Instruction set

Instruction set

An instruction set, or instruction set architecture (ISA), is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O. An ISA includes a specification of the set of opcodes (machine language), and the native commands implemented by a particular processor.

Instruction set architecture is distinguished from the microarchitecture, which is the set of processor design techniques used to implement the instruction set. Computers with different microarchitectures can share a common instruction set. For example, the Intel Pentium and the AMD Athlon implement nearly identical versions of the x86 instruction set, but have radically different internal designs.

Some virtual machines that support bytecode for Smalltalk, the Java virtual machine, and Microsoft's Common Language Runtime virtual machine as their ISA implement it by translating the bytecode for commonly used code paths into native machine code, and executing less-frequently-used code paths by interpretation; Transmeta implemented the x86 instruction set atop VLIW processors in the same fashion.



This concept can be extended to unique ISAs like TIMI (Technology-Independent Machine Interface) present in the IBM System/38 and IBM AS/400. TIMI is an ISA that is implemented by low-level software translating TIMI code into "native" machine code, and functionally resembles what is now referred to as a virtual machine. It was designed to increase the longevity of the platform and applications written for it, allowing the entire platform to be moved to very different hardware without having to modify any software except that which translates TIMI into native machine code, and the code that implements services used by the resulting native code. This allowed IBM to move the AS/400 platform from an older CISC architecture to the newer POWER architecture without having to rewrite or recompile any parts of the OS or software associated with it other than the aforementioned low-level code.

Machine language

Machine language is built up from discrete statements or instructions. On the processing architecture, a given instruction may specify:

  • Particular registers for arithmetic, addressing, or control functions
  • Particular memory locations or offsets
  • Particular addressing modes used to interpret the operands

More complex operations are built up by combining these simple instructions, which (in a von Neumann architecture) are executed sequentially, or as otherwise directed by control flow instructions.

Instruction types

Some operations available in most instruction sets include:

  • Data handling and Memory operations
    • set a register (a temporary "scratchpad" location in the CPU itself) to a fixed constant value
    • move data from a memory location to a register, or vice versa. This is done to obtain the data to perform a computation on it later, or to store the result of a computation.
    • read and write data from hardware devices
  • Arithmetic and Logic
    • add, subtract, multiply, or divide the values of two registers, placing the result in a register, possibly setting one or more condition codes in a status register
    • perform bitwise operations, taking the conjunction and disjunction of corresponding bits in a pair of registers, or the negation of each bit in a register
    • compare two values in registers (for example, to see if one is less, or if they are equal)
  • Control flow
    • branch to another location in the program and execute instructions there
    • conditionally branch to another location if a certain condition holds
    • indirectly branch to another location, while saving the location of the next instruction as a point to return to (a call)

Complex instructions

Some computers include "complex" instructions in their instruction set. A single "complex" instruction does something that may take many instructions on other computers. Such instructions are typified by instructions that take multiple steps, control multiple functional units, or otherwise appear on a larger scale than the bulk of simple instructions implemented by the given processor. Some examples of "complex" instructions include:

  • saving many registers on the stack at once
  • moving large blocks of memory
  • complex and/or floating-point arithmetic (sine, cosine, square root, etc.)
  • performing an atomic test-and-set instruction
  • instructions that combine ALU with an operand from memory rather than a register

A complex instruction type that has become particularly popular recently is the SIMD or Single-Instruction Stream Multiple-Data Stream operation or vector instruction, an operation that performs the same arithmetic operation on multiple pieces of data at the same time. SIMD have the ability of manipulating large vectors and matrices in minimal time. SIMD instructions allow easy parallelization of algorithms commonly involved in sound, image, and video processing. Various SIMD implementations have been brought to market under trade names such as MMX, 3DNow! and AltiVec.

Parts of an instruction

One instruction may have several fields, which identify the logical operation to be done, and may also include source and destination addresses and constant values. This is the MIPS "Add" instruction which allows selection of source and destination registers and inclusion of a small constant.

On traditional architectures, an instruction includes an opcode specifying the operation to be performed, such as "add contents of memory to register", and zero or more operand specifiers, which may specify registers, memory locations, or literal data. The operand specifiers may have addressing modes determining their meaning or may be in fixed fields.

In very long instruction word (VLIW) architectures, which include many microcode architectures, multiple simultaneous opcodes and operands are specified in a single instruction.

Some exotic instruction sets do not have an opcode field (such as Transport Triggered Architectures (TTA) or the Forth virtual machine), only operand(s). Other unusual "0-operand" instruction sets lack any operand specifier fields, such as some stack machines including NOSC [1].

