Processor with data format-independent instructions

Abstract: A processor (102) specially adapted for use as a coprocessor. The processor is implemented as a microprocessor. The adaptations include the following: The microprocessor has a master-slave pin (207) which receives an input which determines whether the microprocessor operates as a bus master (301) or a bus slave (303). Certain output pins have three-state bus drivers (1001) which employ feedback to ensure that a signal on a line being driven by the driver has gone inactive before the driver is turned off. Instructions (1225, 1233, 1241) executed by the microprocessor permit specification of portions of the internal registers ( 1104) as sources and destinations and specification of the size of an ALU operation, permitting easy operation on data ranging from bytes through 24-bit pointers. Instructions are executed in an instruction pipeline and a separate I/O instruction pipeline. Some pipeline stalls are avoided by means of a special MOVE instruction (1233, 1241) which differs from an ordinary MOVE instruction in that it does not cause pipeline stall when it reads data from a register loaded by a preceding READ instruction. The microprocessor also has an Intel/Motorola pin (309) whose input specifies the type of host processor (105) the coprocessor is working with and further executes I/O instructions which permit the same code to be used with more than one type of host processor.

Description: Technical Field The invention relates to digital data processors generally and more specifically to processors employed as coprocessors. Background of the Invention Field of the Invention As processors have gotten cheaper, more and more digital data processing systems have appeared in which several processors operate as coprocessors. A coprocessor is a processor which cooperates with another processor to process data. Classic examples of coprocessors are floating-point units for performing floating point arithmetic and I/O processors for handling the flow of data between peripheral devices such as terminals and the system memory. The relationship between two coprocessors lies along a continuum whose ends are described by the notions tightly coupled and loosely coupled . One coprocessor is tightly coupled to another when there is a high degree of direct interaction between the coprocessors. For example, floating point units are typically tightly coupled. The processor served by the floating point unit provides the operands to the floating point unit, indicates the operation to be performed, and receives the results directly from the floating point unit. The results typically not only include the result value, but also signals indicating the status of the floating point operation. I/O processors, on the other hand, are typically loosely coupled. Communication with the processor they serve is generally through the system memory. When the processor requires the assistance of the I/O processor to output data, the processor places the output data and a description of what the I/O processor is to do with it in memory at a location known to the I/O processor and then indicates to the I/O processor that the data is in memory. The I/O processor thereupon responds to the indication by retrieving the data from memory and outputting it to the desired peripheral device. When it is finished, it puts a record of the status of the operation in memory at a location known to the processor and indicates to the processor that it has finished the memory operation. The processor then responds to the indication by reading the data at the location to determine the status of the output operation. Description of the Prior Art A problem with memory sharing is that the processors which share the memory must agree on the format of the data which they jointly process. For example, two popular architectures for host processors are the Motorola 680x0 architecture and the Intel 80x86 architecture. These two architectures have different formats for pointers and for the order in which consecutively written bytes are stored in memory. A coprocessor which is to be used with either kind of host must some how deal with these differences. One way of dealing with them in the prior art has been a pin on the coprocessor whose input indicates whether the host is a 680x0 machine or an Intel 80x86 machine. This solution works well as long as the coprocessor reads data in exactly the same way as the host wrote it and vice-versa. The problem with such a solution is that it gives the system designer the choice of slowing the coprocessor down or writing code for the coprocessor which is different for the different kinds of hosts. If the coprocessor reads words as well as bytes and the host has written a sequence of bytes in memory, then the fastest way for the coprocessor to read the bytes is as words; however, if it does so, it will have to process the words differently depending on the host machine, and consequently different code will be necessary for the different host machines. Speed and the cost of writing code are both important considerations for the system designer, and consequently both alternatives are unattractive. It is an object of the apparatus disclosed herein to provide a processor which can both read sequences of bytes as words and employ the same code for both host machines. Summary of the Invention The invention disclosed herein is a processor which can share memory with other processors employing different formats for data stored in the memory. The processor executes instructions including at least one instruction which has data format dependency indicating means. The data format dependency indicating means indicate that the operation is dependent on a data format employed by the other processor with which the processor of the invention is sharing memory. The processor of the invention is characterized by data format signal receiving means for receiving a data format signal indicating the data format and operation means responsive to the data format signal and the data format dependency indicating means for performing the operation specified by the instruction as required by the data format. Brief Description of the Drawing FIG. 1 is a block diagram of a system in which the microprocessor of the present invention is employed; FIG. 2 is a diagram showing the pinouts of a preferred embodiment of the microprocessor; FIG. 3 is a byte or word read timing diagram showing how the AS and ERE and ORE lines go high before they are turned off; FIG. 4 is a block diagram showing the instruction architecture of a preferred embodiment of the microprocessor; FIG. 5 is a block diagram showing addressing of general registers in the microprocessor; FIG. 6 is a diagram showing the format and manner of execution of two types of MOVE instructions for the microprocessor; FIG. 7 is a diagram showing the format and manner of execution of a third type of MOVE instruction; FIG. 8 is a diagram showing the format of system memory I/O instructions for the microprocessor; FIG. 9 is a diagram of data formats employed by certain host processors with which the microprocessor is employed; FIG. 10 is a logic diagram showing control logic in the I/O subsystem of a preferred embodiment; FIG. 11 is a logic diagram showing an implementation of input latch 1149 in a preferred embodiment; FIG. 12 is a logic diagram showing an implementation of output latch 1151 and shadow output latch 1155 in a preferred embodiment; FIG. 13 is a truth table for READ and READ-MODIFY-WRITE I/O operations in a preferred embodiment; and FIG. 14 is a truth table for WRITE I/O operations in a preferred embodiment. Detailed Description The following Detailed Description of a preferred embodiment of the microprocessor of the present invention will first show how the microprocessor might be used as a coprocessor in a digital data processing system, will then disclose the interface between the microprocessor and such a system, and will continue with discussions. Coprocessor-dependent execution of system word read operations and system word write operations. System Employing the Microprocessor as a Coprocessor: FIG. 1 FIG. 1 shows a system 101 in which the microprocessor of the present invention is employed as a coprocessor. Components of the system