Within the computer, each instruction is represented by a sequence of bits. The instruction is divided into fields, corresponding to the constituent elements of the instruction. The instruction format is highly machine specific and it mainly depends on the machine architecture. A simple example of an instruction format is shown in the Figure 4.8. It is assume that it is a 16-bit CPU. 4 bits are used to provide the operation code. So, we may have to 16 (24 = 16) different set of instructions. With each instruction, there are two operands. To specify each operands, 6 bits are used. It is possible to provide 64 ( 26 = 64 ) different operands for each operand reference.
It is difficult to deal with binary representation of machine instructions. Thus, it has become common practice to use a symbolic representation of machine instructions.
Opcodes are represented by abbreviations, called mnemonics, that indicate the operations.
Common examples include:
