Basic of Computer Organization and Design [UPDATED]


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We all know this is very vast topic under Computer Architecture and Organization in computer science so today I am writing not all but relevant about computer organization and design. Before starting the topic I am writing down the topics I will cover under this:-



First of all I want to make you clear that every processor in digital system has its own design i.e. design means the registers, buses, micro-operations, instructions etc, used by different processor is different.

If i say about processors in today era, these are very complex device because the work these processors can do are also complex like doing multiple arithmetic operations on both integer and floating point calculations, modern processors having multiple registers for different works, and the new ability of pipelining the instructions for having speed in their execution is also making them different, etc.

So, if you want to understand how processors work everyone takes simple processor model which is similar to the real processors before 20 years ago.

We simply call simple processor model as BASIC COMPUTER.

The basic computer has two components, first one called as processor and second one is called as memory.

The memory has 4096 words in it i.e. 12 bits are used to select a word in memory, because
4096 = 212

And every word uses 16 bits to store value.

basic computer organization and design-basic computer


These Instructions are very similar to the instructions given by an instructor to instruct someone. So here instructions are given by programmer to instruct machine or say processor. Here especially instructions are called machine instructions because these instructions are used to instruct a processor which is a machine.

The group of these machine instructions is known as program as like a page of English lines having group of multiple English lines.

Machine instruction is defined as the group of bits that tells computer to perform a specific operation, or a sequence of micro-operations.

These instructions can contain data into them on which we can perform any desired operation. So these instructions and any data needed stored in memory (second component of basic computer).

If I say deeper then the Central Processing Unit (CPU) is totally using these instructions and executes them one by one. So the question arises that, from where CPU gets the address of the instruction it should execute that is from IR (Instruction Register). IR instruction address to CPU.

Basic computer is always executing instructions by converting them into micro operations and this translation is being done by control circuitry in control unit.


Instruction is basically divided into two parts,

basic computer organization and design-instruction

Operation Code is the code or part in instruction from where machine understands which operations are to execute through this instruction.

Address is that part which specifies the register or any location in memory which we can use for that operation written in operation code.

If I say about Basic Computer it contains 4096 words in memory and as previously explained in this type of memory 12 bits are used to specify any address.

And, this memory can store 16 bits at one location in which 3 bits are reserved for instruction opcode(Operation Code), and 1 bit is reserved for Addressing Mode(will explain later), rest  12 bits are used to store any address of data, register or any other location could use in instruction.

basic computer organization and design-instruction format


In this sub part I will only define Addressing Modes not explain the types of Addressing Modes.

Mainly addressing modes are used to specify how we are explaining address in address field of the instructions, i.e. how we are placing value of address field in instruction like in 16 bits of instruction one bit (15th bit) is used to represent the addressing mode.

Here I m writing only about two addressing modes i.e. Direct and Indirect

Direct Addressing also represents by 0 bit in addressing mode, in direct addressing mode the address used in instruction shows that on that address we can find data and can perform operations.

Indirect Addressing also represents by 1 bit (as shown in above diagram), indirect addressing mode shows that at the address used in instruction, data is not present instead on that address another memory location is present and on that second memory location our data is there which can be use by instruction to perform operation. (It is quite complicated to understand).

In this the term is also used named as Effective Address (EA), Effective address is the address on which directly the operand can be accessed without any modification to access for computation with instruction.


We know a processor has lot of registers to handle work given to it. Registers are used to hold instruction , addresses, data, etc.

Program Counter (PC) is the register in processor which holds  memory address of next instruction to be executed, PC in  Basic Computer needs only 12 bits to store address of next instruction for execution.

Address Register (AR) is that register of processor which keeps track of memory locations used for addressing. It is also 12 bit register. i.e. it is used to store memory addresses.

Data Register (DR), this register is used to store data on which operations  performed by processor. Or we can say this register is used to store operand found on memory locations given in instructions.

Accumulator (AC) this is also a register of the processor and this register comes under general purpose register.  General purpose registers can directly be used in instructions.
e.g. like,  LOAD AC this instruction will save content from specific memory location to Accumulator.

Temporary Register (TR) a register used to store temporary data or intermediate results while executing instruction.

For performing Input/Output operations processor has Input Register (INPR) and Output Register (OUTR).

The INPR holds an 8 bit character it got from any input device attached to processor.

The OUTR holds an 8 bit character to be send to an output device attached to processor.

registers in basic computer-computer organization and design

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