Concept of Program Execution

The instructions constituting a program to be executed by a computer are loaded in sequential locations in its main memory. To execute this program, the CPU fetches one instruction at a time and performs the functions specified. Instructions are fetched from successive memory locations until the execution of a branch or a jump instruction.

The CPU keeps track of the address of the memory location where the next instruction is located through the use of a dedicated CPU register, referred to as the program counter (PC). After fetching an instruction, the contents of the PC are updated to point at the next instruction in sequence.

For simplicity, let us assume that each instruction occupies one memory word. Therefore, execution of one instruction requires the following three steps to be performed by the CPU:

Fetch the contents of the memory location pointed at by the PC. The contents of this location are interpreted as an instruction to be executed. Hence, they are stored in the instruction register (IR). Symbolically this can be written as:

IR = [ [PC] ]

Increment the contents of the PC by 1.

PC = [PC] + 1

Carry out the actions specified by the instruction stored in the IR.

The first two steps are usually referred to as the fetch phase and the step 3 is known as the execution phase. Fetch cycle basically involves read the next instruction from the memory into the CPU and along with that update the contents of the program counter.

In the execution phase, it interprets the opcode and perform the indicated operation. The instruction fetch and execution phase together known as instruction cycle. The basic instruction cycle is shown in the Figure below

Basic Instruction cycle
In cases, where an instruction occupies more than one word, step 1 and step 2 can be repeated as many times as necessary to fetch the complete instruction. In these cases, the execution of a instruction may involve one or more operands in memory, each of which requires a memory access.

Further, if indirect addressing is used, then additional memory access are required.

The fetched instruction is loaded into the instruction register. The instruction contains bits that specify the action to be performed by the processor. The processor interprets the instruction and performs the required action. In general, the actions fall into four categories:

Processor-memory: Data may be transferred from processor to memory or from memory to processor.

Processor-I/O: Data may be transferred to or from a peripheral device by transferring between the processor and an I/O module.

Data processing: The processor may perform some arithmetic or logic operation on data.

Control: An instruction may specify that the sequence of execution be altered.
The main line of activity consists of alternating instruction fetch and instruction execution activities. After an instruction is fetched, it is examined to determine if any indirect addressing is involved. If so, the required operands are fetched using indirect addressing.

The execution cycle of a particular instruction may involve more than one reference to memory. Also, instead of memory references, an instruction may specify an I/O operation. With these additional considerations the basic instruction cycle can be expanded with more details view in the Figure below The figure is in the form of a state diagram.


Instruction cycle state diagram.

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