Definitions

Machine code

Machine code

Machine code or machine language is a system of instructions and data executed directly by a computer's central processing unit. Machine code may be regarded as a primitive (and cumbersome) programming language or as the lowest-level representation of a compiled and/or assembled computer program. Programs in interpreted language (often BASIC, Matlab, Smalltalk, Python, Ruby, etc), are not represented by machine code however, although their interpreter (which may be seen as a processor executing the higher level program) often is. Machine code is also referred to as native code, a term that, in the context of an interpreted language, may refer to the platform-dependent implementation of language features and libraries.

Machine code instructions

Instructions are patterns of bits with different patterns corresponding to different commands to the machine.

Every CPU model has its own machine code, or instruction set. Successor or derivative processor designs may completely include all the instructions of a predecessor and may add additional instructions. Some nearly completely compatible processor designs may have slightly different effects after similar instructions. Occasionally a successor processor design will discontinue or alter the meaning of a predecessor's instruction code, making migration of machine code between the two processors more difficult. Even if the same model of processor is used, two different systems may not run the same example of machine code if they differ in memory arrangement, operating system, or peripheral devices because the machine code has no embedded information about the configuration of the system.

A machine code instruction set may have all instructions of the same length, or may have variable-length instructions. How the patterns are organized depends largely on the specification of the machine code. Common to most is the division of one field (the opcode) which specifies the exact operation (for example "add"). Other fields may give the type of the operands, their location, or their value directly (operands contained in an instruction are called immediate). Some exotic instruction sets do not have an opcode field (such as Transport Triggered Architectures or the Forth virtual machine), only operand(s). Other instruction sets lack any operand fields, such as NOSCs

Programs

A computer program is a sequence of instructions that are executed by a CPU. While simple processors execute instructions one after the other, superscalar processors are capable of executing several instructions at once.

Program flow may be influenced by special 'jump' instructions that transfer execution to an instruction other than the following one. Conditional jumps are taken (execution continues at another address) or not (execution continues at the next instruction) depending on some condition.

Assembly languages

A much more readable rendition of machine language, called assembly language, uses mnemonic codes to refer to machine code instructions, rather than simply using the instructions' numeric values. For example, on the Zilog Z80 processor, the machine code 00000101, which causes the CPU to decrement the B processor register, would be represented in assembly language as DEC B.

Example

The MIPS architecture provides a specific example for a machine code whose instructions are always 32 bits long. The general type of instruction is given by the op (operation) field, the highest 6 bits. J-type (jump) and I-type (immediate) instructions are fully specified by op. R-type (register) instructions include an additional field funct to determine the exact operation. The fields used in these types are:

   6      5     5     5     5      6 bits
[ op  |  rs |  rt |  rd |shamt| funct]  R-type
[ op  |  rs |  rt | address/immediate]  I-type
[ op  |        target address        ]  J-type

rs, rt, and rd indicate register operands; shamt gives a shift amount; and the address or immediate fields contain an operand directly.

For example adding the registers 1 and 2 and placing the result in register 6 is encoded:

[ op  |  rs |  rt |  rd |shamt| funct]
    0     1     2     6     0     32     decimal
 000000 00001 00010 00110 00000 100000   binary

Load a value into register 8, taken from the memory cell 68 cells after the location listed in register 3:

[ op  |  rs |  rt | address/immediate]
   35     3     8           68           decimal
 100011 00011 01000 00000 00001 000100   binary

Jumping to the address 1024:

[ op  |        target address        ]
    2                 1024               decimal
 000010 00000 00000 00000 00100 000000   binary

Relationship to microcode

In some computer architectures, the machine code is implemented by a more fundamental underlying layer of programs called microprograms, providing a common machine language interface across a line or family of different models of computer with widely different underlying dataflows. This is done to facilitate porting of machine language programs between different models. An example of this use is the IBM System/360 family of computers and their successors. With dataflow path widths of 8 bits to 64 bits and beyond, they nevertheless present a common architecture at the machine language level across the entire line.

Using a microcode layer to implement an emulator enables the computer to present the architecture of an entirely different computer. The System/360 line used this to allow porting programs from earlier IBM machines to the new family of computers, e.g. an IBM 1401/1440/1460 emulator on the IBM S/360 model 40.

See also

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