include memory 102, system bus 103, host processor 105, microprocessor 107 of the present invention with its private memory (PMEM) 108, and serial controller (SCTL) 109. All of the components are connected by system bus 103, and host processor 105, microprocessor 107, and serial controller 109 all have access to memory 102 via system bus 103. Private memory 108 is connected to microprocessor 107 by private bus (PB) 110. Private memory 108 is accessible only to microprocessor 107. Contents of private memory 108 include programs executed by microprocessor 107 and may also include data used in the execution of those programs. In a preferred embodiment, private memory 108 is a read-write memory and the programs executed by microprocessor 107 are downloaded by microprocessor 107 from memory 102 into private memory 108. In the system of FIG. 1, microprocessor 107 operates as a coprocessor for dealing with data which is sent or received according to a serial communications protocol. As such, it mediates between host processor 105, which is the source of, or destination for, the data which is sent or received via the protocol, and serial controller 109, which is connected to serial link 111 and which transmits and receives serial protocols over link 111. Operation of system 101 is as follows: host processor 105 maintains the area SDATA 115 in memory 102 which it uses for data to be sent over SL 111 and RDATA 117 for data which is received over SL 111. When host processor 105 wishes to send data, it places a command to that effect in host command queue (HCQ) 119. The command includes at least a destination address for the data and the location of the data in SDATA 115. Host command queue 119 is at a location in memory 102 which is known to microprocessor 107, and microprocessor 107 periodically polls host command queue 119 to determine whether a new command has arrived. When one has, microprocessor 107 locates the data to be sent in SDATA 115, puts it into the proper form for serial controller 109, and places a pointer to the data and a command into microprocessor command queue (MPCQ) 123. That queue is in a location in memory which is known to serial controller 109, and serial controller 109 periodically polls microprocessor command queue 123. When serial controller 109 finds a command and a pointer in queue 123, it outputs the data at the location specified by the pointer to serial link 111 in the manner specified by the command. When serial controller 109 receives data on serial link 111, the reverse of the above takes place. Serial controller 109 places the data it has received in RDATA 117 and then places a pointer to the data in microprocessor interrupt queue (MPIQ) 125; microprocessor 107 periodically polls queue 125, and when there is a pointer in queue 125, microprocessor 107 puts the data indicated by the pointer into the proper format for host processor 105. Thereupon, microprocessor 107 places an interrupt message including a pointer to the data in host interrupt queue (HIQ) 121 and provides an interrupt to host processor 105 via interrupt line 113. Host processor 105 responds to the interrupt by examining host interrupt queue 121, and determines from the interrupt message that incoming data has been placed in RDATA 117. Interface between System 101 and Microprocessor 107: FIG 2 FIG. 2 is a diagram showing the pins of a preferred embodiment 201 of microprocessor 107 which is implemented as a single integrated circuit in a 132-pin package. Other embodiments may multiplex some pins, and in other embodiments, microprocessor 107 might be a component of a larger integrated circuit; for example, microprocessor 107 and one or more serial controllers 109 might be combined to form a single integrated circuit for handling serial protocols. While microprocessor 107 would have no pins of its own in such an embodiment, it would still send and receive signals substantially equivalent to those sent and received via the pins of preferred embodiment 201. Certain of the pins of preferred embodiment 201 are of particular interest in the following discussion. They are MASTER/ SLAVE pin 207, BUSACK pin 203, BUSREQ pin 205, I/ M pin 209, DTACK pin 211, and CLOCK pin 213. The functions of these pins, as well as all of the others, are summarized in the following table: As may be seen from the above table, the preferred embodiment is intended for use in a system 101 in which memory 102 has 16-bit words and is byte addressable; system bus 103 consequently includes 23 address lines, 16 data lines, and control lines. In some embodiments, the control signals BUSACK , BUSREQ , BUSERR , CLOCK, DTACK , RESET , INTIN , INTOUT , EWE , OWE , ERE , ORE , and AS may be part of system bus 103; in others, they may be provided to or received from other logic external to microprocessor 107 which mediates between system 101 and microprocessor 107. Implementation of the IC of the Preferred Embodiment The integrated circuit of the preferred embodiment was implemented using CMOS standard cell technology. Much of the control logic for microprocessor 107 was specified as follows: first, a state machine was defined for a given portion of the control logic. The state machine consisted of the states of the portion of the system being controlled by the given portion of the control logic and the transitions between the states. Next, an emulator for the state machine was written in the "C" language. The emulator was tested to confirm the correctness of the state machine. Thereupon, the emulator code was used as an input to a so-called "silicon compiler". The output of the silicon compiler was a specification of circuits in the integrated circuit of the preferred embodiment which implemented the state machine described by the emulator code. The tools used to implement microprocessor 107 were well within the current state of the art. As can be seen from this fact and the foregoing disclosure of the manner in which control logic in microprocessor 107 was implemented, a description of a state machine is all that is required by one skilled in the art to make and use circuitry which implements the state machine. The Instruction Architecture of Microprocessor 107: FIG. 11 The innovations in microprocessor 107 described thus far have concerned the interface between microprocessor 107 and a system 101 in which the microprocessor 107 is employed; other innovations concern certain instructions executed by microprocessor 107. The following discussion will first provide an overview of the instruction architecture of microprocessor 107 and then discuss those instructions which are of particular interest in detail. FIG. 11 shows instruction architecture 1101 of microprocessor 107, i.e., the figure shows microprocessor 107 as it appears to a programmer who is writing code consisting of instructions from microprocessor 107's instruction set. Each of the registers and other components of instruction architecture 1101 represents a logical device whose behavior may be specified in instructions of the instruction set. The description begins with the registers. REGISTERS There are 256 general registers 1103 (0 to 255) and 14 special purpose registers in instruction architecture 1101. GENERAL REGISTERS 1103 The 256 general registers 1103 make up general register file 1104. General registers 1103 are employed generally to store data being processed within microprocessor 107. Each general register is 24 bits wide, but the registers may be subdivided for addressing purposes. Addressing of general registers 1103 is shown in FIG. 11A. Each general register may be specified directly by an address between 0 and 255 or may be specified by means of an offset from the value contained in frame pointer (FRMPTR) register 1117. Within the addressed register, the following subdivisions may be specified: low byte 1157: bits 0-7 of the addressed register; middle byte 1159: bits 8-15 of the addressed register; high byte 1161: bits 16-23 of the addressed register; low word 1163: bits 0- 15 of the addressed register; high word 1165: bits 08-23 of the addressed register; and entire register 1167: bits 0-23 of the addressed register. With reference to internal data bus (IDB) 1115, the general registers can be either a source (read operation) or a destination (write operation). When a general