Comparison of instruction set architectures
Computer architectures are often described as n-bit architectures. Today n is often 8, 16, 32, or 64, but other sizes have been used. This is actually a strong simplification. A computer architecture often has a few more or less "natural" datasizes in the instruction set, but the hardware implementation of these may be very different. Many architectures have instructions operating on half and/or twice the size of respective processors' major internal datapaths. Examples of this are the 8080, Z80, MC68000 as well as many others. On this type of implementations, a twice as wide operation typically also takes around twice as many clock cycles (which is not the case on high performance implementations). On the 68000, for instance, this means 8 instead of 4 clock ticks, and this particular chip may be described as a 32-bit architecture with a 16-bit implementation. The external databus width is often not useful to determine the width of the architecture; the NS32008, NS32016 and NS32032 were basically the same 32-bit chip with different external data buses. The NS32764 had a 64-bit bus, but used 32-bit registers.
The width of addresses may or may not be different from the width of data. Early 32-bit microprocessors often had a 24-bit address, as did the System/360 processors.
The number of operands is one of the factors that may give an indication about the performance of the instruction set. A three-operand architecture will allow
A := B + C
to be computed in one instruction.
A two-operand architecture will allow
A := A + B
to be computed in one instruction, so two instructions will need to be executed to simulate a single three-operand instruction
A := B A := A + C
An architecture may use "big" or "little" endianness, or both, or be configurable to use either. Little endian processors order bytes in memory with the least significant byte of a multi-byte value in the lowest-numbered memory location. Big endian architectures instead order them with the most significant byte at the lowest-numbered address. The x86 architecture as well as several 8-bit architectures are little endian. Most RISC architectures (SPARC, Power, PowerPC, MIPS) were originally big endian (ARM was little endian), but many (including ARM) are now configurable.
Endianness only applies to processors that allow individual addressing of units of data (such as bytes) that are smaller than the basic addressable machine word.
Usually the number of registers is a power of two, e.g. 8, 16, 32. In some cases a hardwired-to-zero pseudo-register is included, as "part" of register files of architectures, mostly to simplify indexing modes. This table only counts the integer "registers" usable by general instructions at any moment. Architectures always include special-purpose registers such as the program pointer (PC). Those are not counted unless mentioned. Note that some architectures, such as SPARC, have register window; for those architectures, the count below indicates how many registers are available within a register window. Also, non-architected registers for register renaming are not counted.
Note, a common type of architecture, "load-store", is a synonym for "Register Register" below, meaning no instructions access memory except special – load to register(s) – and store from register(s) – with the possible exceptions of atomic memory operations for locking.
The table below compares basic information about instruction sets to be implemented in the CPU architectures:
|Instruction encoding||Branch evaluation||Endian-
|6502||8||1975||1||Register Memory||CISC||3||Variable (8- to 32-bit)||Condition register||Little|
|65k||64 (8→64)||2006?||1||Memory Memory
|CISC||1||Variable (8-bit to 256 bytes)||Compare and branch
|68000 / 680x0||32||1979||2||Register Memory||CISC||8 data and 8 address||Variable||Condition register||Big|
|8080||8||1974||2||Register Memory||CISC||8||Variable (8 to 24 bits)||Condition register||Little|
|8051||32 (8→32)||1977?||1||Register Register||CISC||
||Variable (8-bit to 128 bytes)||Compare and branch||Little|
|8086 / x86||16, 32, 64
||Variable (8086: 8- to 48-bit)||Condition code||Little||x87, IA-32, MMX, 3DNow!, SSE,
SSE2, PAE, x86-64, SSE3, SSE4,
BMI, AVX, AES, FMA, XOP, F16C
|Alpha||64||1992||3||Register Register||RISC||32 (including "zero")||Fixed (32-bit)||Condition register||Bi||, , ,||No|
|ARC||16/32||ARCv2||1996||3||Register Register||RISC||16 or 32 including SP
user can increase to 60
|Variable (16- and 32-bit)||Compare and branch||Bi||APEX User-defined instructions|
||Fixed (32-bit), Thumb: Fixed (16-bit), Thumb-2:
Variable (16- and 32-bit)
|Condition code||Bi||NEON, Jazelle, ,
|ARMv8-A||64/32||ARMv8-A||2011||3||Register Register||RISC||32 (including the stack pointer/"zero" register)||Fixed (32-bit). In ARMv7 compatibility mode: Thumb:
Fixed (16-bit), Thumb-2: Variable (16- and 32-bit), A64
|Condition code||Bi||none: all ARMv7
extensions are non-optional
16 on "reduced architecture"
|Variable (mostly 16-bit, four instructions are 32-bit)||Condition register,
on an I/O or
compare and skip
|AVR32||32||Rev 2||2006||2–3||RISC||15||Variable||Big||Java Virtual Machine|
|CDC Cyber||60||1970s||3||Register Memory||RISC||24 (8 18-bit address reg.,
8 18-bit index reg.,
8 60-bit operand reg.)
