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
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