Abstract:
Methods and apparatuses are disclosed for implementing a processor with a split stack. In some embodiments, the processor includes a main stack and a micro-stack. The micro-stack preferably is implemented in the core of the processor, whereas the main stack may be implemented in areas that are external to the core of the processor. Operands are preferably provided to an arithmetic logic unit (ALU) by the micro-stack, and in the case of underflow (micro-stack empty), operands may be fetched from the main stack. Operands are written to the main stack during overflow (micro-stack full) or by explicit flushing of the micro-stack. By optimizing the size of the micro-stack, the number of operands fetched from the main stack may be reduced, and consequently the processor&#39;s power consumption may be reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Application Ser. No. 60/400,391 titled “JSM Protection,” filed Jul. 31, 2002, incorporated herein by reference. This application also claims priority to EPO Application No. 03291908.6, filed Jul. 30, 2003 and entitled “A Processor With A Split Stack,” incorporated herein by reference. This application also may contain subject matter that may relate to the following commonly assigned co-pending applications incorporated herein by reference: “System And Method To Automatically Stack And Unstack Java Local Variables,” Ser. No. 10/632,228, filed Jul. 31, 2003, “Memory Management Of Local Variables,” Ser. No. 10/632,067, filed Jul. 31, 2003, “Memory Management Of Local Variables Upon A Change Of Context,” Ser. No. 10/632,076, filed Jul. 31, 2003, “Using IMPDEP2 For System Commands Related To Java Accelerator Hardware,” Ser. No. 10/632,069, filed Jul. 31, 2003, “Test With Immediate And Skip Processor Instruction,” Ser. No. 10/632,214, filed Jul. 31, 2003, “Test And Skip Processor Instruction Having At Least One Register Operand,” Ser. No. 10/632,084, filed Jul. 31, 2003, “Synchronizing Stack Storage,” Ser. No. 10/631,422, filed Jul. 31, 2003, “Methods And Apparatuses For Managing Memory,” Ser. No. 10/631,252, filed Jul. 31, 2003, “Write Back Policy For Memory,” Ser. No. 10/631,185, filed Jul. 31, 2003, “Methods And Apparatuses For Managing Memory,” Ser. No. 10/631,205, filed Jul. 31, 2003, “Mixed Stack-Based RISC Processor,” Ser. No. 10/631,308, filed Jul. 31, 2003, “Processor That Accommodates Multiple Instruction Sets And Multiple Decode Modes,” Ser. No. 10/631,246, Jul. 31, 2003, “System To Dispatch Several Instructions On Available Hardware Resources,” Ser. No. 10/631,585, filed Jul. 31, 2003, “Micro-Sequence Execution In A Processor,” Ser. No. 10/632,216, filed Jul. 31, 2003, “Program Counter Adjustment Based On The Detection Of An Instruction Prefix,” Ser. No. 10/632,222, filed Jul. 31, 2003, “Reformat Logic To Translate Between A Virtual Address And A Compressed Physical Address,” Ser. No. 10/632,215, filed Jul. 31, 2003, “Synchronization Of Processor States,” Ser. No. 10/632,024, filed Jul. 31, 2003, “Conditional Garbage Based On Monitoring To Improve Real Time Performance,” Ser. No. 10/631,195, filed Jul. 31, 2003, “Inter-Processor Control,” Ser. No. 10/631,120, filed Jul. 31, 2003, “Cache Coherency In A Multi-Processor System,” Ser. No. 10/631,229, filed Jul. 31, 2003, “Concurrent Task Execution In A Multi-Processor, Single Operating System Environment,” Ser. No. 10/632,077, filed Jul. 31, 2003, and “A Multi-Processor Computing System Having A Java Stack Machine And A RISC-Based Processor,” Ser. No. 10/631,939, filed Jul. 31, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates generally to processors and more particularly to a processor capable of executing a stack-based instruction set and a non-stack based instruction set. 
   2. Background Information 
   Many types of electronic devices are battery operated and thus preferably consume as little power as possible. An example is a cellular telephone. Further, it may be desirable to implement various types of multimedia functionality in an electronic device such as a cell phone. Examples of multimedia functionality may include, without limitation, games, audio decoders, digital cameras, etc. It is thus desirable to implement such functionality in an electronic device in a way that, all else being equal, is fast, consumes as little power as possible and requires as little memory as possible. Improvements in this area are desirable. 
