2.3 Module 4: instruction set: characteristics and functions  (Page 2/7)

Operands are also represented symbolically. For example, the instruction

ADD R, Y

may mean add the value contained in data location Y to the contents of register R. In this example. Y refers to the address of a location in memory, and R refers to a particular register. Note that the operation is performed on the contents of a loca­tion, not on its address.

Simple Instruction Format

1.3 instruction types

Consider a high-level language instruction that could be expressed in a language such as BASIC or FORTRAN. For example,

X = X+Y

This statement instructs the computer lo add the value stored in Y to the value Stored in X and put the result in X. How might this be accomplished with machine instructions? Let us assume that the variables X and Y correspond lo locations 513 and 514. If we assume a simple set of machine instructions, this operation could be accomplished with three instructions:

1. Load a register with the contents of memory location 513.

2. Add the contents of memory location 514 to the register.

3. Store the contents of the register in memory location 513.

As can be seen, the single BASIC instruction may require three machine instructions. This is typical of the relationship between a high-level language and a machine language. A high-level language expresses operations in a concise algebraic form, using variables. A machine language expresses operations in a basic form involving the movement of data to or from registers.

With this simple example to guide us, let us consider the types of instructions that must be included in a practical computer. A computer should have a set of instructions that allows the user to formulate any data processing task. Another way to view it is to consider the capabilities of a high-level programming language. Any program written in a high-level language must be translated into machine language to be executed. Thus, the set of machine instructions must be sufficient to express any of the instructions from a high-level language. With this in mind we can cate­gorize instruction types as follows:

• Data processing: Arithmetic and logic instructions
• Data storage: Memory instructions
• Data movement: I/O instructions
• Control: Test and branch instructions

1.4 number of addresses

What is the maximum number of addresses one might need in an instruction? Evidently, arithmetic and logic instructions will require the most operands. Virtually all arithmetic and logic operations are either unary (one operand) or binary (two operands). Thus, we would need a maximum of two addresses to reference operands. The result of an operation must be stored, suggesting a third address. Finally, after com­pletion of an instruction, the next instruction must be fetched, and its address is needed.

This line of reasoning suggests that an instruction could plausibly be required to contain four address references: two operands, one result and the address of the next instruction. In practice, four-address instructions are extremely rare. Most instructions have one, two, or three operand addresses, with the address of the next instruction being implicit (obtained from the program counter).