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

2. Implementing Complex Behaviors
A Datapath is needed to perform arithmetic/boolean operations on data
+, -, *, /, mod, AND, OR, <<, <, >>, >, 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.
Datapath
Controller
Control bits
Status bits
Register
load
reset

3. Simple Controller/Datapath
Reg A
load D Q
Reg B +
Reg C
Reg E
Reg D
in1
in2
ldA
ldC
ldB
ldD
ldE
in3
ldA=1
ldB=1
ldC=1
ldD=1
ldE=1
A = in1;
B = in2;
D = in3;
C = A + B;
E = C + D;
Behavior
Controller
Datapath
One-to-one mapping from behavior to datapath components
var-reg, op-op

4. Alternate Controller, Same Datapath
ldA=1
ldB=1
ldC=1
ldD=1
ldE=1
A = in1;
B = in2;
D = in3;
C = A + B;
E = C + D;
ldE=1
ldA=1
ldB=1
ldD=1
ldC=1
Behavior
Controller
Optimized Controller
Optimize performance by merging many operations into one state
Merging continues until a data dependency (or conditional) is reached

5. 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; ld
Reg A ld
Reg B ld
Reg C + ld
Reg E ld
Reg D +

6. 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 ld
Reg A
Reg B
Reg C
Reg D
Reg E +
ldA
ldB
ldC
ldD
ldE
A = in1;
B = in2;
D = in3;
C = A + B;
E = C + D;

7. 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
A = B + C;
If (A > 2) then
X = Y + Z;
Else
P = Q + R;
Result = X + P;
A = in1;
B = in2;
D = in3;
C = A + B;
E = C + D;
st1
st2
st3
No conditional
With Conditional

8. Controller Definition Continued
4. Draw FSM edges based on control flow
A = B + C;
If (A > 2) then
X = Y + Z;
Else
P = Q + R;
Result = X + P;
A = in1;
B = in2;
D = in3;
C = A + B;
E = C + D;
A>2
!A>2
No conditional
With Conditional

9. Controller Definition Continued
5. Label controller states with outputs
ldA=1
ldB=1
ldD=1
A = in1;
B = in2;
D = in3;
C = A + B;
E = C + D;
ldC=1
ldE=1