register 1103 is a source, the specified portion of the general register is output to the least significant bits of internal data bus 1115; the remaining bits of bus 1115 are set to 0; when a general register 1103 is a destination, the number of bits of internal data bus 1115 required for the portion are written to the portion, beginning with the least significant bit of internal data bus 1115. As may be seen from the foregoing, it is possible to write any byte or word from one general register 1103 to any byte or word of another general register 1103 without disturbing the remaining portions of either the source or destination general registers 1103. Continuing with the special purpose registers, most of these registers are 24 bits wide. Generally, all of the special registers can be either source or destination for most instructions. One exception is accumulator (ACC) 1109 - it cannot be a direct destination for many register moves. PSW 1119 PSW 1119 is the Processor Status Word register. The format of PSW 1119 is as follows: The meaning of the bits is given in the following table: Mnemonic Full Name Explanation C Carry Carry flag from ALU operations. Z Zero Zero flag from ALU operations. EX Enable eXternal Enables external interrupts. ET Enable Timer Enables internal timer interrupts. L LIFO interrupt Set when the internal LIFO overflows/underflows. F FRMPTR interrupt Set when FRMPTR add/sub overflows/underflows. B Bus Error exception Set when the BUSERR pin is asserted. A Address Error exception Set when an odd address is used for word or long I/O. X External Interrupt Set when an external interrupt is recognized. I Illegal instruction Set when an illegal instruction is decoded. T Timer interrupt Set when the internal timer interrupts. SWB Byte swap bit Set to reverse the effect of I/ M At reset, all bits in PSW 1119 are cleared to zero. Thus, the Carry and Zero flags are cleared and the external interrupts and timer interrupts are not enabled. PSW 1119 may be read or written just like the other special-purpose registers, with a write often being used to change one of the interrupt enable flags or the SWB bit. However, for bits 4-10, the interrupt status flag bits, a move instruction can only clear these bits - they cannot be set by a move instruction. In other words, writing zeros will clear the interrupt status bits, and writing ones will leave the interrupt status bits unchanged. Therefore, to clear a particular interrupt status bit, the interrupt status bits which it is desired to NOT clear should be set to a 1 in the word which will be written to PSW 1119 to modify it. The interrupt status bit which is desired to be cleared should have a 0 in the corresponding bit position of the word written to PSW 1119. ACC 1109 ACC 1109 is a 24 bit register. It stores the result of operations by ALU 1107. In the case of 8 or 16 bit ALU operations, all 24 bits of ACC 1109 are affected, but only the 8 or 16 least significant bits will have valid information. With reference to internal data bus 1115, ACC 1109 is a source only, but an instruction exists to allow loading ACC 1109 from a general register 1103 by having ALU 1107 execute no operation on its input data. Also, ACC 1109 may be loaded from B register 1105, again by having ALU 1107 perform no operation on its input value. B Register 1105 B register 1105 is a 24 bit register. It provides one of the operands to ALU 1107 for ALU operation instructions. ALR 1145 ALR 1145 is a 24 bit address latch. It is used to hold the effective address for read operations of system memory 102. ALW 1146 ALW 1146 is a 24 bit address latch. It is used to hold the effective address for write operations to system memory 102. IL 1149 IL 1149 is a 24 bit register used to hold the input data from an external system memory read operation. The instruction set defines an 8 bit read, a 16 bit read and a 24 bit read which actually does two system reads to consecutive address locations in system memory 102. OL 1151 OL 1151 is a 24 bit register used to hold the output data for an external system memory write operation. The instruction set defines an 8 bit write, a 16 bit write and a 24 bit write which actually does two system writes to consecutive address locations. FRMPTR 1117 FRMPTR 1117 is an 8 bit register. General registers 1103 may be addressed by offsets ranging from +127 to -127 from the value stored in FRMPTR 1117. FRMPTR 1117 may be read and written from internal data bus 1115, and consequently, may be saved and set to a new value when a subroutine is entered and restored to its old value on return from the subroutine. This in turn makes it possible to use general registers 1103 as a stack of local variables used by a subroutine. PC 1127 PC 1127 is a 16 bit program counter. It is used to control the program flow through access to the instructions stored in both internal ROM and/or private memory 108. On each clock cycle, the output of the PC is temporarily stored in a 16 bit latch. From that latch, it can be stored in the last-in-first-out (LIFO) stack 1125. This stack is used to implement subroutine returns and interrupt returns. Input to PC 1127 is from LIFO 1125 and internal data bus 1115. Output is to LIFO 1125, IDB 1115, and private data bus 108. Note: When performing a move from PC instruction, since it is only 16 bits wide, the 6 bits (21-16) above the PC bits will contain the current value of the internal stack pointer, which is really a count of the number of addresses on the internal stack. These bits are provided so that during a post-mortem analysis of a program with a fatal error, it is possible to tell how many addresses to pop off of LIFO 1125 for examination. A move to PC will only affect the 16 bits of the PC. IT 1123 IT 1123 is the 24 bit internal timer register. It contains the value used to initialize and reload the down-counter when it has reached zero and possibly generated an interrupt. STACKREG 1133 STACKREG 1133 is a 16 bit register used to store the output from LIFO 1125 when a "popstack" instruction is executed. The popstack instruction and STACKREG 1133 are intended to be used only for the interrupt handler for catastrophic exceptions - i.e. to handle FRMPTR or LIFO overflow/underflow exceptions, or Bus Error or Address Error exceptions. STACKREG 1133 allows the contents of LIFO 1125 to be examined during a post-mortem analysis to determine why the catastrophic failure occurred. Instructions in Microprocessor 107 Certain aspects of the instructions executed by microprocessor 107 are of particular interest. Discussion of these aspects will begin with a description of instruction execution generally in microprocessor 107 and will then deal with the aspects as they appear in certain of the instructions. Instruction Execution in Microprocessor 107: FIG. 19 One technique which is widely used to speed up instruction execution in processors is pipelining . The execution of any instruction has a number of phases. For example, with a MOVE instruction which moves data from one register to another, it is typically necessary to fetch the MOVE instruction, to decode it to determine that it is a MOVE instruction and to determine which register is the source of the data being moved and which the destination, and to actually execute the instruction by moving the data from the source register to the destination register. When instruction execution is pipelined, the phases of execution are performed in parallel on different instructions. For instance, if a pipelined processor is executing three instructions in the order n1, n2, and n3, it may be simultaneously performing the execution phase on n1, the decode phase on n2, and the fetch phase on n3. Microprocessor 107 has two pipelines. The first pipeline is the instruction pipeline . The execution of every instruction is divided into three phases, fetch, decode, and execute, and these phases are performed in parallel. Consequently, microprocessor 107 is generally able to execute one instruction per cycle of CLOCK signal 501. The second pipeline is an I/O instruction pipeline, which handles the later phases of execution of I/O instructions independently of the instruction pipeline. FIG. 19 shows the operation of the two pipelines. The operation of each pipeline is shown in the form of a table, with rows corresponding to the