An instruction set architecture (ISA) is an abstract model of a computer, also referred to as computer architecture. A realization of an ISA is called an implementation. An ISA permits multiple implementations that may vary in performance, physical size, and monetary cost (among other things); because the ISA serves as the interface between software and hardware. Software that has been written for an ISA can run on different implementations of the same ISA. This has enabled binary compatibility between different generations of computers to be easily achieved, and the development of computer families. Both of these developments have helped to lower the cost of computers and to increase their applicability. For these reasons, the ISA is one of the most important abstractions in computing today.
An ISA defines everything a machine language programmer needs to know in order to program a computer. What an ISA defines differs between ISAs; in general, ISAs define the supported data types, what state there is (such as the main memory and registers) and their semantics (such as the memory consistency and addressing modes), the instruction set (the set of machine instructions that comprises a computer's machine language), and the input/output model.
In the early decades of computing, there were computers that used binary, decimal[1] and even ternary.[2] [3] Contemporary computers are almost exclusively binary.
Characters are encoded as strings of bits or digits, using a wide variety of character sets; even within a single manufacturer there were character set differences.
Integers are encoded with a variety of representations, including Sign_magnitude, Ones' complement, Two's complement, Offset binary, Nines' complement and Ten's complement.
Similarly, floating point numbers are encoded with a variety of representations for the sign, exponent and mantissa. In contemporary machines IBM hexadecimal floating-point and IEEE 754 floating point have largely supplanted older formats.
Addresses are typically unsigned integers generated from a combination of fields in an instruction, data from registers and data from storage; the details vary depending on the architecture.
Computer architectures are often described as n-bit architectures. In the first of the 20th century, n is often 12, 18, 24, 30, 36, 48 or 60. In the last of the 20th century, n is often 8, 16, or 32, and in the 21st century, n is often 16, 32 or 64, but other sizes have been used (including 6, 39, 128). This is actually a simplification as computer architecture often has a few more or less "natural" data sizes in the instruction set, but the hardware implementation of these may be very different. Many instruction set architectures have instructions that, on some implementations of that instruction set architecture, operate on half and/or twice the size of the processor's major internal datapaths. Examples of this are the Z80, MC68000, and the IBM System/360. On these types 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 IBM System/360 instruction set architecture is 32-bit, but several models of the System/360 series, such as the IBM System/360 Model 30, have smaller internal data paths, while others, such as the 360/195, have larger internal data paths. The external databus width is not used 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, and used 32-bit register. Early 32-bit microprocessors often had a 24-bit address, as did the System/360 processors.
In the first of the 20th century, word oriented decimal computers typically had 10 digit[4] [5] [6] words with a separate sign, using all ten digits in integers and using two digits for exponents[7] [5] in floating point numbers.
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 arrange bytes 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 as either.
Endianness only applies to processors that allow individual addressing of units of data (such as bytes) that are smaller than some of the data formats.
In some architectures, an instruction has a single opcode. In others, some instructions have an opcode and one or more modifiers. E.g., on the IBM System/370, byte 0 is the opcode but when byte 0 is a then byte 1 selects a specific instruction, e.g., is store clock (STCK).
Architectures typically allow instructions to include some combination of operand addressing modes
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 (2-in, 1-out) will allow A := B + Cto be computed in one instruction ADD B, C, A
A two-operand architecture (1-in, 1-in-and-out) will allow A := A + Bto be computed in one instruction ADD B, Abut requires that A := B + Cbe done in two instructions MOVE B, A ADD C, A
As can be seen in the table below some instructions sets keep to a very simple fixed encoding length, and other have variable-length. Usually it is RISC architectures that have fixed encoding length and CISC architectures that have variable length, but not always.
The table below compares basic information about instruction set architectures.
