Comparison of instruction set architectures


An instruction set architecture is an abstract model of a computer. It is also referred to as architecture or 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 ; 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 and their semantics, the instruction set, and the input/output model.

Base

In the early decades of computing, there were computers that used binary, decimal and even ternary. Contemporary computers are almost exclusively binary.

Bits

s 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. 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.

Operands

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

Endianness

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 were originally big endian, but many are now configurable.
Endianness only applies to processors that allow individual addressing of units of data that are smaller than the basic addressable machine word.

Instruction sets

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. 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 – and store from register – 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:
Archi-
tecture
BitsVersionIntro-
duced
Max #
operands
TypeDesignRegisters
Instruction encodingBranch evaluationEndian-
ness
ExtensionsOpenRoyalty
free
6502819751Register MemoryCISC3Variable Condition registerLittle
6809819781Register MemoryCISC9Variable Condition registerBig
680x03219792Register MemoryCISC8 data and 8 addressVariableCondition registerBig
8080819742Register MemoryCISC8Variable Condition registerLittle
805132 1977?1Register RegisterCISCVariable Compare and branchLittle
x8616, 32, 64
19782
3
4
Register MemoryCISCVariable Condition codeLittlex87, IA-32, MMX, 3DNow!, SSE,
SSE2, PAE, x86-64, SSE3, SSSE3, SSE4,
BMI, AVX, AES, FMA, XOP, F16C
Alpha6419923Register RegisterRISC32 Fixed Condition registerBi,,,
ARC16/32ARCv219963Register RegisterRISC16 or 32 including SP
user can increase to 60
Variable Compare and branchBiAPEX User-defined instructions
ARM/A3232ARMv1-v819833Register RegisterRISCFixed Condition codeBiNEON, Jazelle,,
TrustZone,
Thumb/T3232ARMv4T-ARMv819943Register RegisterRISCThumb: Fixed, Thumb-2:
Variable
Condition codeBiNEON, Jazelle,,
TrustZone,
Arm64/A6464ARMv8-A20113Register RegisterRISC32 Fixed Condition codeBinone: all ARMv7
extensions are non-optional
AVR819972Register RegisterRISC32
16 on "reduced architecture"
Variable Condition register,
skip conditioned
on an I/O or
general purpose
register bit,
compare and skip
Little
AVR3232Rev 220062–3RISC15VariableBigJava Virtual Machine
Blackfin3220003Register RegisterRISC2 accumulators
8 data registers
8 pointer registers
4 index registers
4 buffer registers
VariableCondition codeLittle
CDC 60006019643Register MemoryRISC24 Variable Compare and branchn/aCompare/Move Unit, additional
Peripheral Processing Units
Crusoe
3220001Register RegisterVLIWVariable Condition codeLittle
Elbrus
64Elbrus-4S20141Register RegisterVLIW8–6464Condition codeLittleJust-in-time dynamic trans-
lation: x87, IA-32, MMX, SSE,
SSE2, x86-64, SSE3, AVX
DLX3219903RISC32Fixed Big
eSi-RISC16/3220093Register RegisterRISC8–72Variable Compare and branch
and condition register
BiUser-defined instructions
Itanium
642001Register RegisterEPIC128Fixed Condition registerBi
Intel Virtualization Technology
M32R3219973Register RegisterRISC16Variable Condition registerBi
Mico323220063Register RegisterRISC32Fixed Compare and branchBigUser-defined instructions
MIPS64 619811–3Register RegisterRISC4–32 Fixed Condition registerBiMDMX, MIPS-3D
MMIX6419993Register RegisterRISC256Fixed Big
NS320xx3219825Memory MemoryCISC8Variable Huffman coded, up to 23 bytes longCondition codeLittleBitBlt instructions
OpenRISC32, 641.320103Register RegisterRISC16 or 32Fixed
PA-RISC
64 2.019863Register RegisterRISC32Fixed Compare and branchBig → BiMAX
PDP-8121966Register MemoryCISC1 accumulator
1 multiplier quotient register
Fixed Condition register
Test and branch
EAE
PDP-111619703Memory MemoryCISC8 Fixed Condition codeLittleFloating Point,
Commercial Instruction Set
POWER, PowerPC, Power ISA32/64 3.0B19903Register RegisterRISC32Fixed, VariableCondition codeBig/BiAltiVec, APU, VSX, Cell
RISC-V32, 64, 1282.220103Register RegisterRISC32 VariableCompare and branchLittle
RX64/32/1620003Memory MemoryCISC4 integer + 4 addressVariableCompare and branchLittle
S+core16/322005RISCLittle
SPARC64 OSA201719853Register RegisterRISC32 Fixed Condition codeBig → BiVIS
SuperH 3219942Register Register
Register Memory
RISC16Fixed, VariableCondition code
Bi
System/360
System/370
z/Architecture
64 19642
3
4
Register Memory
Memory Memory
Register Register
CISC16Variable Condition code, compare and branchBig
Transputer32 19871Stack machineMISC3 Variable Compare and branchLittle
VAX3219776Memory MemoryCISC16VariableCompare and branchLittle
Z80819762Register MemoryCISC17Variable Condition registerLittle
Archi-
tecture
BitsVersionIntro-
duced
Max #
operands
TypeDesignRegisters
Instruction encodingBranch evaluationEndian-
ness
ExtensionsOpenRoyalty
free