Computer History
Tracing the History of the Computer - Machine Code (Machine Language)

Machine code or machine language is a system of instructions and data directly understandable by a computer's central processing unit.

Machine code instructions

The "words" of a machine language are called instructions, each of which cause an elementary action by the CPU, such as reading from a memory location. 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, although there is considerable overlap between some. If CPU A understands the full language of CPU B it is said that A is compatible with B. CPU B may not be compatible with CPU A, as A may know a few codes that B does not.

Some machine languages give all their instructions the same number of bits, while the instruction length differs in others. How the patterns are organised depends largely on the specification of the machine code. Common to most is the division of an instruction into fields. Typically the value of one field (the opcode) 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 NOSC ("no operand stack computers").

Programs

A 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.