ST. ALOYSIUS INSTITUTE OF TECHNOLOGY

COMPUTER SCIENCE ENGINEERING

4th SEMESTER

COMPUTER SYSTEM ORGAINZATION

SUBMITTED TO :- SHWETA AGRAWAL

SUBMITTED BY :- ANN MARIYA, DEEKSHA, SONA

HARDWIRED CONTROL UNIT

A control unit can be implemented using a variety of techniques. Most of these fall into one of the two categories:-

1. Hardwired implementation

2. Micro programmed implementation.

Hardwired control unit is implemented as a sequential logic circuit or a finite state machine that generates a specific sequence of control signals to execute an instruction.

In Micro programmed control unit, micro-instructions are stored in a special memory called control memory. In response to a machine instruction, a set of micro-instruction are executed. Each micro- instructions generates a set of control signals. Execution of micro-program resembles execution of a conventional program.

• Each state defines current control signalValues

Control execution– determines what happens in next clock cycle

• Current state and current register/flag values

Control sequencing-determine next state

STATE MACHINE

STRUCTURE OF A HARDWIRED CONTROL UNIT

IR

Sequential logic circuit

Control signals

clock

Status signal

Instruction register

Hardwired control unit uses fixed logic to interpret an instruction and generate appropriate control signals. While designing a control unit, we have to consider various factors like :

1)Amount of hardware used .

2)The speed of operation.

3)Cost of design.

There are four techniques for design of hardwired control unit. They are as follows:

1)State -table method 2)Delay-element method3)Sequence-counter method4)PLA method

STATE TABLE METHOD

It is a classical method of sequential circuit design. It attempts to minimize the amount of hardware.

DELAY-ELEMENT METHOD

It is heuristic method based on the use of clocked delay element (D-Flip Flop) for control signal timings.

SEQUENCE COUNTER METHOD

It uses counter for timing purposes.

PLA METHOD

It uses programmable Logic Array.

HARDWIRED CONTROL LOGIC IMPLEMENTATION

Finite statediagram

Explicitnext statefunction

Logicequations

PLA

Initial representation

Sequencing control

Logic representation

Implementation:

STATE TABLE METHOD:-

In the state table technique of hardwired control unit, we start with the construction of state transition table. In every state the control unit generates a set of control signals.

Control unit transmits from one state to another state depending on its:

1. Current state2. Input to the controller

Above concept is being explained with the help of an example of hardwired control unit for multiplication.

Hardwired Control Logic

• Circuitry becomes very big and complex very soon.

– may be unnecessarily slow– simpler is smaller, and thus faster

• Many lines (states) exactly or almost similar.

• Have methods to find similar lines (states) and combine them

– not simple– save space, may lose in speed

HP 9100 Calculator (1968), 20 kg,$5000, 16 regs (data or 14 instructions/reg),32Kb ROM, 2208 bit RAM magnetic core memory

Hardwired Control Logic board

REFRENCES-

Dilip Kumar Sultania

&

Google