For a processor to be able to process an instruction, it requires to be able to determine what the instruction is asking to be carried out. For this to take place, the CPU requires to know what actions it may be asked to perform, and have pre-determined methods accessible to carry out these actions. It is this idea which is the reasoning behind the ''instruction set''.
When a processor is executing a program, at that time the program is in a machine language. Programmers mainly never write their programs in this form. While it may not have been at first written in this way, it is converted to a machine language at some point previous to execution so that it is comprehensible by the CPU. Machine language can be unswervingly interpreted by the hardware itself, and is able to be without difficulty encoded as a string of binary bits and sent simply via electrical signals.
The instruction set is a set of pre-defined machine codes, which the CPU is designed to expect and be able to operate upon when detected. Dissimilar processors have dissimilar instruction sets, to allow for easier coding, greater aspects and to cope with changes in the genuine architecture of the processor itself. Each machine code of an instruction set consists of two separate fields:
• Opcode and operands
The opcode is a short code which points to what operation is expected to be performed. Each and every operation has a unique opcode. The operand, or operands, points to where the data required for the operation can be found and how it can be accessed (the addressing mode, which is conversed in full later). The length of a machine code can vary - ordinary lengths vary from one to twelve bytes in size.
The exact format of the machine codes is again CPU dependant.
Opcodes are also known mnemonics (short names) so that they can be simply referred to in code listings and similar documentation. for instance, an instruction to store the data of the accumulator in a given memory address could be specified the binary opcode 000001, which may then be referred to using the mnemonic STA (short for Store Accumulator).
Now we identify what form the data is in when it is read by the CPU, it is essential to learn about the cycle by which the instructions of a program are executed.