RX DATA register • Bit 11 (TX) is a data transmit bit that indicates the processor core 104 is ready to transmit data so that the host system 111 can read the data from the TX D ATA register Commumcation between an operating system (OS) and Applications via the JTAG Debug port 100 is imtiated by the host system 111 wπting the DEBUG instruction to the JTAG instruction register m the JTAG TAP controller 204 Wnting of the DEBUG instruction causes the Seπal Debug Shifter 212 to connect to the JTAG TDI-TD0 seπal interface of the JTAG TAP conttoller 204 The seπal debug port 100 includes two debug registers for transmitting (TX_DATA register) and receiving (RX DATA register) data TX_DATA and RX DATA are accessed using the soft address and senal/ parallel debug register commands The processor core 104 initiates a data transmission by first testing the read-only TX bit in the ITCR register If the TX bit is set to 0 then the processor core 104 executes an X86 instruction to ttansfer the data to the TX_DATA register The seπal port 100 sets the TX bit in the DCSR and ITCR registers indicating to the host system 111 data is ready for transmission The seπal port 100 also sets the STOPTX pin to high After the host system 111 completes reading the transmit data from the TX_DATA register, the TX bit is set to 0 A TXINTEN bit in the ITCR register, when set, generates a signal to interrupt the processor core 104 The interrupt is generated only when TX bit in the ITCR register makes a transition to 0. When the TXINTEN bit is not set, the processor core 104 polls the TX bit of the ITCR register to further ttansmit the data. The host system 111 sends data to the processor core 104 by first testing the read-only RX bit in the DCSR register. If the RX bit is set to 0, then the host system 111 writes the data to the receive data (RX DATA) register and the serial port 100 sets the RX bit to 1 in the DCSR and ITCR registers. The RXINTEN bit in the ITCR register, when set, generates a signal to interrupt the processor core 104. The interrupt is only generated when RX bit in the DCSR makes a transition to 1. When the RXINTEN bit is not set, the processor core 104 polls the RX bit of the ITCR register. If the RX bit is set to 1, the processor core 104 executes an X86 instruction to read the data from the receive data (RX DATA) register. After data is read by the processor core 104 from the RX DATA register, the RX bit is set to 0 by the serial port 100. The host system 111 continuously reads the DCSR register and momtors the RX bit to continue sending data. While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiments described are possible. For example, those skilled in the art will readily implement the steps necessary to provide the structures and methods disclosed herein, and will understand that the process parameters, materials, and dimensions are given by way of example only and can be varied to achieve the desired structure as well as modifications which are within the scope of the invention. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.

Claims: WE CLAIM: 1. A debug interface in a target processor for debug interfacing to a host system, the debug interface comprising: a ttace conttoller for coupling to the target processor and receiving ttace information from the target processor, the trace conttoller detecting selected trace information and formatting samples of the selected trace information; and a ttace buffer coupled to the ttace conttoller, the ttace buffer including a plurality of ttace data storage elements including a trace code (TCODE) field indicative of a type of trace information and a ttace data (TDATA) field indicative of a type of ttace information data. 2. An interface according to Claim 1 wherein: the ttace data storage elements of the ttace buffer further include a verify bit indicative of whether the ttace data storage element specifies a valid trace record. 3. An interface according to Claim 1 wherein: a ttace code (TCODE) designates a missed ttace type of ttace information indicative that the target processor missed reporting some ttace records. 4. An interface according to Claim 1 wherein: a ttace code (TCODE) designates a conditional branch type of ttace information indicative of an occurrence of a conditional branch instruction; and a ttace data (TDATA) coπesponding to the conditional branch TCODE designating a plurality of bits indicating taken branches and not taken branches. 5. An interface according to Claim 4 wherein: a ttace data (TDATA) coπesponding to the conditional branch TCODE designates a plurality of bits that are initially cleared except for a set left-most bit and either set or cleared to indicate an outcome of up to 15 conditional branch events in which as a new conditional branch is encountered, a new bit is added on the left and other entries are shifted right one-bit. 6. An interface according to Claim 1 wherein: a ttace code (TCODE) designates a branch target type of trace information indicative of a target address of a data-dependent branch instruction; and a ttace data (TDATA) coπesponding to the conditional branch TCODE designating a value indicative of an Extended Instruction Pointer of the data-dependent branch instruction. 7. An interface according to Claim 1 wherein: a trace code (TCODE) designates a previous segment base type of trace information indicative of a segment base address of a previous code segment; and a ttace data (TDATA) coπesponding to the previous segment base TCODE designating the segment base address and segment attributes of the previous code segment, the segment attributes reporting a status for paging, operand size, and addressing mode. 8. An interface according to Claim 1 wherein: a ttace code (TCODE) designates a current segment base type of ttace information indicative of a segment base address of a cuπent code segment; and a trace data (TDATA) coπesponding to the previous segment base TCODE designating the segment base address and segment attributes of the cuπent code segment, the segment attributes reporting a status for paging, operand size, and addressing mode. 9. An interface according to Claim 1 wherein: a ttace code (TCODE) designates an interrupt type of ttace information indicative of an asynchronous or synchronous event such as an interrupt or trap; and a trace data (TDATA) coπesponding to the interrupt TCODE designating a vector number of an exception or interrupt. 10. An interface according to Claim 1 wherein: a ttace code (TCODE) designates a trace synchronization type of ttace information indicative of a cuπently executed instruction; and a ttace data (TDATA) coπesponding to the ttace synchronization TCODE designating an address of a cuπently executed instruction. 11. An interface according to Claim 1 wherein: a trace code (TCODE) designates a multiple ttace entry type of ttace information indicative of a ttace data extending to additional ttace data storage elements; and a ttace data (TDATA) coπesponding to the multiple trace entry TCODE and designating an additional record of a trace entry. 12. An interface according to Claim 1 wherein: a trace code (TCODE) designates a ttace stop type of trace information indicative of a directive to stop ttace capture; and a trace data (TDATA) coπesponding to the ttace stop TCODE and designating an instruction address at which trace capture is stopped. 13. An interface according to Claim 1 further comprising: a serial parallel interface coupled to the trace conttoller, the trace conttoller selectively sending the trace code (TCODE) and the ttace data (TDATA) serially or in parallel to the host system. 14. A processor comprising: an integrated circuit chip; a processor fabricated on the integrated circuit chip; an I/O interface coupled to the processor; and a debug interface fabricated on the integrated circuit chip coupled to the processor and coupled to the I O interface, the debug interface for debug interfacing to a host system, the debug interface including: a ttace conttoller for coupling to the target processor and receiving ttace information from the target processor, the trace conttoller detecting selected ttace information and formatting samples of the selected ttace information; and a ttace buffer coupled to the ttace conttoller, the ttace buffer including a plurality of ttace data storage elements including a ttace code (TCODE) field indicative of a type of ttace information and a ttace data (TDATA) field indicative of a type of trace information data. 15. An processor according to Claim 14 wherein: the ttace data storage elements of the ttace buffer further include a verify bit indicative of whether the ttace data storage element specifies a valid ttace record.

Inventors: MANN DANIEL P, MANN, DANIEL, P.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1998
Main classification: G06F11
IPC codes: G06F11