|Variable (15, 30, and 60-bit)||Compare and branch||n/a||Compare/Move Unit, additional
Peripheral Processing Units
|32||2000||1||Register Register||VLIW||Variable (64- or 128-bit)||Condition code||Little|
|64||Elbrus-4S||1976||1||Register Register||VLIW||8-64||64||Condition code||Little||Just-in-time dynamic trans-
lation: x87, IA-32, MMX, SSE,
SSE2, x86-64, SSE3, AVX
|eSi-RISC||16/32||2009||3||Register Register||RISC||8–72||Variable (16- or 32-bit)||Compare and branch
and condition register
|64||2001||Register Register||EPIC||128||Fixed (128 bit bundles with 5 bit template tag
and 3 instructions, each 41 bit long)
|Intel Virtualization Technology||No||No|
|M32R||32||1997||RISC||16||Fixed (16- or 32-bit)||Bi|
|Mico32||32||2006||3||Register Register||RISC||32||Fixed (32-bit)||Compare and branch||Big||User-defined instructions||Yes||Yes|
|MIPS||64 (32→64)||5||1981||1–3||Register Register||RISC||4–32 (including "zero")||Fixed (32-bit)||Condition register||Bi||MDMX, MIPS-3D||No|
|MMIX||64||1999||3||Register Register||RISC||256||Fixed (32-bit)||Big||Yes||Yes|
|NS320xx||32||1982||5||Memory Memory||CISC||8||Variable Huffman coded, up to 23 bytes long||Condition code||Little||BitBlt instructions|
|OpenRISC||32, 64||2010||3||Register Register||RISC||16 or 32||Fixed||Yes||Yes|
|64 (32→64)||2.0||1986||3||Register Register||RISC||32||Fixed (32-bit)||Compare and branch||Big → Bi||MAX||No|
|PDP-11||16||1970–1990||3||Memory Memory||CISC||8 (includes stack pointer,
though any register can
act as stack pointer)
|Fixed (16)||Condition code||Little||Floating Point,
Commercial Instruction Set
|PowerPC||32/64 (32→64)||2.07||1991||3||Register Register||RISC||32||Fixed (32-bit), Variable||Condition code||Big/Bi||AltiVec, APU, VSX, Cell||Yes||No|
|RISC-V||32, 64, 128||2010||Register Register||RISC||32 (including "zero")||Variable||Compare and branch||Little||Yes||Yes|
|RX||64/32/16||2000||3||Memory Memory||CISC||4 integer + 4 address||Variable||Compare and branch||Little||No|
|SPARC||64 (32→64)||OSA2015||1985||3||Register Register||RISC||32 (including "zero")||Fixed (32-bit)||Condition code||Big → Bi||VIS||Yes||Yes|
|SuperH (SH)||32||1990s||2||Register Register
|RISC||16||Fixed (16- or 32-bit), Variable||Condition code
|64 (32→64)||1964||2 (most)
3 (FMA, distinct
|Transputer||32 (4→64)||1987||1||Stack machine||MISC||3 (as stack)||Variable (8 ~ 120 bytes)||Compare and branch||Little|
|VAX||32||1977||6||Memory Memory||CISC||16||Variable||Compare and branch||Little|
|Z80||8||1976||2||Register Memory||CISC||17||Variable (8 to 32 bits)||Condition register||Little|
|Instruction encoding||Branch evaluation||Endian-
- "The 65k Project". Advanced 6502. Retrieved 20 December 2013.
- The LEA (8086 & later) and IMUL-immediate (80186 & later) instructions accept three operands; most other instructions of the base integer ISA accept no more than two operands.
- ARMv8 Technology Preview
- "ARM goes 64-bit with new ARMv8 chip architecture". Retrieved 26 May 2012.
- "AVR32 Architecture Document" (PDF). Atmel. Retrieved 2008-06-15.
- "Blackfin Processor Architecture Overview". Analog Devices. Retrieved 2009-05-10.
- "Blackfin memory architecture". Analog Devices. Retrieved 2009-12-18.
- Since memory is an array of 60-bit words with no means to access sub-units, big endian vs. little endian makes no sense. The optional CMU unit uses big endian semantics.
- "Crusoe Exposed: Transmeta TM5xxx Architecture 2". Real World Technologies.
- Alexander Klaiber (January 2000). "The Technology Behind Crusoe Processors" (PDF). Transmeta Corporation. Retrieved December 6, 2013.
- "LatticeMico32 Architecture". Lattice Semiconductor. Retrieved 2009-12-18.
- "Open Source Licensing". Lattice Semiconductor. Retrieved 2009-12-18.
- "Power ISA 2.07". IBM. Retrieved 2013-08-12.
- http://www.oracle.com/technetwork/server-storage/sun-sparc-enterprise/documentation/sparc-processor-2516655.html Oracle SPARC Processor Documentation
- http://sparc.org/technical-documents/#ArchLic SPARC Architecture License