phases of instruction execution and columns corresponding to cycles of CLOCK 501. Table 1901 represents the instruction pipeline and table 1909 represents the I/O pipeline. As already mentioned, there are three phases of execution in instruction pipeline 1901: fetch (F) phase 1903, decode (De) phase 1905, and execute (E) phase 1907. There are two phases in I/O pipeline 1909: a load phase (L) 1911, in which the address required for the operation is loaded into a register in the I/O subsystem of microprocessor 107, indicated in FIG. 11 by box 1135, and an execute I/O phase (EIO) 1913 in which the address and the data are transferred via bus 103. EIO phase 1913 lasts until the transfer of data is complete, i.e. until the DTACK signal is received from memory 102. At a minimum, EIO phase 1913 lasts two clock cycles for byte and word reads and writes and four clock cycles for byte and word read-modify-writes and pointer reads and writes. Additional cycles are required if microprocessor 107 must wait for access to bus 103 or if memory 102 cannot immediately respond. FIG. 19 shows the execution of a sequence of four instructions, SysW1, a system write instruction which writes data to memory 102 from input latch 1149, SysW2, a second such instruction which writes data from output latch 1151, NIO1, which may be any instruction which is not an I/O instruction, and NIO2, a second such instruction. In cycle n, instruction pipeline 1901 executes F phase 1903 of SysW1; in cycle n+1, it executes F phase 1903 of SysW2 and De phase 1905 of SysW1; in cycle n+2, it executes F phase 1903 of NIO1, De phase 1905 of SysW2, and E phase 1907 of NIO1, and so forth. I/O pipeline 1909 receives SysW1 in cycle n+2, and performs load phase L 1911 on it in the same cycle. In cycle n+3, I/O pipeline 1909 performs phase L on SysW2 and EIO phase 1913 on SysW1. That phase lasts two cycles for SysW1, or until cycle n+4. Consequently, SysW2 cannot enter EIO phase 1913 until cycle n+5; EIO phase 1913 lasts more than three cycles for SysW2, and SysW2 is still in that phase in cycle n+7, the last cycle shown in FIG. 19. Since I/O pipeline 1909 operates independently of instruction pipeline 1901, execution of instructions in instruction pipeline 1901 need not wait until EIO phase 1913 is completed for SysW1 or SysW2. While the I/O is going on, instruction pipeline 1901 executes NIO1 and NIO2. Of course, it may at times be necessary to stop instruction pipeline 1901 to wait for I/O pipeline 1909. One such situation would occur if there were already an I/O instruction in L phase 1911 when a following I/O instruction entered E phase 1907; another such situation would occur if an NIO instruction required data in its E phase 1907 which was being provided by an I/O instruction which was still being executed by I/O pipe 1909. Instruction Formats: FIG. 12 FIG. 12 shows the MOVE instructions executed by microprocessor 107. As may be seen from that figure, microprocessor 107's instructions are made up of 24-bit instruction words 1201. Most of the instructions consist of a single instruction word 1201; however, some MOVE instructions and LOAD instructions for 24-bit literal constants require two words. Except when instruction pipeline 1901 is halted, microprocessor 107 fetches one instruction word 1201 per clock cycle. MOVE Instructions: FIGS. 12 and 12A The MOVE instructions move data between registers of microprocessor 107. The register from which the data is moved is the source and the register to which it is moved is the destination . There are four types of move instructions: Type 0, in which both the source and the destination are general registers 1103; Type 1, in which a general register 1103 is the source and one of the special registers is the destination; Type 2, in which one of the special registers is the source and one of the general registers 1103 is the destination; and Type 3, in which a special register is the source and another special register is the destination. Type 0 instructions are shown in the portion of FIG. 12 labeled 1225. The instructions have two words, instruction word 1 (IW1) 1215 and instruction word 2 (IW2) 1217. Beginning with IW1 1215, the most significant 4 bits contain an operation code (OPCODE) 1203; the next 4 bits are a don't care field 1205; the following 4 bits, TYPE 1207, specify the type of move instruction. In type 0, TYPE 1207 has the value 0000; SIZE field 1209 indicates what portion of the contents of the source general register 1103 is to be moved to the destination general register 1103. The codes and the portion of general register 1103 which they specify are the following: SIZE code Register Portion 001 low byte 1157 010 middle byte 1159 100 high byte 1161 011 low word 1163 110 high word 1165 111 entire register 1167 The reference numbers refer to FIG. 11A. The remaining 9 bits specify the general register 1103 which is the source. F bit 1211 indicates whether the general register 1103 is being specified by means of an offset from frame pointer 1117; if it is, general register source field 1213 contains the offset; if it is not, general register source field 1213 contains the number of general register 1103. The 12 most significant bits of instruction word 2 1217 are don't care bits 1219; bits 9-11 are SIZE field 1209; the code in SIZE 1209 in instruction word 2 1217 specifies the portion of the destination general register 1103 which will receive the data. If the portion specified by SIZE in instruction word 2 1217 is larger than that specified by SIZE in instruction word 1 1215, the most significant bits are 0 filled; if the portion is smaller, the excess most significant bits are discarded. The remaining 9 bits contain F field 1211 and general register destination field 1221, which together specify destination general register 1103 in the same fashion as the corresponding fields specify the source general register in instruction word 1 1215. Execution of type 0 move instruction 1225 is shown in pipeline diagram 1223. In cycle n, IW1 1215 is fetched; in cycle n+1, IW2 1217 is fetched and IW1 1215 is decoded; the decoding involves computing which general register 1103 is the source register and reading the portion thereof specified by SIZE field 1209 into a latch connected to internal data bus 1115. In cycle n+2, IW2 1217 is decoded to determine which general register 1103 is the destination register, and in the same cycle, the contents of the latch are read into the portion of the destination general register 1103 specified by size field 1209. Move instructions of types 1 and 2 are shown in the portion of FIG. 12 labeled 1233. The type 1 instruction has a general register 1103 as its source and a special register as its destination; the type 2 instruction has a special register as its source and a general register 1103 as its destination. Except for the value of type field 1207, the format of the two types is the same. The most significant 4 bits are opcode 1203; the next four bits, SPR 1227, are a code specifying one of the special registers; then comes SIZE field 1209, and finally F field 1211 and GR field 1229 specifying a general register 1103. Whether the special and general registers are source and destination or vice-versa depends on the value of TYPE field 1207. The effect of SIZE is the following: If a general register 1103 is the source and the portion specified by SIZE 1209 is less than 24 bits, the specified portion is placed on the least significant bits of IDB 1115 and the remaining bits are set to 0. If a general register 1103 is the destination and the portion specified by SIZE 1209 is less than 24 bits, the specified portion is written from the least significant bits of IDB 1115. Execution of a type 1 or 2 MOVE instruction in instruction pipeline 1901 is shown in diagram 1231. At cycle n, the instruction is fetched; at cycle n+1, the source and destination are determined; at cycle n+2, the data is written via IDB 1115 from the source to the destination. The type 3 move instruction moves data from a source special register to a destination special register. The instruction is shown at reference number 1241 in FIG. 12A. The instruction has an opcode 1203 in bits 20-23, a code in bits 16-19 specifying the destination special register, TYPE 1207 in bits 12-15, and a code in bits 0-3 