Computer Function
- The basic function performed by the computer is the execution of a program.
- A program consists of a set of instructions stored in memory.
- The processor does the actual work by executing instructions specified in the program.
- Instruction processing consists of two steps:
- The processor reads (fetches) instructions from memory one at a time and executes each instruction. (and repeats the fetch cycle)
- Program execution consists of repeating the steps of instruction fetch and instruction execution. (until the last line of the code)
- The instruction execution may involve several operations and depends on the nature of the instruction.
Instruction Cycle
The processing required for a single instruction is called an instruction cycle.
Instruction cycle two steps are referred to as the:
1) Fetch cycle 2) Execute cycle
- Fetch cycle is the process by which a computer retrieves a program instruction from its memory.
- Then it determines what actions the instruction dictates, and carries out those actions called execute cycle.
1. Fetch Cycle
- At the beginning of each instruction cycle, the processor fetches an instruction from memory.
- A register called the Program Counter (PC) holds the address of the instruction to be fetched next.
- The processor fetches the instruction from memory location pointed to by PC unless program flow control is altered by a jump instruction.
- The processor always increments the PC after each instruction fetch, so that it will fetch the next instruction in sequence.
- The Next instruction is the instruction located at the next higher memory address.
Fetch Example
- For example, consider a computer in which each instruction occupies one 16-bit word of memory.
- Assume that the program counter (PC) is set to memory location 300, where the location address (300) refers to a 16-bit word.
- The processor will next fetch the instruction at location 300.
- On succeeding instruction cycles, it will fetch instructions from locations 301, 302, 303, and so on.
2. Execute Cycle
- In ‘Instruction Fetch’ we load the ‘PC (program counter)’ memory location value into the ‘IR (instruction register)’ and increments the PC.
- The fetched instruction is loaded into a register in the processor known as the Instruction Register (IR).
- The instruction contains the bits (opcode) that specify the action the processor is to take.
- The processor interprets the instruction (matches the opcode from its instruction set) and performs the required action.
General Categories of Functions Specified by Computer Instructions
An instruction execution may involve a combination of these actions:
Processor-memory: Data may be transferred from processor to memory or from memory to processor. E.g. move instruction.
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. E.g. Add, Sub, Mul, Div etc.
Control: An instruction may specify that the sequence of execution is altered. E.g. Jump instruction.
Execute Example
- For example, the processor may fetch an instruction from memory location 149.
- The location 149 specifies the next instruction to be from location 182. (Jump instruction).
- The processor will remember this fact by setting the program counter to 182.
- Thus, on the next fetch cycle, the instruction will be fetched from location 182 rather than 150.
Example of a Hypothetical Machine
- This machine includes the characteristics listed in Figure next slide.
- The processor contains a single data register, called an accumulator (AC). Both instruction and data are 16 bits long.
- Thus, the memory is organized using 16-bit words.
- The instruction format provides 4 bits for the opcode so that there can be as many as 24 = 16 different opcodes.
- And up to 212 = 4096(4K) words of memory can be directly addressed.
Characteristics of a Hypothetical Machine