   BRIEF SUMMARY 
   Methods and apparatuses are disclosed for implementing a multi-stack processor. In some embodiments, the processor includes a main stack and a micro-stack. The micro-stack preferably is implemented in the core of the processor, whereas the main stack may be implemented in areas that are external to the core of the processor. Operands are preferably provided to an arithmetic logic unit (ALU) by the micro-stack, and in some cases, operands may be fetched from the main stack. By optimizing the size of the micro-stack, the number of operands fetched from the main stack may be reduced, and consequently the processor&#39;s power consumption may be reduced. 
   NOTATION AND NOMENCLATURE 
   Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, semiconductor companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the preferred embodiments of the present invention, reference will now be made to the accompanying drawings, wherein: 
       FIG. 1  shows a diagram of a system in accordance with preferred embodiments of the invention and including a Java Stack Machine (“JSM”) and a Main Processor Unit (“MPU”); 
       FIG. 2  shows a block diagram of the JSM of  FIG. 1  in accordance with preferred embodiments of the invention; 
       FIG. 3  shows various registers used in the JSM of  FIGS. 1 and 2 ; 
       FIGS. 4A–C  depict stack management in the event of an overflow condition; 
       FIG. 5A–C  depict stack management in the event of an underflow condition; and 
       FIG. 6  depicts an exemplary embodiment of the system described herein. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims, unless otherwise specified. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
   The subject matter disclosed herein is directed to a programmable electronic device such as a processor. The processor described herein is particularly suited for executing Java™ bytecodes or comparable, code. As is well known, Java is particularly suited for embedded applications. Java is a relatively “dense” language meaning that on average each instruction may perform a large number of functions compared to various other programming languages. The dense nature of Java is of particular benefit for portable, battery-operated devices that preferably include as little memory as possible to save space and power. The reason, however, for executing Java code is not material to this disclosure or the claims that follow. The processor described herein may be used in a wide variety of electronic systems. By way of example and without limitation, the Java-executing processor described herein may be used in a portable, battery-operated cell phone. Further, the processor advantageously includes one or more features that reduce the amount of power consumed by the Java-executing processor. 
   Referring now to  FIG. 1 , a system  100  is shown in accordance with a preferred embodiment of the invention. As shown, the system includes at least two processors  102  and  104 . Processor  102  is referred to for purposes of this disclosure as a Java Stack Machine (“JSM”) and processor  104  may be referred to as a Main Processor Unit (“MPU”). System  100  may also include an external memory  106  coupled to both the JSM  102  and MPU  104  and thus accessible by both processors. The external memory  106  may exist on a separate chip than the JSM  102  and the MPU  104 . At least a portion of the external memory  106  may be shared by both processors meaning that both processors may access the same shared memory locations. Further, if desired, a portion of the external memory  106  may be designated as private to one processor or the other. System  100  also includes a Java Virtual Machine (“JVM”)  108 , compiler  110 , and a display  114 . The JSM  102  preferably includes an interface to one or more input/output (“I/O”) devices such as a keypad to permit a user to control various aspects of the system  100 . In addition, data streams may be received from the I/O space into the JSM  102  to be processed by the JSM  102 . Other components (not specifically shown) may include, without limitation, a battery and an analog transceiver to permit wireless communications with other devices. As noted above, while system  100  may be representative of, or adapted to, a wide variety of electronic systems, an exemplary electronic system may comprise a battery-operated, mobile cell phone. 
   As is generally well known, Java code comprises a plurality of “bytecodes”  112 . Bytecodes  112  may be provided to the JVM  108 , compiled by compiler  110  and provided to the JSM  102  and/or MPU  104  for execution therein. In accordance with a preferred embodiment of the invention, the JSM  102  may execute at least some, and generally most, of the Java bytecodes. When appropriate, however, the JSM  102  may request the MPU  104  to execute one or more Java bytecodes not executed or executable by the JSM  102 . In addition to executing Java bytecodes, the MPU  104  also may execute non-Java instructions. The MPU  104  also hosts an operating system (“O/S”) (not specifically shown), which performs various functions including system memory management, the system task management that schedules the JVM  108  and most or all other native tasks running on the system, management of the display  114 , receiving input from input devices, etc. Without limitation, Java code may be used to perform any one of a variety of applications including multimedia, games or web based applications in the system  100 , while non-Java code, which may comprise the O/S and other native applications, may still run on the system on the MPU  104 . 
   The JVM  108  generally comprises a combination of software and hardware. The software may include the compiler  110  and the hardware may include the JSM  102 . In accordance with preferred embodiments of the invention, the JSM  102  may execute at least two instruction sets. One instruction set may comprise standard Java bytecodes. As is well-known, Java bytecode is a stack-based intermediate language in which instructions generally target a stack. For example, an integer add (“IADD”) Java instruction pops two integers off the top of the stack, adds them together, and pushes the sum back on the stack. As will be explained in more detail below, the JSM  102  comprises a stack-based architecture with various features that accelerate the execution of stack-based Java code, where the stack may include multiple portions that exist in different physical locations. 
