Start Lecture #13
We start with our last figure, which shows the data path and then add the missing mux and show how the instruction is broken down.
We need to set the muxes.
We need to generate the three ALU cntl lines: 1-bit Bnegate and 2-bit OP
And 0 00 Or 0 01 Add 0 10 Sub 1 10 Set-LT 1 11
Homework: What happens if we use 1 00 for the three ALU control lines? What if we use 1 01?
What information can we use to decide on the muxes and alu cntl lines?
So no problem, just do a truth table.
We will let the main control (to be done later)
the opcode for us.
From this summary we determine the control lines for the muxes.
Specifically, the main control will generate a 2-bit field ALUOp
ALUOp Action needed by ALU 00 Addition (for load and store) 01 Subtraction (for beq) 10 Determined by funct field (R-type instruction) 11 Not used