Instruction length

The size or length of an instruction varies widely, from as little as four bits in some microcontrollers to many hundreds of bits in some VLIW systems. Processors used in personal computers, mainframes, and supercomputers have instruction sizes between 8 and 64 bits(The longest possible instruction on x86 is 15 bytes, that is 120 bits). Within an instruction set, different instructions may have different lengths. In some architectures, notably most reduced instruction set computers (RISC), instructions are a fixed length, typically corresponding with that architecture's word size. In other architectures, instructions have variable length, typically integral multiples of a byte or a halfword.


The instructions constituting a program are rarely specified using their internal, numeric form; they may be specified by programmers using an assembly language or, more commonly, may be generated by compilers.


The design of instruction sets is a complex issue. There were two stages in history for the microprocessor. The first was the CISC (Complex Instruction Set Computer) which had many different instructions. In the 1970s, however, places like IBM did research and found that many instructions in the set could be eliminated. The result was the RISC (Reduced Instruction Set Computer), an architecture which uses a smaller set of instructions. A simpler instruction set may offer the potential for higher speeds, reduced processor size, and reduced power consumption. However, a more complex set may optimize common operations, improve memory/cache efficiency, or simplify programming.

Some instruction set designers reserve one or more opcodes for some kind of system call or software interrupt For example, MOS Technology 6502 uses 00H, Zilog Z80 uses the eight codes C7,CF,D7,DF,E7,EF,F7,FFH[1] while Motorola 68000 use codes in the range A000..AFFFH.

Fast virtual machines are much easier to implement if an instruction set meets the Popek and Goldberg virtualization requirements.

The NOP slide used in Immunity Aware Programming is much easier to implement if the "unprogrammed" state of the memory is interpreted as a NOP.

On systems with multiple processors, non-blocking synchronization algorithms are much easier to implement if the instruction set includes support for something like "fetch-and-increment" or "load linked/store conditional (LL/SC)" or "atomic compare and swap".

Instruction set implementation

Any given instruction set can be implemented in a variety of ways. All ways of implementing an instruction set give the same programming model, and they all are able to run the same binary executables. The various ways of implementing an instruction set give different tradeoffs between cost, performance, power consumption, size, etc.

When designing the microarchitecture of a processor, engineers use blocks of "hard-wired" electronic circuitry (often designed separately) such as adders, multiplexers, counters, registers, ALUs etc. Some kind of register transfer language is then often used to describe the decoding and sequencing of each instruction of an ISA using this physical microarchitecture. There are two basic ways to build a control unit to implement this description (although many designs use middle ways or compromises):

  1. Early computer designs and some of the simpler RISC computers "hard-wired" the complete instruction set decoding and sequencing (just like the rest of the microarchitecture).
  2. Other designs employ microcode routines and/or tables to do this—typically as on chip ROMs and/or PLAs (although separate RAMs have been used historically).

There are also some new CPU designs which compile the instruction set to a writable RAM or flash inside the CPU (such as the Rekursiv processor and the Imsys Cjip),[2] or an FPGA (reconfigurable computing). The Western Digital MCP-1600 is an older example, using a dedicated, separate ROM for microcode.

An ISA can also be emulated in software by an interpreter. Naturally, due to the interpretation overhead, this is slower than directly running programs on the emulated hardware, unless the hardware running the emulator is an order of magnitude faster. Today, it is common practice for vendors of new ISAs or microarchitectures to make software emulators available to software developers before the hardware implementation is ready.

Often the details of the implementation have a strong influence on the particular instructions selected for the instruction set. For example, many implementations of the instruction pipeline only allow a single memory load or memory store per instruction, leading to a load-store architecture (RISC). For another example, some early ways of implementing the instruction pipeline led to a delay slot.

The demands of high-speed digital signal processing have pushed in the opposite direction—forcing instructions to be implemented in a particular way. For example, in order to perform digital filters fast enough, the MAC instruction in a typical digital signal processor (DSP) must be implemented using a kind of Harvard architecture that can fetch an instruction and two data words simultaneously, and it requires a single-cycle multiply–accumulate multiplier.

Code density

In early computers, program memory was expensive, so minimizing the size of a program to make sure it would fit in the limited memory was often central. Thus the combined size of all the instructions needed to perform a particular task, the code density, was an important characteristic of any instruction set. Computers with high code density also often had (and have still) complex instructions for procedure entry, parameterized returns, loops etc. (therefore retroactively named Complex Instruction Set Computers, CISC). However, more typical, or frequent, "CISC" instructions merely combine a basic ALU operation, such as "add", with the access of one or more operands in memory (using addressing modes such as direct, indirect, indexed etc.). Certain architectures may allow two or three operands (including the result) directly in memory or may be able to perform functions such as automatic pointer increment etc. Software-implemented instruction sets may have even more complex and powerful instructions.