   Another instruction set executed by the JSM  102  may include instructions other than standard Java instructions. In accordance with at least some embodiments of the invention, other instruction sets may include register-based and memory-based operations to be performed. This other instruction set generally complements the Java instruction set and, accordingly, may be referred to as a complementary instruction set architecture (“C-ISA”). By complementary, it is meant that the execution of more complex Java bytecodes may be substituted by a “microsequence” sequence comprising one or more C-ISA instructions that permit address calculation to readily “walk through” the JVM data structures. A micro-sequence also may include one or more bycode instructions. The execution of Java may be made more efficient and run faster by replacing some sequences of bytecodes by preferably shorter and more efficient sequences of CISA ISA instructions. The two sets of instructions may be used in a complementary fashion to obtain satisfactory code density and efficiency. As such, the JSM  102  generally comprises a stack-based architecture for efficient and accelerated execution of Java bytecodes combined with a register-based architecture for executing register and memory based C-ISA instructions. Both architectures preferably are tightly combined and integrated through the C-ISA. 
     FIG. 2  shows an exemplary block diagram of the JSM  102 . As shown, the JSM includes a core  120  coupled to a data storage  122  and an instruction storage  130 . Storage  122  and  130  are preferably integrated, along with core  120 , on the same JSM chip. Integrating storage  122  and  130  on the same chip as the core  120  may reduce data transfer time from storage  122  and  130  to the core  120 . The core  120  may include one or more components as shown. Such components preferably include a plurality of registers  140 , three address generation units (“AGUs”)  142 ,  147 , micro-translation lookaside buffers (micro-TLBs)  144 ,  156 , a multi-entry micro-stack  146 , an arithmetic logic unit (“ALU”)  148 , a multiplier  150 , decode logic  152 , and instruction fetch logic  154 . In general, operands may be retrieved from a main stack and processed by the ALU  148 , where the main stack may include multiple portions that exist in different physical locations. For example, the main stack may reside in external memory  106  and/or data storage  122 . Selected entries from the main stack may exist on the micro-stack  146 . In this manner, selected entries on the micro-stack  146  may represent the most current version of the operands in the system  100 . Accordingly, operands in external memory  106  and data storage  122  may not be coherent with the versions contained on the micro-stack  146 . A plurality of flags  158  preferably are coupled to the micro-stack  146 , where the flags  158  indicate the validity of data on the micro-stack  146  and whether data on the micro-stack  146  has been modified. Also, stack coherency operations may be performed by examining the flags  158  and updating the main stack with valid operands from the micro-stack  146 . 
   The micro-stack  146  preferably comprises, at most, the top n entries of the main stack that is implemented in data storage  122  and/or external memory  106 . The micro-stack  146  preferably comprises a plurality of gates in the core  120  of the JSM  102 . By implementing the micro-stack  146  in gates (e.g., registers) in the core  120  of the JSM  102 , access to the data contained on the micro-stack  146  is generally very fast. Therefore data access time may be reduced by providing data from the micro-stack  146  instead of the main stack. General stack requests are provided by the micro-stack  146  unless the micro-stack  146  cannot fulfill the stack requests. For example, when the micro-stack  146  is in an overflow condition or when the micro-stack  146  is in an underflow condition (as will be described below), general stack requests may be fulfilled by the main stack. By analyzing trends of the main stack, the value of n, which represents the size of the micro-stack  146 , may be optimized such that a majority of general stack requests are fulfilled by the micro-stack  146 , and therefore may provide requested data in fewer cycles. As a result, power consumption of the system  102  may be reduced. Although the value of n may vary in different embodiments, in accordance with at least some embodiments, the value of n may be the top eight entries in the main stack. In this manner, about 98% of the general stack accesses may be provided by the micro-stack  146 , and the number of accesses to the main stack may be reduced. 
   Instructions may be fetched from instruction storage  130  by fetch logic  154  and decoded by decode logic  152 . The address generation unit  142  may be used to calculate addresses based, at least in part on data contained in the registers  140 . The AGUs  142  may calculate addresses for C-ISA instructions. The AGUs  142  may support parallel data accesses for C-ISA instructions that perform array or other types of processing. AGU  147  couples to the micro-stack  146  and may manage overflow and underflow conditions on the micro-stack  146  preferably in parallel. The micro-TLBs  144 ,  156  generally perform the function of a cache for the address translation and memory protection information bits that are preferably under the control of the operating system running on the MPU  104 . 
