Microprogrammed Control Unit

Methods of designing a control unit

1. Encoded state assignment method 2. One-Hot state assignment method3. Microprogrammed Control Unit method

4. Pipeline Control <== in EE457

Microprogram method of designing control unit ==> suitable for “large” control applications

example: state machine with 500 statesMotorola 68020 processor uses this method

Conventional encoded state assignment control unit method ==> unwieldy for large state machines ( > 64 states)

sometimes called Hard-Wired method

Microprogrammed Control Unit method ==> more like software design

can be easily corrected/revised

Microprogram method -- was used in mechanical systems

Numerical Control Machinesautomatic weaving machines

punched paper tape with lines of holes/no-holes

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

?

A line of Holes ( ) / No-holes ( )

Presence of a HOLE ==> activate a tool/control signal

No-Hole ==> do not activate the tool / control signal

A line of Holes / No-holes ==> a micro-operation

==> a micro-instruction==> a control word

==> a state

A sequence of micro-operations==> a recipe

==> a microprogram==> a state machine

Paper Tape==> control store / control memory

==> microprogram memory / uROM

Paper Tape has width and lengthWidth ==>Length ==>

uROM has width and depthWidth ==>Depth ==>

0 1 2

012

uROM

Paper advancing mechanismSprocket and motor

in the case of a Microprogram memoryuPC (micro-program counter)(an incrementable address register)

Straight-line sequencingSprocket and uPC

Branchingloadable uPC either it increments or it loads

012

uROM

uPC

012

01

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