machine code

The representation of a computer program which is actually

read and interpreted by the computer. A program in machine

code consists of a sequence of machine instructions (possibly

interspersed with data). Instructions are binary strings

which may be either all the same size (e.g. one 32-bit word for

many modern RISC microprocessors) or of different sizes,

in which case the size of the instruction is determined from

the first word (e.g. Motorola 68000) or byte (e.g. Inmos

transputer). The collection of all possible instructions

for a particular computer is known as its "instruction set".

Execution of machine code may either be hard-wired into the

central processing unit or it may be controlled by

microcode. The basic execution cycle consists of fetching

the next instruction from main memory, decoding it

(determining which operation it specifies and the location of

any arguments) and executing it by opening various gates

(e.g. to allow data to flow from main memory into a CPU

register) and enabling functional units (e.g. signalling to

the ALU to perform an addition).

Humans almost never write programs directly in machine code.

Instead, they use a programming language which is translated

by the computer into machine code. The simplest kind of

programming language is assembly language which usually has

a one-to-one correspondence with the resulting machine code

instructions but allows the use of mnemonics (ASCII strings)

for the "op codes" (the part of the instruction which

encodes the basic type of operation to perform) and names for

locations in the program (branch labels) and for variables and

constants.

(1995-02-15)