Control unit

How to control the datapath?

Need to generate the control signals for MUXes, registers, and memory

Look at instruction formats

R-type

000000

$rs

$rt

$rd

shamt

function

b31-26

b25-21

b20-16

b15-11

b10-6

b5-0

memory

35 or 43

$rs

$rt

address

b31-26

b25-21

b20-16

b15-0

branch

4

$rs

$rt

address

b31-26

b25-21

b20-16

b15-0

Observations:

Opcode is always in upper 6 bits: Op[5-0], function in lower 6 bits: F[5-0]

Use to select operation to perform

Registers to read ($rs, $rt) always in bits 25-21 and 20-16

Use as indexes to register file for read

Base register for load/store always in bits 25-21

Offset always in bits 15-0

Destination register may be in 2 places:

bits 15-11 for R-type

bits 20-16 for load

Use as index to register file to write: need MUX to select