2.3 Module 4: instruction set: characteristics and functions  (Page 6/7)

3.7 transfer of control

For all of the operation types discussed so far. The next instruction to be performed is the one that immediately follows, in memory, the current instruction. However, a significant fraction of the instructions in any program have as their function chang­ing the sequence of instruction execution. For these instructions, the operation per­formed by the CPU is to update the program counter to contain the address of some instruction in memory.

There are a number of reasons why transfer-of-control operations are re­quired. Among the most important are the following:

1. In the practical use of computers, it is essential to be able to execute each instruction more than once and perhaps many thousands of times. It may require thousands or perhaps millions of instructions to implement an appli­cation. This would be unthinkable if each instruction had to be written out sep­arately. If a table or a list of items is to be processed, a program loop is needed. One sequence of instructions is executed repeatedly to process all the data.

2. Virtually all programs involve some decision making. We would like the com­puter to do one thing if one condition holds, and another thing if another con­dition holds. For example, a sequence of instructions computes the square root of a number. At the start of the sequence, the sign of the number is tested, If the number is negative, the computation is not performed, but an error con­dition is reported.

3. To compose correctly a large or even medium-size computer program is an exceedingly difficult task. It helps if there are mechanisms for breaking the task up into smaller pieces that can be worked on one at a time.

We now turn to a discussion of the most common transfer-of-control opera­tions found in instruction sets: branch, skip, and procedure call.

Branch instruction

A branch instruction, also called a jump instruction, has as one of its operands the address of the next instruction to be executed. Most often, the instruction is a conditional branch instruction. That is, the branch is made (update program counter to equal address specified in operand) only if a certain condition is met. Otherwise, the next instruction in sequence is executed (increment program counter as usual).

Skip instructions

Another common form of transfer-of-control instruction is the skip instruc­tion. The skip instruction includes an implied address. Typically, the skip implies that one instruction be skipped; thus, the implied address equals the address of the next instruction plus one instruction-length.

Procedure call instructions

Perhaps the most important innovation in the development of programming languages is the procedure, a procedure is a self-contained computer program that is incorporated into a larger program. At any point in the program the procedure may he invoked, or called. The processor is instructed to go and execute the entire procedure and then return to the point from which the call took place.

The two principal reasons for the use of procedures are economy and modu­larity. A procedure allows the same piece of code to be used many times. This is important for economy in programming effort and for making the most efficient use of storage space in the system (the program must be stored). Procedures also allow large programming tasks to be subdivided into smaller units. This use of modularity greatly eases the programming task.