2.3 Module 4: instruction set: characteristics and functions  (Page 31/7)

1. machine instruction characteristics

1.1 what is an instruction set?

From the designer's point of view, the machine instruction set provides the functional requirements for the CPU: Implementing the CPU is a task that in large part involves implementing the machine instruction set.

From the user's side, the user who chooses to program in machine language (actually, in assembly language) becomes aware of the register and memory structure, the types of data directly supported by the machine, and the functioning of the ALU.

1.2 elements of an instruction

Each instruction must have elements that contain the information required by the CPU for execution. These elements are as follows

• Operation code: Specifies the operation to be performed (e.g.. ADD, I/O). The operation is specified by a binary code, known as the operation code, or opcode.
• Source operand reference: The operation may involve one or more source operands, that is, operands that are inputs for the operation.
• Result operand reference: The operation may produce a result.
• Next instruction reference: This tells the CPU where to fetch the next instruc­tion after the execution of this instruction is complete.

The next instruction to be fetched is located in main memory or, in the case of a virtual memory system, in either main memory or secondary memory (disk). In most cases, the next instruction to be fetched immediately follows the current instruction. In those cases, there is no explicit reference to the next instruction. Source and result operands can be in one of three areas:

• Main or virtual memory: As with next instruction references, the main or vir­tual memory address must be supplied.
• CPU register: With rare exceptions, a CPU contains one or more registers that may be referenced by machine instructions. If only one register exists, reference to it may be implicit. If more than one register exists, then each register is assigned a unique number, and the instruction must contain the number of the desired register.
• I/O device: The instruction must specify (he I/O module and device for the operation. If memory-mapped I/O is used, this is just another main or virtual memory address.

Instruction cycle state diagram

Instruction representation

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. During instruction execution, an instruction is read into an instruction register (IR) in the CPU. The CPU must be able to extract the data from the various instruction fields to perform the required operation.

It is difficult for both the programmer and the reader of textbooks to deal with binary representations 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 operation. Common examples include