Complete Computer Description

The complete computer description includes both, the instruction cycle and the interrupt cycle. Interrupt cycle, because there may arise a case of I/O operation anytime during normal operation. Instruction cycle, because in absence of interrupts, the CPU is always busy with stored program instructions. A flip flop R is used as a condition to determine the type of operation. When R=0, the instruction cycle continues. Similarly when R=1, the computer goes through an interrupt cycle.
Here it must be noted that an interrupt is signaled by IEN and R is just to make it sure that another interrupt does not get entertained while processing of an interrupt.

Consider the following diagram:

A basic computer consists of the following hardware components:

• A memory unit of 4096 X 16 bits
• Nine Registers- PC, AR, IR, DR, AC, TR, INPR, OUTR and SC
• Five Flip-Flops- I, R, IEN, FGI and FGO
• Two decoders- a 3 to 8 operation decoder and a 4 to 16 timing decoder
• A 16-bit common bus
• Control Logic Gates
• Arithmetic and Logic Unit connected to AC

Control functions and microoperations for a basic computer are as follows:

Instruction Cycle

Fetch Phase:

Decode Phase:

Indirect address in Memory Reference Instructions:

Interrupt Cycle