Reduced instruction-set computers, RISC, were first widely implemented during a period of rapidly growing memory subsystems and sacrifice code density in order to simplify implementation circuitry and thereby try to increase performance via higher clock frequencies and more registers. RISC instructions typically perform only a single operation, such as an "add" of registers or a "load" from a memory location into a register; they also normally use a fixed instruction width, whereas a typical CISC instruction set has many instructions shorter than this fixed length. Fixed-width instructions are less complicated to handle than variable-width instructions for several reasons (not having to check whether an instruction straddles a cache line or virtual memory page boundary[3] for instance), and are therefore somewhat easier to optimize for speed. However, as RISC computers normally require more and often longer instructions to implement a given task, they inherently make less optimal use of bus bandwidth and cache memories.

Minimal instruction set computers (MISC) are a form of stack machine, where there are few separate instructions (16-64), so that multiple instructions can be fit into a single machine word. These type of cores often take little silicon to implement, so they can be easily realized in an FPGA or in a multi-core form. Code density is similar to RISC; the increased instruction density is offset by requiring more of the primitive instructions to do a task.[citation needed]

There has been research into executable compression as a mechanism for improving code density. The mathematics of Kolmogorov complexity describes the challenges and limits of this.

Number of operands

Instruction sets may be categorized by the maximum number of operands explicitly specified in instructions.

(In the examples that follow, a, b, and c are (direct or calculated) addresses referring to memory cells, while reg1 and so on refer to machine registers.)

  • 0-operand (zero-address machines), so called stack machines: All arithmetic operations take place using the top one or two positions on the stack; 1-operand push and pop instructions are used to access memory: push a, push b, add, pop c.
  • 1-operand (one-address machines), so called accumulator machines, include early computers and many small microcontrollers: most instructions specify a single right operand (that is, constant, a register, or a memory location), with the implicit accumulator as the left operand (and the destination if there is one): load a, add b, store c. A related class is practical stack machines which often allow a single explicit operand in arithmetic instructions: push a, add b, pop c.
  • 2-operand — many CISC and RISC machines fall under this category:
    • CISC — often load a,reg1; add reg1,b; store reg1,c on machines that are limited to one memory operand per instruction; this may be load and store at the same location
    • CISC — move a->c; add c+=b.
    • RISC — Requiring explicit memory loads, the instructions would be: load a,reg1; load b,reg2; add reg1,reg2; store reg2,c
  • 3-operand, allowing better reuse of data:.[3]
    • CISC — It becomes either a single instruction: add a,b,c, or more typically: move a,reg1; add reg1,b,c as most machines are limited to two memory operands.
    • RISC — arithmetic instructions use registers only, so explicit 2-operand load/store instructions are needed: load a,reg1; load b,reg2; add reg1+reg2->reg3; store reg3,c; unlike 2-operand or 1-operand, this leaves all three values a, b, and c in registers available for further reuse[3].
  • more operands—some CISC machines permit a variety of addressing modes that allow more than 3 operands (registers or memory accesses), such as the VAX "POLY" polynomial evaluation instruction.

Due to the large number of bits needed to encode the three registers of a 3-operand instruction, RISC processors using 16-bit instructions are invariably 2-operand machines, such as the Atmel AVR, the TI MSP430, and some versions of the ARM Thumb. RISC processors using 32-bit instructions are usually 3-operand machines, such as processors implementing the Power Architecture, the SPARC architecture, the MIPS architecture, the ARM architecture, and the AVR32 architecture.

Each instruction specifies some number of operands (registers, memory locations, or immediate values) explicitly. Some instructions give one or both operands implicitly, such as by being stored on top of the stack or in an implicit register. When some of the operands are given implicitly, the number of specified operands in an instruction is smaller than the arity of the operation. When a "destination operand" explicitly specifies the destination, the number of operand specifiers in an instruction is larger than the arity of the operation. Some instruction sets have different numbers of operands for different instructions.

List of ISAs

This list is far from comprehensive as old architectures are developed and new ones invented. There are many commercially available microprocessors and microcontrollers implementing ISAs. Customized ISAs are also quite common in some applications, e.g. ASIC, FPGA, and reconfigurable computing.

  • List of instruction sets

ISAs implemented in hardware

ISAs commonly implemented in software with hardware incarnations

ISAs only implemented in software

ISAs never implemented in hardware

See also

Categories of ISA

Applications where specialized instruction sets are used

Device types that implement some ISA



  1. ^ Ganssle, Jack. "Proactive Debugging". Published February 26, 2001.
  2. ^
  3. ^ a b c The evolution of RISC technology at IBM by John Cocke – IBM Journal of R&D, Volume 44, Numbers 1/2, p.48 (2000)
  4. ^

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