Basic Computer Organization and Design

Introduction to Basic Computer

We introduce here a basic computer whose operation can be specified by the resister transfer statements. Internal organization of the computer is defined by the sequence of microoperations it performs on data stored in its resisters. Every different processor type has its own design (different registers, buses, microoperations, machine instructions, etc). Modern processor is a very complex device. It contains:

– Many registers

– Multiple arithmetic units, for both integer and floating point calculations

– The ability to pipeline several consecutive instructions for execution speedup.

However, to understand how processors work, we will start with a simplified processor model. M. Morris Mano introduces a simple processor model; he calls it a “Basic Computer”. The Basic Computer has two components, a processor and memory.

• The memory has 4096 words in it

        – 4096 = 212, so it takes 12 bits to select an address in memory

• Each word is 16 bits long

Stored Program Organization

• The program (instruction) as well as data (operand) is stored in the same memory.

Instruction Format 

A computer instruction is often divided into two parts:

• An op-code (Operation Code) that specifies the operation for that instruction.
• An address that specifies the registers and/or locations in memory to use for that operation.

Addressing Modes

The address field of an instruction can represent either

• Direct address: the address operand field is effective address (the address of the operand).
• Indirect address: the address operand field contains the memory address of effective address.

Basic Computer Registers and Common Bus System >>