specifying the source special register. Bits 4-11 are "don't care" bits. As implied by the absence of SIZE field 1209, the instruction always moves 24 bits. Execution in the pipeline is substantially the same as for the type 1 and 2 instructions. System Memory I/O in Microprocessor 107: FIGS. 2 and 9 Another area in which microprocessor 107 is particularly adapted for efficient and flexible use in a variety of systems is system memory I/O. In system memory I/O, data is read from and written to memory 102 via system bus 103. In preferred embodiment 201 of microprocessor 107, microprocessor 107 is connected to system bus 103 by 16 data lines and 23 address lines. Each address specifies a 16-bit word in memory 102. The system I/O operations performed by microprocessor 107 include reads and writes for bytes, 16-bit words, and 24-bit pointers and read-modify-writes for bytes and words. In the byte reads, the signals ERE and ORE (see FIG. 2) specify whether the even or odd byte of the word specified by the address is being read. In the byte writes, the corresponding signals are EWE and OWE . FIG. 9 shows the timing of a system memory byte read: Once microprocessor 107 has gained access to bus 103, the byte read requires a minimum of two cycles of clock 501 on bus 103: one to place the address, the address strobe signal AS and either ERE or ORE on the bus and one to receive the data from bus 103. The timing is substantially the same as shown above for the byte write and the word read and write. The 24-bit pointer read and write operations require 4 cycles: two as described above for two bytes of the pointer and two more for the third byte. The read-modify-write instructions require a minimum of four cycles: two for the read and two for the write. More time may be required for any of the I/O operations if memory 102 is unable to respond immediately. For this reason, memory 102 provides the DTACK signal to microprocessor 107 when it has provided the data in a read operation or received the data in a write operation. Microprocessor 107 waits for the DTACK signal before it concludes the second cycle of a read or write operation. I/O Subsection 1135: FIG. 11 The instruction architecture for microprocessor 107's system I/O instructions is shown in I/O subsection 1135 of FIG. 11. Registers in Section 1135 are accessible by means of internal data bus 1115. Programmer-specifiable registers include ALR 1145, which is loaded with a 24-bit address on a read operation, ALW 1146, which is loaded with an address on a write operation, input latch 1149, which receives data from system bus 103 on a read operation, and output latch 1151, which receives data to be written to system bus 103 on a write operation. Both input latch 1149 and output latch 1151 are 24-bit registers; byte I/O operations read or write the 8 least significant bits, word operations the 16 least significant bits, and pointer operations the entire register. Section 1135 further includes registers which are part of I/O pipeline 1909. Shadow address latch (SAL) 1141 holds the address for an I/O instruction which has entered load phase 1911 before the preceding I/O instruction has left EIO phase 1913. Shadow output latch (SOL) 1155 holds the data to be output by a write instruction which is in EIO phase 1913 when a following I/O instruction enters load phase 1911, thus making OL 1151 available to be loaded before the write operation is complete. Operation of I/O pipeline 1909 is as follows: if the pipeline is empty when an I/O instruction enters load phase 1911, ALR 1145 is loaded if the I/O instruction is a read instruction. If it is a write instruction, OL 1151 has been loaded by a previous MOVE instruction and load phase 1911 loads ALW 1146. On the first cycle of EIO phase 1913 for the write instruction, the contents of OL 1151 are moved to SOL 1155, freeing OL 1151 to be loaded by a MOVE instruction in that same cycle or later. On the first cycle of EIO phase 1913 after microprocessor 107 has gained access to bus 103, the data in SOL 1155 and the address in ALW 1146 are output to bus 103 until memory 102 responds with the DTACK signal. If there is already an I/O instruction in pipeline 1909 when the next I/O instruction enters load phase 1911, the address is loaded into SAL 1141. Finally, if SAL 1141 is full when an I/O instruction whose execute phase 1907 would load that register reaches decode phase 1905, instruction pipeline 1901 is halted until the register becomes available. In preferred embodiment 201, address and data processing typical of I/O operations is performed in I/O subsection 1135, so that ALU 1107 may concurrently perform other operations. One example of such processing is offset adder 1139. When an address is loaded into ALR 1145 or ALW 1146, an offset 1137 which is specified in the I/O instruction may be added to the address; moreover, the instruction may specify either ALR 1145 or ALW 1146 as the source of the address for an I/O operation and use offset adder 1139 to add the offset to the current contents of the specified register. As may be seen from FIG. 11, this is implemented by means of buses connecting the outputs of ALR 1145 and ALW 1146 to internal data bus 1115, which in turn provides the address input to offset adder 1139. Another example of such processing is the provision of both ALR 1145 and ALW 1146 and the provision of a connection between the output of input latch 1149 and the input of shadow output latch 1155. These features taken together permit efficient movement of data from one location in memory 102 to another location in memory 102. Since both the read and write addresses are available in ALR 1145 and ALW 1146 and input latch 1149 is available as a source of data for the write operation, the write operation may immediately follow the read operation and the transfer of a word or byte from one location to another in memory 102 requires a minimum of four cycles. Finally, AND/OR select logic is provided between IL 1149 and OL 1151 on the one hand and SOL 1155 on the other. This permits the contents of OL 1151 (loaded by a previous MOVE instruction) to be ANDed or ORed with the contents of IL 1149, and thus permits reading of data from memory 102, modifying it by means of a mask in OL 1151, and writing the data back to memory 102 in a minimum of four cycles. I/O Instructions: FIGS. 13, 17, and 18 Microprocessor 107 has three I/O instructions: a READ instruction, a WRITE instruction, and a READ-MODIFY-WRITE instruction. The formats of all three instructions are shown in FIG. 13. The READ instruction is identified by reference number 1301, the WRITE instruction by reference number 1313, and the READ-MODIFY-WRITE instruction by reference number 1319. As is apparent in that FIG., all of the I/O instructions have a number of features in common. Beginning with the most significant bits, bits 20-23 are the opcode, which is different for each of the three instructions; bits 12-19 are an 8-bit OFFSET value which is added to the value of a pointer to obtain the address used in the I/O operation. I/O Size (IOS) field 1307 (bits 11 and 10) indicates the size of the data to be read or written and, in conjunction with I/ M pin 209, the format of 16-bit words and 24-bit pointers. Bits 0 through 8 make up pointer (PTR) field 1311, which specifies a register of microprocessor 107 which contains a value to be used as a pointer, i.e., a value representing a location in memory 102, in the I/O operation. The specified register may either be a general register 1103 or one of the special registers, most generally ALR 1145 or ALW 1146. In the case of a general register 1103, the register is either specified directly by means of its number in GR field 1229 or indirectly by means of an offset from the value in frame pointer 1117. As with the MOVE instructions, F bit 1211 indicates which mode is being used. In the case of a special register, SR field 1312 contains a code for the special register, while bits 4-8 are a don't care field, here DC 1310. As is clear from the common fields described above, microprocessor 107 executes I/O instructions generally as follows: in decode phase 1905, the pointer indicated by pointer fields 1311 is made the source for internal data bus 1115; in execute phase 1907, the pointer value passes via internal data bus 1115 