COMP541 Datapaths II

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COMP541Datapaths II

• Montek Singh
• Mar 22, 2007

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Topics

• Continue datapath
• Inside ALU
• Addition and Subtraction
• Implementing add/sub in ALU
• Shifting

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Datapath

• Fairly complete datapath
• Arithmetic
• Logic
• Shifter
• One bit, R or L

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ALU

• This one has 2 control lines for arithmetic
• S2 selects logical ops

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Addition and Subtraction

• Lets take side trip into how these are done at
  low level
• Then come back to look at detailed design of ALU

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Looking Inside

• Cin also an input

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Design of B Select Logic

• Just straightforward truth table

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4-Bit Circuit

• Weve seen similar before

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Now Add Logic Section

• Mux to choose which

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Logic Section Design

• Generous number of operations

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Resulting Control
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Shifter First Thought

• Make a parallel-load, bidirectional shift
  register
• Anything wrong with that?

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Takes Three Clocks

• First clock loads shift reg
• Second makes the shift
• Third loads to destination reg
• Any alternatives?

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Mux as Shift Register

• Combinational no clock

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Is This Better?

• Three short clocks may be faster than one limited
  by gate delays
• But complex control
• We have long ALU delay, so maybe this doesnt
  matter
• Our one-clock shifter becomes multi instruction
  to shift n (gt1) bits

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Shifting More Than 1 Bit

• Barrel shifter

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Impractical As-Is

• When n gets larger, this becomes huge
• Can use levels of muxes
• Similar to carry-lookahead adder
• Can design with transistors

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Datapath

• Higher-level view for hierarchical design
• Can replace modules with same interface but
  different implementation

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Register File

• Can be fairly large (32 registers)
• Memory with three addresses
• D for write
• A and B for read
• What are
• Inputs
• Outputs

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Function Unit

• Arithmetic and shifter
• Can be bypassed at output

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Control Lines

• Control inputs have been blended together into
  one control word (see next slide)

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Operations of Func. Unit
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Particular 8-bit Datapath

• For examples to follow
• Note that all control lines now have sizes and
  names
• Look at each in next slide

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Control Word

• Collect all lines in a control word for
  convenience
• Has 7 fields

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Table of Fields (for reference)
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Microoperation

• Microoperation
• Maps to

Field DA AA BA MB FS MD RW
Symb. R1 R2 R3 Reg Func Wr
Num. 001 002 003 0 00101 0 1
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Other Examples
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Using Numeric Notation
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Have Simple Machine

• With part of an instruction set
• No branches, for example

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Timing

• Note that regs are latched on next clock

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Next Time

• Either look at control
• or specifically at how to add memory to your MIPS
  design, and implement load and store