Key
HW 5.13 in the text.
We can do better than the answer we gave in class by using figure 5.18.
In figure 5.18 we can see that the MemRead column has the same values as the
Memtoreg column for R-format and lw instructions. However for sw and beq they differ,
however for both of those cases the Memtoreg does not matter ( since regwrite is 0).
Thus all the instructions would work properly if MemRead were also used to control the
Memtoreg MUX.
In figure 5.18 we can see that the AluSrc column has the same values as the Memtoreg
column for R-format and lw instructions. However for sw and beq they differ, however
for both of those cases the Memtoreg does not matter ( since regwrite is 0). Thus all the
instructions would work properly if AluSrc were also used to control the Memtoreg
MUX.
However we cannot substitute ALUSrc for MemWrite since this would cause the CPU to
write into memory on a lw instruction and we do not want that to happen. Also we could
not substitute the Memwite control signal for the ALUSrc signal as the input to the ALU
would then be wrong on a lw instruction. NOTE THAT WE DID THIS PART WRONG
IN CLASS BECAUSE WE CONCENTRATED ON FIGURE 5.17 AND DID NOT
THINK THROUGH ALL THE ALTERNATIVES.