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COMP541COMP541

Sequencing and Control -- Sequencing and Control -- IIII

Montek SinghMontek Singh

April 5, 2007April 5, 2007

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TopicsTopics Still on Chapter 8Still on Chapter 8 Last classLast class

Hardwired controlHardwired control

NowNow Microprogrammed control of multiplierMicroprogrammed control of multiplier

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Microprogrammed ApproachMicroprogrammed Approach Control values stored in a memoryControl values stored in a memory

Job of instructions is to generate control Job of instructions is to generate control signals to datapath and outputsignals to datapath and output

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Nomenclature and CharacteristicsNomenclature and Characteristics Word of memory called Word of memory called microinstructionmicroinstruction The set of instructions called The set of instructions called microprogrammicroprogram Sometimes in ROM, sometimes loadableSometimes in ROM, sometimes loadable Often wide wordOften wide word

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Recall DatapathRecall Datapath

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Microprogrammed Control UnitMicroprogrammed Control Unit Control Address Control Address

Register (CAR) Register (CAR) equivalent to PCequivalent to PC

SequencerSequencer Part of instruction sent Part of instruction sent

to next-address to next-address generator to determine generator to determine next instruction addrnext instruction addr

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Control Data RegisterControl Data Register Pipelining approach to Pipelining approach to

break up the delay in break up the delay in the addr gen and ROMthe addr gen and ROM

Not used in exampleNot used in example

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Status BitsStatus Bits Notice that they go only to Notice that they go only to

sequencersequencer Can only affect next control Can only affect next control

wordword So, conditional output boxes So, conditional output boxes

not allowed in this not allowed in this architecture architecture

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ASM – old vs. newASM – old vs. new

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MicroinstructionMicroinstruction Word formatWord format

Addresses of potential next instructionsAddresses of potential next instructions Fields for next instruction selectionFields for next instruction selection Fields for datapath controlFields for datapath control

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Datapath Control SignalsDatapath Control Signals Doesn’t include load reg inst.Doesn’t include load reg inst. Look at ASM to see where they are assertedLook at ASM to see where they are asserted

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Mapping to MicroinstructionMapping to Microinstruction Have only 4 signalsHave only 4 signals Could encode (2 bits?)Could encode (2 bits?) Would cost a decoderWould cost a decoder

Just a design tradeoffJust a design tradeoff This design has tiny ROM anywayThis design has tiny ROM anyway

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Sequencer DesignSequencer Design Probably most important part of this processProbably most important part of this process

This design provides 2 addrsThis design provides 2 addrs SEL field and control logic choose oneSEL field and control logic choose one

Other possibility is one addr fieldOther possibility is one addr field Choice is to go to next sequential addr (like PC), orChoice is to go to next sequential addr (like PC), or Using control signals go to addr specifiedUsing control signals go to addr specified

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SEL FieldSEL Field

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ResultResult 5 words in ROM5 words in ROM ROM is 12 bits wideROM is 12 bits wide Design nextDesign next

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Microprog Design for MultMicroprog Design for Mult

Mux chooses addr1 or addr2

Mux control from datapath status and external signals.

Next slide

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Detail of ControlDetail of Control

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MicroprogramMicroprogram

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Is it necessary?Is it necessary? Microprogramming less useful now that HLLs Microprogramming less useful now that HLLs

used for designused for design Can use Verilog to generate very complex Can use Verilog to generate very complex

state machinestate machine

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TodayToday Looked at ways to implement Looked at ways to implement

microprogrammed control for the multipliermicroprogrammed control for the multiplier

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NextNext Look at computerLook at computer

SequencingSequencing PipeliningPipelining

We’ll also return to look at programmable logicWe’ll also return to look at programmable logic We’d skipped earlierWe’d skipped earlier