to either of ALR 1145 or ALW 1146, depending on the kind of I/O instruction. As the pointer is being moved, the offset in bits 12-19 is added to it by offset adder 1139. Execute phase 1907 is thus simultaneously load phase 1911 for instruction pipeline 1909. EIO phase 1913 of instruction pipeline 1901 begins in the cycle following the loading of the pointer into ALR 1145 or ALW 1146. f instruction pipeline 1901 already has an I/O instruction in it, the pointer is loaded into SAL 1141 instead of ALR 1145 or ALW 1146. WRITE instruction 1313 additionally has latch source (LS) field 1317. One setting of the single-bit field indicates that the source of the data to be written is output latch 1151; the other setting indicates that it is input latch 1149. The utility of this arrangement for performing back-to-back memory reads and writes has already been described. READ-MODIFY-WRITE instruction 1319 also has an additional one-bit field: AND/OR field 1323. One setting of the bit indicates that logic 1153 is to AND the contents of IL 1149 with those of OL 1151; the other indicates that logic 1153 is to OR the contents of the two registers. Again, the utility of this arrangement for performing read-modify-writes in four cycles has already been described. As previously indicated, READ-MODIFY-WRITE only writes bytes or words; consequently, IOS field 1307 in this instruction cannot specify a 24-bit data item. Reads and Writes with Different Data Formats: FIGS. 11 and 17 As shown in FIG. 1, in many applications, microprocessor 107 will be cooperating with a host processor 105. In some applications, host processor 105 may be another microprocessor 107; however, in others, it will be a different processor or microprocessor. In particular, it is highly probable that host processor 105 will have either the Intel 80x86 architecture or the Motorola 680x0 architecture. Two of the ways in which these architectures differ are the format of their pointers and the manner in which bytes are stored in 16-bit words. Three features of microprocessor 107, I/ M pin 209, a code in IOS field 1307 of the I/O instructions, and the SWB bit in program status word 1119, permit microprocessor 107 to be easily employed with either an 80x86 host or a 680x0 host. The code in IOS field 1307 further permits a programmer to write a single program which will execute properly in both the 80x86 and 680x0 environments. FIG. 17 shows the different ways the 80x86 architecture and the 680x0 architecture organize bytes in memory 102 and the different ways in which they format pointers. In the byte and word examples, the figure presumes that a sequence of 4 bytes with the values AA, BB, CC, and DD are being stored. When an 80x86 performs a sequence of byte write operations to a memory organized into 16-bit words and the sequence begins on a word boundary, the results are shown at 1701: the first byte AA is written to the 8 least significant bits of the first word, the next, BB, to the 8 most significant bits of the first word, the next, CC, to the 8 least significant bits of the second word, and the next, DD, to the 8 most significant bits of the second word. When a 680x0 performs the same operation, the results are shown at 1703: the first byte AA is written to the 8 most significant bits of the first word, the next byte BB is written to the 8 least significant bytes of that word, the third byte CC is written to the 8 most significant bytes of the second word, and the fourth byte DD is written to the 8 least significant bytes of the second word. On the other hand, when the same four bytes are written by word write operations, with the first word write writing bytes AA and BB and the second word write writing the bytes CC and DD, there is no difference between the 80x86 and 680x0 organizations, as shown by 1705. In both cases, AA is in the 8 most significant bits of the first word to be written, BB is in the 8 least significant bits, CC is in the 8 most significant bits of the second word, and DD is in the 8 least significant bits of that word. Continuing with the pointer formats, both architectures employ 32-bit pointers which are stored in two adjacent words. Processor 107 uses 24-bit addresses and is consequently only concerned with the 24 least significant bits of the 32-bit pointers. Presuming that the 24 least significant bits have the value AABBCC, with AA being the most significant byte and CC the least, the 80x86 format is that shown at 1707 and the 680x0 format is that shown at 1709. In the 80x86 format, the most significant byte of the pointer is in the 8 least significant bits of the second word, the next most significant byte is in the 8 most significant bits of the first word, and the least significant byte is in the least significant bits of the first word. In the 680x0 format, the most significant byte of the pointer is in the least significant 8 bits of the first word, the next most significant byte is in the most significant 8 bits of the second word, and the least significant 8 bits are in the least significant 8 bits of the second word. Microprocessor 107 deals with some of the inconsistencies between the 80x86 and 680x0 architectures by means of I/ M pin 209. When microprocessor 107 is operating with an 80x86 host 105, Vcc is input to pin 207; when microprocessor 107 is operating with a Motorola 680x0 host 105, pin 207 is grounded. Generally, pin 207 will be either tied to ground or Vcc, since the type of host processor 105 is not likely to change with any frequency. The input to the pin 207 could, however, be made switchable in order to permit system reconfiguration. Beginning with the byte operations, these are specified in IOS field 1307 of the instruction by means of the code "00". In the byte write operation, the byte to be written is contained in the least significant 8 bits of SOL 1155; in both 80x86 and 680x0 modes, the byte is output to both bits 0-7 and 8-15 of data lines 313 of bus 103. The input at the I/ M pin and the value of the least significant bit of the address then determine whether the EWE signal or the OWE signal is enabled to write the byte to the upper or lower byte of the addressed word, as required by the 80x86 and 680x0 memory formats. In the byte read operations, the odd byte will be on lines data 8-15 and the even byte on data lines 0-7; in the second case, the reverse will be true. Which byte is read into bits 7-0 of input latch 1149 in response to a given odd or even byte address depends on the input which microprocessor 107 receives on I/ M pin 209 and the SWB bit in PSW 1119. If SWB is set to 0 and the input to I/ M is low, indicating a 680x0 host, a read of an even byte results in the byte on lines 8-15 being read into input latch 1149 [7-0], while a read of an odd byte results in the byte on lines 0-7 being read into that portion of input latch 1149. If SWB is set to 0 and the input to I/ M is high, indicating an 80x86 host, the reverse of the above occurs. If SWB is set to 1, finally, microprocessor 107's response to the input to I/ M pin 209 is the reverse of that just described. SWB thus gives the programmer a way of overriding the effect of I/ M on byte reads. In the read-modify-write for byte data, the read works as just described and the write as just described for the byte write, except that the data in SOL 1155 is the contents of input latch 1149 as ANDed or ORed with the mask in OL 1151. Word operations are specified in I/O size field 1307 by the code 01. There is no difference between the 80x86 and 680x0 formats for 16-bit memory words, and consequently, I/ M pin 209 does not affect the manner in which these operations are performed. In the write, the lower 16 bits of SOL 1155 are output to data lines 313; in the read, the bits on data lines 313 are latched into the lower 16 bits of input latch 1149. The same is the case for the read portion of the read-modify-write and the write portion of that operation. A pointer read operation, indicated by the code 10 in I/O size field 1307, requires that two words be fetched from memory 102. If the host is a 680x0, the lower byte of the first word contains the most significant byte of the pointer, while the upper and lower bytes of the next word contain the next less significant byte and the least significant byte. Thus, when I/ M pin 209 indicates