The representation of a computer program that is read and interpreted by the computer hardware (rather than by some other machine code program). A program in machine code consists of a sequence of "instructions" (possibly interspersed with data). An instruction is a binary string, (often written as one or more octal, decimal or hexadecimal numbers). Instructions may be 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".
Each instruction typically causes the Central Processing Unit to perform some fairly simple operation like loading a value from memory into a register or adding the numbers in two registers. An instruction consists of an op code and zero or more operands. Different processors have different instruction sets - the collection of possible operations they can perform.
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 action the operation code 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 programming languages. 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. Other languages are either translated by a compiler into machine code or executed by an interpreter