Controllers and Datapaths

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Controllers and Datapaths
CS 153, Spring 2007 Ian G. Harris Department of
Computer Science University of California Irvine
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Implementing Complex Behaviors

• A Datapath is needed to perform
  arithmetic/boolean operations on data
• , -, , /, mod, AND, OR, ltlt, lt, gtgt, gt, etc.
• A Controller is needed to tell datapath
  components what to do and when to do it
• Status information from the datapath may be
  needed
• A controller drives the control inputs of the
  datapath components
• load, reset, shiftleft, ALUOp, etc.

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Simple Controller/Datapath
in3
A in1 B in2 D in3 C A B E C D
Controller
Datapath
Behavior

• One-to-one mapping from behavior to datapath
  components
• var-reg, op-op

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Alternate Controller, Same Datapath
A in1 B in2 D in3 C A B E C D
Behavior
Optimized Controller
Controller

• Optimize performance by merging many operations
  into one state
• Merging continues until a data dependency (or
  conditional) is reached

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Algorithm to Make Datapath and Controller

• Determine all datapath components needed by
  looking at each step of the behavior
• Assume no hardware sharing

• 5 Registers - one for each variable
• 2 adders - one for each addition

A in1 B in2 D in3 C A B E C D


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Datapath/Controller Algorithm Continued

• 2. Connect datapath components by identifying
  connections needed for each step in the behavior.
• Be sure to label control inputs, status outputs

in1
in2
in3
A in1 B in2 D in3 C A B E C D
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Datapath/Controller Algorithm Controller
Definition

• 3. Group steps of the behavior into states
• Pack as many steps as possible into a state
• No data dependency can exist in a state
• No conditional can exist in a state - all ops in
  state MUST occur together

st1
st2
st3
No conditional
With Conditional
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Controller Definition Continued
4. Draw FSM edges based on control flow
Agt2
!Agt2
No conditional
With Conditional
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Controller Definition Continued
5. Label controller states with outputs
ldA1 ldB1 ldD1
ldC1
ldE1