Notes:
| Archi- tecture | Bits | Version | Intro- duced | Max # operands | Type | Design | Registers (excluding FP/vector) | Instruction encoding | Branch evaluation | Endian- ness | Extensions | Open | Royalty free | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6502 | 8 | 1975 | 1 | Register–Memory | CISC | 3 | Variable (8- to 24-bit) | Condition register | Little | |||||
| 6800 | 8 | 1974 | 1 | Register–Memory | CISC | 3 | Variable (8- to 24-bit) | Condition register | Big | |||||
| 6809 | 8 | 1978 | 1 | Register–Memory | CISC | 5 | Variable (8- to 32-bit) | Condition register | Big | |||||
| 680x0 | 32 | 1979 | 2 | Register–Memory | CISC | 8 data and 8 address | Variable | Condition register | Big | |||||
| 8080 | 8 | 1974 | 2 | Register–Memory | CISC | 7 | Variable (8 to 24 bits) | Condition register | Little | |||||
| 8051 | 32 (8→32) | 1977? | 1 | Register–Register | CISC | Variable (8 to 24 bits) | Compare and branch | Little | ||||||
| x86 | 16, 32, 64 (16→32→64) | v4 (x86-64) | 1978 | 2 (integer) 3 (AVX) 4 (FMA4 and VPBLENDVPx)[8] | Register–Memory | CISC | Variable (8086 ~ 80386: variable between 1 and 6 bytes /w MMU + intel SDK, 80486: 2 to 5 bytes with prefix, pentium and onward: 2 to 4 bytes with prefix, x64: 4 bytes prefix, third party x86 emulation: 1 to 15 bytes w/o prefix & MMU . SSE/MMX: 4 bytes /w prefix AVX: 8 Bytes /w prefix) | Condition code | Little | x87, IA-32, MMX, 3DNow!, SSE, SSE2, PAE, x86-64, SSE3, SSSE3, SSE4, BMI, AVX, AES, FMA, XOP, F16C | ||||
| Alpha | 64 | 1992 | 3 | Register–Register | RISC | 32 (including "zero") | Fixed (32-bit) | Condition register | Bi | ,,, | ||||
| ARC | 16/32/64 (32→64) | ARCv3[9] | 1996 | 3 | Register–Register | RISC | 16 or 32 including SP user can increase to 60 | Variable (16- or 32-bit) | Compare and branch | Bi | APEX User-defined instructions | |||
| ARM/A32 | 32 | ARMv1–v9 | 1983 | 3 | Register–Register | RISC | Fixed (32-bit) | Condition code | Bi | NEON, Jazelle,, TrustZone, | ||||
| Thumb/T32 | 32 | ARMv4T-ARMv8 | 1994 | 3 | Register–Register | RISC | Thumb: Fixed (16-bit), Thumb-2: Variable (16- or 32-bit) | Condition code | Bi | NEON, Jazelle,, TrustZone, | ||||
| Arm64/A64 | 64 | v8.9-A/v9.4-A,[10] Armv8-R[11] | 2011[12] | 3 | Register–Register | RISC | 32 (including the stack pointer/"zero" register) | Fixed (32-bit), Variable (32-bit or 64-bit for FMA4 with 32-bit prefix[13]) | Condition code | Bi | SVE and SVE2 | |||
| AVR | 8 | 1997 | 2 | Register–Register | RISC | 32 16 on "reduced architecture" | Variable (mostly 16-bit, four instructions are 32-bit) | Condition register, skip conditioned on an I/O or general purpose register bit, compare and skip | Little | |||||
| AVR32 | 32 | Rev 2 | 2006 | 2–3 | RISC | 15 | Variable[14] | Big | Java virtual machine | |||||
| Blackfin | 32 | 2000 | 3[15] | Register–Register | RISC[16] | 2 accumulators8 data registers 8 pointer registers 4 index registers 4 buffer registers | Variable (16- or 32-bit) | Condition code | Little[17] | |||||
| CDC Upper 3000 series | 48 | 1963 | 3 | Register–Memory | CISC | 48-bit A reg., 48-bit Q reg., 6 15-bit B registers, miscellaneous | Variable (24- or 48-bit) | Multiple types of jump and skip | Big | |||||
| CDC 6000 Central Processor (CP) | 60 | 1964 | 3 | Register–Register | n/a | 24 (8 18-bit address reg., 8 18-bit index reg., 8 60-bit operand reg.) | Variable (15-, 30-, or 60-bit) | Compare and branch | n/a | Compare/Move Unit | ||||
| CDC 6000 Peripheral Processor (PP) | 12 | 1964 | 1 or 2 | Register–Memory | CISC | 1 18-bit A register, locations 1–63 serve as index registers for some instructions | Variable (12- or 24-bit) | Test A register, test channel | n/a | additional Peripheral Processing Units | ||||
| Crusoe (native VLIW) | 32[18] | 2000 | 1 | Register–Register | VLIW[19] | Variable (64- or 128-bit in native mode, 15 bytes in x86 emulation) | Condition code | Little | ||||||
| Elbrus 2000 (native VLIW) | 64 | v6 | 2007 | 1 | Register–Register | VLIW | 8–64 | 64 | Condition code | Little | Just-in-time dynamic translation: x87, IA-32, MMX, SSE, SSE2, x86-64, SSE3, AVX | |||
| DLX | 32 | 1990 | 3 | RISC | 32 | Fixed (32-bit) | Big | |||||||
| eSi-RISC | 16/32 | 2009 | 3 | Register–Register | RISC | 8–72 | Variable (16- or 32-bit) | Compare and branch and condition register | Bi | User-defined instructions | ||||
