Basic Computer Organization and Design

Description of basic ComputerInstruction Format and their executionCommon Bus SystemInstruction CycleHardwired Control UnitIO Configuration and IO handling

Basic Computer Organization and Design

Description of basic computer

Computers are the digital devices that performs the various computational task.

Digital means there is the process of representing the information by the help of the certain discrete values.

Information is represented in digital computers in terms of bits.

By various coding techniques these groups of bits can not only represent numbers but also other discrete symbols.


A computer system is sub-divided into two functional entities: hardware and software.

Hardware consists of all electronics components.

Software consists of instruction and data that the computer manipulate to perform various tasks.



Application software is all the computer software that causes a computer to perform useful tasks beyond the running of the computer itself.

System software is computer software designed to operate and control the computer hardware and to provide a platform for running application software.

Computer hardware is the collection of physical elements that comprise a computer system



Hardware consist of three major parts: CPU: It has ALU for manipulating data, registers for

storing data and control circuit for generating control signals.

Memory: It store instruction and data. I/O Processor: It contain electronic circuit for

communicating and controlling information flow between computer and input or output devices.


Computer organization is concerned with the way hardware operate and the way the way they are connected together to form the computer system.

Computer architecture is concerned with the structure and behavior of computers as seen by the user. It includes information formats, the instruction set and technique for addressing memory.

Instruction Code

It is a group of bits that instruct the computer to perform a specific task.

It contains various parts one of them is the opcode to specify the type of operation to be performed.

Another field is the address field which is used to specify the operand address on which the operation is to be done.

Stored Program Organization

Indirect Address

In direct address the address field in the instruction gives the address of the operand,

In case of the indirect address, the address in the address field gives an address of the memory word in which address operand is found.

Direct and indirect address are separated by bit I. I is 1 for indirect and o for direct.

Effective Address

It is the address of the actual operand.In direct address, content of the address field

gives the effective address which is 457 in the given example.

In indirect address, the word in the memory address specified in the address field gives the effective address which is 1350 in the given example.

Computer Register

Computer Registers

Common Bus System

The basic computer has eight register, memory unit and control unit.

Path must be provided to transfer information from one register to another and from memory to the register.

The number of wires will be excessive if the connection is made between the output of each register and input of other.

A more efficient scheme in transferring information in a system having many register is to use a common bus.

Common Bus System

The outputs of seven registers and memory are connected to the common bus. The specific output that is selected for the bus

lines at any given time is determined from the binary value of the selection variables S2,S1 and S0.

The numbers along each output shows the decimal equivalent of the required binary selection.

The lines from the common bus are connected to the inputs of each register and data input of the memory.

Common Bus System

The particular register whose LD(load) input is enabled receives the data from the bus during the next clock transition.

The memory receives the content of the bus when its write input is activated.

Basic Instruction Format


A memory reference instruction uses 12 bits to specify the an address and one bit to specify the addressing mode.

A register reference instruction specifies an operation on or a test of the AC register. 12 bits are used to specify the operation or test to be executed.

In the case of i/o instruction also 12 bits are used to specify the type of i/o operation or test performed.


Hex code of i/o instruction starts with F.Hex code of register reference instruction

starts with the 7.Hex code of direct memory reference

instruction starts with 0-6.Hex code of indirect memory reference

instruction starts with 8-E.

Instruction Set Completeness

The set of instruction are said to complete if the computer includes sufficient number of instruction in each of the following categories. Arithmetic, logical and shift instruction. Instruction for moving information to and from

memory and processor registers. Program control instructions together with instruction

that check the status conditions. Input and output instructions.

Timing and Control

The timing for all registers in the basic computer is controlled by the master clock generator.

The clock pulsed are applied to all the flip flops and registers in the system.

The clock pulses do not change the state of register unless enabled by control signals.

These signals are generated in the control unit.

There are two major types of control organization: hardwired control and micro programmed control.

Timing and Control

In hardwired control, the control logic is implemented with gates, flip flops, decoders and other digital circuits.

In micro programmed organization, the control information is stored in a control memory. The control memory is programmed to initiate the required sequence of micro operation.

Hardwired Control Unit


It consists of two decoders , sequence counter and number of control logic gates.

An instruction read from memory is places in IR.

Opcode of the instruction derives one of the output of opcode decoder high.

At first SC is cleared to zero. As the first clock pulse hits the output of counter and hence counter decoder change and produce sequence of timing signals T0,T1,T2 and so on.


The output of counter decoder and decoded opcode of the instruction causes the control logic to generate a particular control word for the operation.

At the end of each instruction, counter is cleared.

For the next instruction, the opcode produces different output on the opcode decoder and the control will be different and the control unit will perform the different tasks

Instruction Cycle

The program executed in the computer by going through a cycle for each instruction.

The process of fetching, decoding and executing the instruction is called instruction cycle.

In basic computer each instruction cycle consists of the following phases: Fetch an instruction from memory. Decode instruction Read effective address if the instruction has indirect

address. Execute the instruction

Instruction Cycle

Instruction Cycle

Fetch and Decode SC is cleared to 0. It is incremented by one so that

timing signal goes through T0,T1 and so on. PC is loaded with the address of the first instruction. Micro operation are listed below:

Instruction Cycle

Determine the type of instruction Decoder output D7 equals to 1 if instruction is

register reference or input/output reference.(opcode 111)

Decoder output D7 equals to o if the instruction is memory reference.(opcode 000-110)

Control then inspects the first bit. If D7=0 & I=1 it is memory reference with indirect address. If D7=0 and I=0 it is memory reference with direct address.

If D7=1 and I=0 it is register reference instruction. If D7=1 and I=1 it is input/output reference

instruction.

Instruction Cycle

Execute the Instruction Three instruction types are divided into four paths. The operation related to timing signal T3 are :

Input Output Configuration

A computer serves no useful purpose unless it communicates with the external environment.

Instruction and data stored in memory must come from some input device.

Computational results must be transmitted to the user through some output device



Input Register(INPR) consist of eight bits and hold alphanumeric input information.

The one bit input flag(FGI) is control flip flop. It is 1 when information is available in INPR and cleared to 0 when information is accepted by computer.

The output register(OUTR) works in similar manner.

When computer found FGO is 1, the information from AC is transferred to OUTR and FGO is cleared to 0.

The output device receives the information and set FGO to 1.

Program Interrupt

The speed of the peripheral devices are very slow in comparison to that of the computer.

So during i/o operation computer will waste time while checking the flag instead of doing some useful tasks.

One solution for this problem is that computer won’t check the flag continuously but get interrupted whenever the flag is set.

Computer than deviates from what it was doing and take care of the i/o operation.

Interrupt Cycle

The interrupt handled by the program can be explained with the help of flow chart.

When R=0, computer goes through an instruction cycle.

During execute phase IEN is checked if it is 1, control checks for flags if both are zero next instruction is executed. If either flag is 1 when IEN is 1, R is set to 1 and it goes through the interrupt cycle.

Interrupt Cycle

In interrupt cycle address in PC is stored in some location so that it can be found later.

Here we choose memory location at address 0 to store the return address.

Interrupt Cycle

Interrupt Cycle

Engineering

Basic Computer Organization and Design