   Referring now to  FIG. 3 , the registers  140  may include 16 registers designated as R 0 –R 15  Registers R 0 –R 3 , R 5 , R 8 –R 11  and R 13 –R 14  may be used as general purposes (“GP”) registers usable for any purpose by the programmer. Other registers, and some of the GP registers, may be used for specific functions. For example, registers R 4  and R 12  may be used to store two program counters. Register R 4  preferably is used to store the program counter (“PC”) and register R 12  preferably is used to store a micro-program counter (“micro-PC”). In addition to use as a GP register, register R 5  may be used to store the base address of a portion of memory in which Java local variables may be stored when used by the current Java method. The top of the micro-stack  146  is reflected in registers R 6  and R 7 . The top of the micro-stack  146  has a matching address in external memory  106  pointed to by register R 6 . The operands contained on the micro-stack  146  are the latest updated values, while their corresponding values in external memory  106  may or may not be up to date. Register R 7  provides the data value stored at the top of the micro-stack  146 . Registers R 8  and R 9  may also be used to hold an address index 0 (“AI0”) and an address index 1 (“AI1”), which may be used in calculating addresses in memory generated by various bytecodes, for example, the result of an IADD instruction. Register R 14  may also be used to hold the indirect register index (“IRI”) that also may be used in calculating memory addresses. Register R 15  may be used for status and control of the JSM  102 . As an example, one status/control bit (called the “Micro-Sequence-Active” bit) may indicate if the JSM  102  is executing a “simple” instruction or a “complex” instruction through a “micro-sequence.” This bit controls in particular, which program counter is used R 4  (PC) or R 12  (micro-PC) to fetch the next instruction. A “simple” bytecode instruction is generally one in which the JSM  102  may perform an immediate operation either in a single cycle (e.g., an IADD instruction) or in several cycles (e.g., “dup2 — ×2”). A “complex” bytecode instruction is one in which several memory accesses may be required to be made within the JVM data structure for various verifications (e.g., NULL pointer, array boundaries). Because these data structure are generally JVM-dependent and thus may change from one JVM implementation to another, the software flexibility of the micro-sequence provides a mechanism for various JVM optimizations now known or later developed. 
   The second, register-based, memory-based instruction set may comprise the C-ISA instruction set introduced above. The C-ISA instruction set preferably is complementary to the Java bytecode instruction set in that the C-ISA instructions may be used to accelerate or otherwise enhance the execution of Java bytecodes. 
   The ALU  148  adds, subtracts, and shifts data. The multiplier  150  may be used to multiply two values together in one or more cycles. The instruction fetch logic  154  generally fetches instructions from instruction storage  130 . The instructions may be decoded by decode logic  152 . Because the JSM  102  is adapted to process instructions from at least two instruction sets, the decode logic  152  generally comprises at least two modes of operation, one mode for each instruction set. As such, the decode logic unit  152  may include a Java mode in which Java instructions may be decoded and a C-ISA mode in which C-ISA instructions may be decoded. 
   The data storage  122  generally comprises data cache (“D-cache”)  124  and data random access memory (“D-RAM”)  126 . Reference may be made to copending applications U.S. Ser. No. 09/591,537 filed Jun. 9, 2000 , Ser. No. 09/591,656 filed Jun. 9, 2000, and Ser. No. 09/932,794 filed Aug. 17, 2001, all of which are incorporated herein by reference. The main stack, arrays and non-critical data may be stored in the D-cache  124 , while Java local variables, critical data and non-Java variables (e.g., C, C++) may be stored in D-RAM  126 . The instruction storage  130  may comprise instruction RAM (“I-RAM”)  132  and instruction cache (“I-cache”)  134 . The I-RAM  132  may be used for “complex” micro-sequence bytecodes or micro-sequences or predetermined sequences of code, as will be described below. The I-cache  134  may be used to store other types of Java bytecode and mixed Java/C-ISA instructions. 
   As noted above, the C-ISA instructions generally complement the standard Java bytecodes. For example, the compiler  110  may scan a series of Java bytes codes  112  and replace one or more of such bytecodes with an optimized code segment mixing C-ISA and bytecodes and which is capable of more efficiently performing the function(s) performed by the initial group of Java bytecodes. In at least this way, Java execution may be accelerated by the JSM  102 . 
