Patterns & Instructions

Machine code or machine language is a system of instructions and data directly executed by a computer's central processing unit. Machine code is the lowest-level of abstraction for representing a computer program. more...

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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; 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 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.

Read more at Wikipedia.org