CS2100: Processor - Control

Control

Control unit

We will have the control unit that is plug into all the components mention in the previous slides (MUX/REGISTERS/DATAMEMORY)

This will allow us to control when we want each component to accept each specific data

Control Signals

RegDst

Controls the multiplexer.

False (0) - Write register = Inst[20:16] //Register Target

True (1) - Write register = Inst[15:11] //Register Destination

(Inst[Bitsfrom instruction])

R-Type -> 1

lw -> 0 //has no third register, rt is the register number

sw-> X //we dont care, we are not even going to write back

beq-> X //we dont care

RegWrite

Controls the write to register

False (0) - No register Write

True (1) - New value will be written

R-Type -> 1

lw -> 1

sw-> 0 

beq-> 0 //because of the previous

ALUSrc

Controls the write to register

False (0) - Operand2 = Register Read Data 2

True (1) - Operand2 = SignExt (Inst [15:0]) //for immediate

R-Type -> 0

lw -> 1

sw-> 1 

beq-> 0 //The signal is send to the adder, we need to ensure that the second register goes to the ALU

Memread

Under datamemory

False (0) - Not performing memory read access

True (1) - Read memory using address

R-Type -> 0

lw -> 1

sw-> 0 

beq-> 0 

Memwrite

Under datamemory

False (0) - Not performing memory write access

True (1) - Write memory using address

R-Type -> 0

lw -> 0

sw-> 1 

beq-> 0 

MemToReg

This mux is reverse input

To select the memory or ALU result to be written back to the register

False (0) - ALU result

True (1) - Memory read data

R-Type -> 0 //write back to register

lw -> 1 //route to back from memory

sw-> X

beq-> X //Due to regwrite control ignoring

PCSrc

Selects the next instruction or the branch target address

However, is this signal generate from the type of pc source

We need to check if its a branch instruction. (It could be SW)

Check if its Branch:

Branch = 1

Not Branch = 0

Check if we can jump:

Taken = 1

Not Taken = 0

We will use an AND gate to and it together to drive this MUX

R-Type -> 0

lw -> 0

sw-> 0 

beq-> 1 //only beq will jump

ALUcontrol signal

We need to know the opcode and the function code to generate the alu signal

Multilevel decoding

Using the control that is looking at the opcode, generate the ALUop signal

Generate ALUCtrl signal from ALUop and optionally function field.

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