   As noted above, the micro-stack  146  includes a finite number of entries, and therefore overflow and underflow conditions may occur.  FIGS. 4A–C  depict an overflow condition of the micro-stack  146 . Note that although the micro-stack  146  shown in  FIGS. 4A–C  is shown containing four entries, preferred embodiments may have any number of entries. As shown in  FIG. 4A , the micro-stack  146  may include data values or operands A and B, for example as the result of pushing A and B on the micro-stack  146 . Stack pointers  162  and  164  reflect the top of the micro-stack  146  and the top of the main stack  160  respectively. When new data values are pushed on the micro-stack  146 , the flags  158  may be enabled (indicated by EN in the Figures) to indicate that the new data is valid. A lack of EN in a flag  158  indicates invalid data. Operands pushed on the micro-stack  146  generally are not pushed on a main stack  160 . Operands A′ and B′ indicate the place of the data in the main stack, but these entries are not coherent with the corresponding micro-stack entries A and B. Coherence may be achieved if A and B are written to main memory during an overflow condition or flushing, as explained below. The main stack pointer  164  is updated at every push or pop. As indicated above, the main stack  160  may exist in external memory  106  and/or data storage  122 , and the main stack  160  may be larger than the micro-stack  146 . 
     FIG. 4B  shows operands C and D pushed on the micro-stack  146 , where the micro-stack  146  is now full. Operands are pushed on the micro-stack  146  in a cyclical manner such that when the micro-stack  146  is full, the data at the bottom of the micro-stack  146  (which in this example is operand A) is overwritten. As operands are pushed into entries of the micro-stack  146 , the flag  158  associated with each entry may be checked for validity. If the flag  158  indicates that the data in an entry, where a new push is performed, is valid (i.e., flag  158  enabled), then the entry must be copied on the main stack  160  prior to pushing the next data operand on the micro-stack  146 .  FIG. 4C  depicts the result of pushing operand E on the full micro-stack  146  shown in  FIG. 4B . When the micro-stack  146  is full (shown in  FIG. 4B ), the bottom of the micro-stack  146  is moved into the main stack  160  at an address value equal to the stack pointer  164  minus the number of entries n on the micro-stack  146 . For example,  FIG. 4B  shows the micro-stack  146  including  4  entries and the stack pointer  164  indicating the top of the main stack  160 . In this example, prior to overwriting operand A on the micro-stack  146  with operand E, operand A is copied to an address that is four entries less than the address indicated by the stack pointer  164 . Thus, operand A from the micro-stack  164  is written to the main stack as indicated in  FIG. 4C . 
     FIGS. 5A–C  depict an underflow condition of the micro-stack  146  shown in  FIG. 4C . Referring to  FIG. 5A , operand E is popped off the micro-stack  146  and then operand D is popped off the micro-stack  146 . As operands are popped off of the micro-stack  146  the corresponding flag  158  is invalidated and the stack pointers  162  and  164  are decremented.  FIG. 5B  illustrates a bytecode that provides operands B and C to the ALU  148 . The ALU  148  produces a result Z, which is placed back on the micro-stack  146 , and the flag  158  is enabled as shown. If a subsequent bytecode requires operand Z as well as another operand that is not on the micro-stack  146 , an underflow occurs. Flag  158  is preferably checked for valid data prior to executing bytecodes to determine whether the required data is present on the micro-stack  146 . For example,  FIG. 5C  depicts a bytecode requiring operand Z in addition to operand A, which is not on the micro-stack  146 . Since flag  158  associated with operand A is not enabled in  FIG. 5B , operand A is fetched from the main stack  160 . In some embodiments, multiple operands may be fetched simultaneously from the main stack  160 . In addition, other embodiments include pre-fetching the operands from the main stack  160 . 
   Flags  158  may include a register with bits allocated for each entry in the micro-stack  146 , or alternatively flags  158  may include a read pointer and a write pointer. The read pointer is preferably updated on each stack instruction execution. For example, during an IADD instruction, the read pointer may decrement itself once for each operand that is popped off the stack, and then increment itself once to write the result of the operand back on the stack. The write pointer is preferably updated during an underflow or an overflow. By comparing the values of the read pointer and the write pointer, overflow and underflow conditions can be detected. 
   As noted previously, system  100  may be implemented as a mobile cell phone such as that shown in  FIG. 6 . As shown, a mobile communication device includes an integrated keypad  412  and display  414 . The JSM  102  and MPU  104  and other components may be included in electronics package  410  connected to the keypad  412 , display  414 , and radio frequency (“RF”) circuitry  416 . The RF circuitry  416  may be connected to an antenna  418 . 
   While the preferred embodiments of the present invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above. Each and every claim is incorporated into the specification as an embodiment of the present invention.