| iAPX 432[20] | 32 | 1981 | 3 | Stack machine | CISC | 0 | Variable (6 to 321 bits) | |||||||
| Itanium (IA-64) | 64 | 2001 | Register–Register | EPIC | 128 | Fixed (128-bit bundles with 5-bit template tag and 3 instructions, each 41-bit long) | Condition register | Bi (selectable) | Intel Virtualization Technology | |||||
| LoongArch | 32, 64 | 2021 | 4 | Register–Register | RISC | 32 (including "zero") | Fixed (32-bit) | Little | ||||||
| M32R | 32 | 1997 | 3 | Register–Register | RISC | 16 | Variable (16- or 32-bit) | Condition register | Bi | |||||
| m88k | 32 | 1988 | 3 | Register–Register | RISC | Fixed (32-bit) | Big | |||||||
| Mico32 | 32 | 2006 | 3 | Register–Register | RISC | 32[21] | Fixed (32-bit) | Compare and branch | Big | User-defined instructions | [22] | |||
| MIPS | 64 (32→64) | 6[23] [24] | 1981 | 1–3 | Register–Register | RISC | 4–32 (including "zero") | Fixed (32-bit) | Condition register | Bi | MDMX, MIPS-3D | [25] [26] | ||
| MMIX | 64 | 1999 | 3 | Register–Register | RISC | 256 | Fixed (32-bit) | Condition register | Big | |||||
| Nios II | 32 | 2000 | 3 | Register–Register | RISC | 32 | Fixed (32-bit) | Condition register | Little | Soft processor that can be instantiated on an Altera FPGA device | ||||
| NS320xx | 32 | 1982 | 5 | Memory–Memory | CISC | 8 | Variable Huffman coded, up to 23 bytes long | Condition code | Little | BitBlt instructions | ||||
| OpenRISC | 32, 64 | 1.4[27] | 2000 | 3 | Register–Register | RISC | 16 or 32 | Fixed | Condition code | Bi | ||||
| PA-RISC (HP/PA) | 64 (32→64) | 2.0 | 1986 | 3 | Register–Register | RISC | 32 | Fixed (32-bit) | Compare and branch | Big → Bi | MAX | |||
| PDP-8[28] | 12 | 1966 | Register–Memory | CISC | 1 accumulator1 multiplier quotient register | Fixed (12-bit) | Condition registerTest and branch | EAE (Extended Arithmetic Element) | ||||||
| PDP-11 | 16 | 1970 | 2 | Memory–Memory | CISC | 8 (includes program counter and stack pointer, though any register can act as stack pointer) | Variable (16-, 32-, or 48-bit) | Condition code | Little | Extended Instruction Set, Floating Instruction Set, Floating Point Processor, Commercial Instruction Set | ||||
| POWER, PowerPC, Power ISA | 32/64 (32→64) | 3.1[29] | 1990 | 3 (mostly). FMA, LD/ST-Update | Register–Register | RISC | 32 GPR, 8 4-bit Condition Fields, Link Register, Counter Register | Fixed (32-bit), Variable (32- or 64-bit with the 32-bit prefix) | Condition code, Branch-Counter auto-decrement | Bi | AltiVec, APU, VSX, Cell, Floating-point, Matrix Multiply Assist | |||
| RISC-V | 32, 64, 128 | 20191213[30] | 2010 | 3 | Register–Register | RISC | 32 (including "zero") | Variable | Compare and branch | Little | ||||
| RX | 64/32/16 | 2000 | 3 | Memory–Memory | CISC | 4 integer + 4 address | Variable | Compare and branch | Little | |||||
| S+core | 16/32 | 2005 | RISC | Little | ||||||||||
| SPARC | 64 (32→64) | OSA2017[31] | 1985 | 3 | Register–Register | RISC | 32 (including "zero") | Fixed (32-bit) | Condition code | Big → Bi | VIS | [32] | ||
| SuperH (SH) | 32 | 1994 | 2 | Register–Register Register–Memory | RISC | 16 | Fixed (16- or 32-bit), Variable | Condition code (single bit) | Bi | |||||
| System/360 System/370 z/Architecture | 64 (32→64) | 1964 | 2 (most) 3 (FMA, distinct operand facility) 4 (some vector inst.) | Register–Memory Memory–Memory Register–Register | CISC | 16 general 16 control (S/370 and later) 16 access (ESA/370 and later) | Variable (16-, 32-, or 48-bit) | Condition code, compare and branch auto increment, Branch-Counter auto-decrement | Big | |||||
| TMS320 C6000 series | 32 | 1983 | 3 | Register-Register | VLIW | 32 on C67x 64 on C67x+ | Fixed (256-bit bundles with 8 instructions, each 32-bit long) | Condition register | Bi | |||||
| Transputer | 32 (4→64) | 1987 | 1 | Stack machine | MISC | 3 (as stack) | Fixed (8-bit) | Compare and branch | Little | |||||
| VAX | 32 | 1977 | 6 | Memory–Memory | CISC | 16 | Variable | Condition code, compare and branch | Little | |||||
| Z80 | 8 | 1976 | 2 | Register–Memory | CISC | 17 | Variable (8 to 32 bits) | Condition register | Little | |||||
| Archi- tecture | Bits | Version | Intro- duced | Max # operands | Type | Design | Registers (excluding FP/vector) | Instruction encoding | Branch evaluation | Endian- ness | Extensions | Open | Royalty free |