that the host is a 680x0, the read of the first word from memory 102 causes the contents of data lines 313 [7-0] to be placed into input latch 1149 [23- 16]. When the second word is read from memory 102, the contents of data lines 313 [15-8] go into input latch 1149 [15-8] and the contents of data lines 313 [7-0] go simultaneously into latch 1149 [7-0]. If the host is an 80x86, the first word contains the least significant byte of the pointer in its lower byte and the next least significant byte of the pointer in its upper byte; the second word contains the most significant byte of the pointer in its lower byte. Consequently, when I/ M pin 209 indicates that the host is an 80x86, the read of the first word from memory 102 results in the contents of data lines 313 [15-8] being placed into input latch 1149 [15-8] and those of data lines 313 [7-0] being placed into input latch 1149 [7-0]. On the next cycle, the contents of data lines 313 [7-0] are placed in input latch 1149 [23-16]. When pointers are being written, if I/ M pin 209 indicates that the host is a 680x0, the write of the first word to memory 102 results in shadow output latch 1155 [23-16] being placed on D 313 [7-0] and zeros being placed on z 313 [15-8], while the write of the second word results in shadow output latch 1155 [15-8] being placed on D 313 [15-8] and shadow output latch 1155 [7-0] being simultaneously placed on D 313 [7-0]. If I/ M pin 209 indicates that the host is an 80x86, the write of the first word results in shadow output latch 1155 [15-8] being placed on D 313 [15-8] and shadow output latch 1155 [7-0] being simultaneously placed on D 313 [7-0]. The write of the second word results in shadow output latch 1155 [23- 16] being placed on D 313 [7-0] and zeros being placed on D 313 [15-8]. The I/ M pin 209 by itself ensures that if host processor 105 does a byte, word, or pointer write of data to memory 102 and microprocessor 107 does a byte, word, or pointer read of the same data from memory 102 or vice-versa, both host processor 105 and microprocessor 107 will receive the data in the expected format. However, if host processor 105 does two byte writes to memory 102 and microprocessor 107 does a word read of the two bytes, the positions of the bytes in the word will depend on whether host processor 105 is an 80x86 or a 680x0, and the code in microprocessor 107 which processes the word will depend on the type of host processor. Since this is the case, a change in host processor would require extensive revision of the code, which in turn would greatly increase the cost of developing systems using microprocessor 107. This problem is solved in microprocessor 107 by the code 11 in I/O size 1307. That code indicates "word with swapped byte" i.e., that word I/O operations are to be employed but that the bytes of the word are to be swapped if I/ M pin 209 indicates that the host is an 80x86 machine. Thus, if "word with swapped byte" is indicated in an I/O instruction, microprocessor 107 will be able to correctly perform word read and write operations on byte data regardless of whether the host is a 680x0 machine or an 80x86 machine, and there will be no need to write different code depending on the type of host machine. The "word with swapped byte" code and I/ M pin 209 interact to produce the result described above as follows: In a read operation, when I/ M specifies a 680x0 host, the contents of D 313 [15-8] go to input latch 1149 [15-8] and those of D 313 [7-0] go to input latch 1149 [7-0]; when I/ M pin 209 specifies an 80x86 host, the contents of D 313 [15-8] go to input latch 1149 [7-0] and those of D 313 [7-0] go to input latch 1149 [15-8]. In a write operation, when I/ M pin 209 specifies a 680x0 host, the contents of shadow output latch 1155 [7-0] go to D 313 [7-0] and those of shadow output latch 1155 [15-8] go to D 313 [15-8]. When I/ M pin 209 specifies an 80x86 host, the contents of shadow output latch 1155 [7-0] go to D 313 [15-8] and those of shadow output latch 1155 [8-15] go to D 313 [7-0]. In the read-modify-write with "word with swapped byte" the read and the write phases of the operation are as just described for the read operation and the write operation. Implementation of Data Input and Output: FIGS. 18, 20, 21-23 FIGS. 18, 20, 21-23 show the implementation in preferred embodiment 201 of input latch 1149, output latch 1151, shadow output latch 1153, AND/OR 1323, and the control logic which controls those devices. Beginning with FIG. 20, that figure shows input latch 1149 and the multiplexers which permit the byte swapping operations just described. Input latch 1149 is made up of three 8-bit latches, latch 2007, containing the most significant byte, latch 2009, containing the next most significant byte, and latch 2011, containing the least significant byte. Output from the latches is to input latch output (ILO) 2013. Input to the latches comes from muxes 2001, 2003, and 2005. Each of these muxes takes input from data lines 313 and ILO 2013. Specifically, mux 2001 selects input for latch 2007 from data lines 313 [7-0] or input latch 1149 [23-16], mux 2003 selects input for latch 2009 from data lines 313 [15-8], data lines 313 [7-0], or input latch 1149 [15-8], and mux 2005 selects input for latch 2009 from data lines 313 [15-8], data lines 313 [7-0], or input latch 1149 [7-0]. If a mux 2001, 2003, or 2005 is not selected, it outputs 0's. Selection is performed by selection lines A, C, D, F, G, J, L, and M according to the table below. The notation source → destination in the table indicates that the data in the source is transferred to the destination when the selection line is active. Continuing with FIG. 21, that figure shows the implementation of output latch 1151, AND/OR logic 1153, and shadow output latch 1155. Inputs for this circuitry come from internal data bus 1115, input latch 1149 via ILO 2013, and shadow output latch 1155. Output is to data lines 313. Selection of inputs is by multiplexer 2101, while selection of outputs is by multiplexers 2109 and 2111. If one of these multiplexers is not selected, it outputs 0's. Beginning with output latch 1151, that latch receives its inputs from internal data bus 1115 and provides outputs to AND/OR logic 1153 and to multiplexer 2101. Multiplexer 2101 additionally receives inputs from ILO 2013, which is connected to the output of input latch 1149, SOL 1155, and AND/OR logic 1153. Output from mux 2101 is to shadow output latch 1155, which has three 8-bit latches, latch 2103, containing bits [23-16], latch 2105, containing bits [15-8], and latch 2107, containing bits [7-0]. Outputs are via muxes 2109 and 2111 to D 313 [7-0] and [15-8]. Selection of inputs is by selection lines U,V,W, and X for mux 2101 and P,Q,R,S, and T for muxes 2109 and 2111. The table shows the line and the result of the selection. The selection lines are controlled by selection logic 1801, shown in FIGS. 18, 22, and 23. The selection lines are the outputs of I/O control logic (I/OCTL) 1819; the inputs for the logic are the following: Signal Meaning RW 1: Read operation; 0: write operation RMW 1: Read/modify/write operation RMWW 0: Read part of read-modify-write operation; 1: write part IOS Codes from IOS 1307 IL/OL 0: write data from OL 1151; 1: write data from IL 1149 LONG2 Second part of pointer read or write A0 Least significant address bit; 0: even byte; 1: odd byte ITL 1: 80x86 mode unless SWB is set; 0: 680x0 mode unless SWB is set (see below) The RW, RMW, IOS, and IL/OL signals are all derived ultimately from the I/O instructions; A0 is derived from the contents of ALR 1145 or ALW 1146, depending on the I/O operation; LONG2 and RMWW are derived from internal control logic of microprocessor 107 which keeps track of the phase of execution of I/O instructions; ITL is produced by ITL logic 1803. When ITL has the value 1, I/O section 1135 operates as if microprocessor 107 was operating in a system with an 80x86 host; when ITL 1817 has the value 0, I/O section 1135 operates as if microprocessor 107 was operating with a 680x0 host. With operations other than byte I/O operations, the value of ITL 1817 is determined solely by the value microprocessor 107 is receiving on I/ M pin 209; with the byte operations, the effect of the I/ M pin 209 may be reversed by setting the SWB bit in program status word 