COMP541%20Sequencing%20and%20Control%20--%20II

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COMP541Sequencing and Control -- II

• Montek Singh
• April 5, 2007

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Topics

• Still on Chapter 8
• Last class
• Hardwired control
• Now
• Microprogrammed control of multiplier

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Microprogrammed Approach

• Control values stored in a memory
• Job of instructions is to generate control
  signals to datapath and output

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Nomenclature and Characteristics

• Word of memory called microinstruction
• The set of instructions called microprogram
• Sometimes in ROM, sometimes loadable
• Often wide word

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Recall Datapath
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Microprogrammed Control Unit

• Control Address Register (CAR) equivalent to PC
• Sequencer
• Part of instruction sent to next-address
  generator to determine next instruction addr

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Control Data Register

• Pipelining approach to break up the delay in the
  addr gen and ROM
• Not used in example

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Status Bits

• Notice that they go only to sequencer
• Can only affect next control word
• So, conditional output boxes not allowed in this
  architecture

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ASM old vs. new
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Microinstruction

• Word format
• Addresses of potential next instructions
• Fields for next instruction selection
• Fields for datapath control

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Datapath Control Signals

• Doesnt include load reg inst.
• Look at ASM to see where they are asserted

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Mapping to Microinstruction

• Have only 4 signals
• Could encode (2 bits?)
• Would cost a decoder
• Just a design tradeoff
• This design has tiny ROM anyway

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Sequencer Design

• Probably most important part of this process
• This design provides 2 addrs
• SEL field and control logic choose one
• Other possibility is one addr field
• Choice is to go to next sequential addr (like
  PC), or
• Using control signals go to addr specified

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SEL Field
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Result

• 5 words in ROM
• ROM is 12 bits wide
• Design next

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Microprog Design for Mult
Mux chooses addr1 or addr2
Mux control from datapath status and external
signals. Next slide
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Detail of Control
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Microprogram
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Is it necessary?

• Microprogramming less useful now that HLLs used
  for design
• Can use Verilog to generate very complex state
  machine

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Today

• Looked at ways to implement microprogrammed
  control for the multiplier

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Next

• Look at computer
• Sequencing
• Pipelining
• Well also return to look at programmable logic
• Wed skipped earlier