11. Appendix: instruction coding

11.1 Principles

An instruction includes a certain amount of information: what the instruction does (usually called the operation code), and the means of specifying the operands and the result. Operands may be in registers, in the instruction (immediate) or in memory. The result may be in a register or stored in memory. Jump or branch instructions manipulate addresses, part of which may be in a register and part in the instruction, in the form of a displacement relative to the contents of the program counter.

Instruction coding consists of specifying a certain number of fields or sub-fields which determine :

• the nature of the instruction through the operation code ;

• a register...