The CPU's functioning is described by the fetch-execute cycle, as discussed elsewhere in this book. Once the instruction has been fetched, it must be executed. However, before the instruction can be executed, it must be decoded. The processor has to decode the instruction to determine what is meant by it. The instruction follows a certain syntax, and the corresponding operation must be determined so that it can be carried out--this is what decoding is.

After the instruction is decoded, the next action of the processor could well be to ask for more data from the memory, or to decide that more fetches are needed before the present instruction is executed. Decoding an instruction thus determines three things:

• What kind of action is called for.
• If data is required to execute the instruction, where that data is to be obtained from--register, memory address, etc.
• If data is...