Understanding the Control Unit in Computing: Structure, Functions, and Implementations, Slides of Computer Architecture and Organization

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Control Unit

Structure - The CPU Computer

ArithmeticRegistersandLogin UnitInternal CPUInterconnectionControlUnit I/O^ CPUSystemBus Memory

CPU docsity.com

Control Unit
The control unit is the main component that directsthe system operations by sending control signals tothe datapath.
These signals control the flow of data within theCPU and between the CPU and external units suchas memory and I/O.
Control busses generally carry signals between thecontrol unit and other computer components in aclock driven manner.
The system clock produces a continuous sequenceof pulses in a specified duration and frequency.

Control Unit
A sequence of steps^ t 0, t1,^ t 2, … ( t 0 <^

t 1 <^ t 2 <…) are used to execute a certain instruction.
The op-code field of a fetched instruction isdecoded to provide the control signal generatorwith information about the instruction to beexecuted.
Step information generated by a logic circuitmodule is used with other inputs to generate controlsignals.
The signal generator can be specified simply by aset of Boolean equations for its output in terms ofits inputs.

Functions of Control Unit
Sequencing ■ Causing the CPU to step through a series ofmicro-operations
Execution ■ Causing the performance of each micro-op
This is done using Control Signals

Control Signals
Clock ■ One micro-instruction (or set of parallel micro-instructions) per clock cycle
Instruction register ■ Op-code for current instruction ■ Determines which micro-instructions are performed
Flags ■ State of CPU ■ Results of previous operations
From control bus ■ Interrupts ■ Acknowledgements

Control Signals - output
Within CPU ■ Cause data movement ■ Activate specific functions
Via control bus ■ To memory ■ To I/O modules

Example Control Signal Sequence -Fetch
^ MAR <- (PC)^ ■^ Control unit activates signal to open gatesbetween PC and MAR
^ MBR <- (memory)^ ■^ Open gates between MAR and address bus^ ■^ Memory read control signal^ ■^ Open gates between data bus and MBR

Intel 8085 PinConfiguration

Intel 8085 OUT InstructionTiming Diagram

Hardwired Implementation
Control unit inputs
Flags and control bus ■ Each bit means something
Instruction register ■ Op-code causes different control signals for eachdifferent instruction ■ Unique logic for each op-code ■ Decoder takes encoded input and producessingle output n^ ■ n binary inputs and 2outputs

Hardwired Implementation…
Clock ■ Repetitive sequence of pulses ■ Useful for measuring duration of micro-ops ■ Must be long enough to allow signal propagation ■ Different control signals at different times withininstruction cycle ■ Need a counter with different control signals fort1, t2 etc.

Problems With Hard Wired Designs
^ Complex sequencing & micro-operation logic
^ Difficult to design and test
^ Inflexible design
^ Difficult to add new instructions

Microprogrammed Control Unit
Microprogramming was motivated by the desire to reducethe complexities involved with hardwired control.
Associated with each micro-operation is a set of controllines that must be activated to carry out the correspondingmicrooperation.
The idea of microprogrammed control is to store the controlsignals associated with the implementation of a certaininstruction as a microprogram in a special memory called acontrol memory (CM).
A microprogram consists of a sequence of microinstructions. ■ A microinstruction is a vector of bits, where each bit is a controlsignal, condition code, or the address of the next microinstruction. ■ Microinstructions are fetched from CM the same way programinstructions are fetched from main memory.