1. CSCI206 - Computer Organization & Programming
Single Cycle Control
zyBook: 11.4

2. Why do we need control?

3. MIPS with control logic shown

4. Morgan Kaufmann Publishers
24 November, 2018
ALU Control
Assume 2-bit ALUOp derived from opcode
Combinational logic derives ALU control
opcode
ALUOp
Operation
funct
ALU function
ALU control
lw (100011) 00
load word
XXXXXX
add
0010
sw (101011)
store word
beq (000100) 01
branch equal
subtract
0110
R-type (000000) 10
100000
100010
AND
100100
0000 OR
100101
0001
set-on-less-than
101010
0111
Chapter 4 — The Processor

5. Meanings of Control Signals
Signal name
Effect when deasserted(0)
Effect when asserted(1)
RegDst
Reg. number for write comes from rt field [20:16].
Reg. number for write comes from rd field [15:11].
Branch
Next instruction address comes from PC+4.
Next instruction addr. comes from ALU (jump or branch).
MemRead
None.
Data memory content designated by the input address are put on Read data output.
MemtoReg
The value fed to the register Write data input comes from ALU.
The value fed to the register Write data input comes from the data memory.
ALUOp
See the ALU Control in previous slides. The 2-bit value combines with [5-0] to determine the value for ALU Control.
MemWrite
Data memory content designated by the input address is replaced by the value on Write data input.
ALUSrc
The second ALU operand comes from the second register file output (Read data 2).
The second ALU operand is the sign-extended, lower 16 bits of the instruction.
RegWrite
The register on the Write register input is written with the value on the Write data input.

6. Truth Table for Control Function
Signal
R-format lw sw
beq
Input
Op5 1
Op4
Op3
Op2
Op1
Op0
Output
RegDst X
ALUSrc
MemtoReg
RegWrite
MemRead
MemWrite
Branch
ALUOp1
ALUOp2
bit 31
bit 26