1119 to 1. This effect is produced by ITL logic 1803 as follows: ITL 1817 is the negated output of XOR gate 1815; the inputs to gate 1815 come from I/ M pin 209 and NAND gate 1811, which has three inputs. Inputs IOS0 1805 and IOS1 1807 are the negations of IOS 1307; PSW_SWB 1809 is derived from the PSW status bit. When either IOS0 1805 or IOS1 1807 is low, indicating that IOS 1307 specifies something other than byte I/O, NAND gate 1811 has a 1 output at 1813 and negated output 1817 of XOR gate 1815 is the same as the input from I/ M pin 209. When both inputs 1807 and 1805 are high, indicating the 00 code for byte I/O, the output of gate 1811 is determined by PSW_SWB 1809. When input 1809 is low, NAND gate 1811 still has a 1 output, and output 1817 is the same as input 209, as just described. When input 1809 is high, i.e., when the SWB bit is set in PSW 1809, NAND gate 1811 has an 0 output and output 1817 is the inverse of input 209, that is, the effect of I/ M pin 209 is reversed. Operation of I/OCTL 1819 is shown in the truth tables of FIGS. 22 and 23. FIG. 22 shows the truth table for READ and READ-MODIFY-WRITE operations. There are sections of the table for byte reads, word reads, word reads with the WSB code in IOS 1307, pointer reads, byte read-modify-writes, word read-modify-writes, and word read-modify-writes with the WSB code in IOS 1307. The columns in the center part of the table indicate values for the input signals to I/OCTL 1819; the columns in the right-hand part of the table indicate values resulting from the inputs on selector lines 1833. In the center portion of the table, "X's" indicate "don't care" bits, i.e., bits whose value makes no difference in the operation indicated by the row. In the right-hand portion of the table, ". " indicates a "don't care" bit. Taking the first row of the table as an example, the row shows that when RW=1, indicating a read operation, RMW=0, indicating that it's not a read-modify-write, IOS indicates a byte operation (i.e., has the code 00), ITL has the value 0, indicating 680x0 mode, and A0 has the value 0, indicating an even byte address, then select line M is high and the rest low. The values on the input lines indicate that the operation specified by the row is a read of an even byte from data that is stored in 680x0 format. Turning to FIG. 20, it may be seen that when select line M is active, mux 2005 selects D 313 [15-8] as the source for input latch 1149 [7-0]. Turning then to FIG. 17, it may be seen that the selected byte is indeed an even byte in 680x0 byte format 1703. When read as just indicated, the truth tables of FIGS. 22 and 23 and the logic diagrams of FIGS. 18, 20, and 21 show how the read and write operations just described are implemented in preferred embodiment 201. Conclusion The foregoing "Detailed Description" has shown how to make and use a processor which shares memory with other processors and in which the same code can be used even though the memory is shared with other processors having different data formats. In the embodiment disclosed in the "Detailed Description", the data formats concerned the ordering of bytes; however, the invention may be employed with data formats which differ in other ways. In the disclosed embodiment, the instructions whose operation is dependent on the data formats are I/O instructions; again, the principles of the invention may be employed with other types of instructions. Instructions which fall within the ambit of the invention may be specified by means other than the.WSB code in the IOSIZE field employed in the disclosed embodiment, and the actual swapping may be achieved by circuitry other than the circuitry disclosed in the present application. For these reasons, the implementation disclosed in the "Detailed Description" is to be regarded in all respects as merely illustrative and exemplary and the invention claimed herein is not defined by the disclosed implementation, but instead by the claims as interpreted in light of the doctrine of equivalents.

Claims: A processor capable of sharing memory with other processors employing different formats for data stored in the memory, the processor executing instructions including at least one instruction including data format dependency indicating means (1307) which indicate that the operation specified by the instruction is dependent on a data format employed by the other processor with which the processor is sharing the memory, the processor being characterized by: data format signal receiving means (209) for receiving a data format signal indicating the data format and operation means (1801) responsive to the data format signal and the data format dependency indicating means for performing the operation specified by the instruction as required by the data format. The processor set forth in claim 1 further characterized in that the data format is the order of a plurality of bytes in the memory; the operation is a read or write of a plurality of bytes in the memory; and the operation means performs the read or write as required by the order of the plurality of bytes in the memory. The processor set forth in claim 2 further characterized in that the plurality of bytes is two; a first one of the other processors places the two bytes in a first order in the memory; a second one of the other processors places the two bytes in a second order in the memory; the data format signal indicates either the first order or the second order; and the read operation places any two bytes being read from the memory in the first order when the data format signal indicates the second order and the write operation places any two bytes being written to the memory in the second order when the data format signal indicates the second order. The processor set forth in claim 3 further characterized in that the first order is that employed by processors of the 680x0 type; and the second order is that employed by processors of the 80x86 type. A data processing system comprising: a memory (102) for storing data; a first processor (105) which is coupled to the memory and processes the data, the first processor belonging to one of a plurality of processor types, each processor type employing a different format for certain of the data; a second processor (107) which is coupled to the memory and processes the data, and executes instructions including at least one instruction including data format dependency indicating means (1307) which indicate that the operation specified by the instruction is dependent on a data format employed by the first processor, said data processing system being characterized in that the second processor further includes data format signal receiving means (209) for receiving a data format signal indicating the data format and operation means (1801) responsive to the data format signal and the data format dependency indicating means for performing the operation specified by the instruction as required by the data format. The system set forth in claim 5 further characterized in that the data format is the order of a plurality of bytes in the memory; the operation is a read or write of a plurality of bytes in the memory; and the operation means performs the read or write as required by the order of the plurality of bytes in the memory. The system set forth in claim 6 further characterized in that the plurality of bytes is two; a first one of the types of processors places the two bytes in a first order in the memory; a second one of the types of processors places the two bytes in a second order in the memory; the data format signal indicates either the first order or the second order; and the read operation places any two bytes read from the memory in the first order when the data format signal indicates the second order and the write operation places any two bytes written to the memory in the second order when the data format signal indicates the second order. The system set forth in claim 7 further characterized in that the first one of the types is the 680x0 type; and the second one of the types is the 80x86 type.

Inventors: CORCORAN GARY T, FAIRFIELD ROBERT CHARLES, SUFI AKKAS T, CORCORAN, GARY T., FAIRFIELD, ROBERT CHARLES, SUFI, AKKAS T.
Applicants: AT & T CORP, AT&T CORP.
Application year: 1991
Main classification: G06F9
IPC codes: G06F9, G06F15, G06F12