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.