Key hw #2
5.2 For full credit one only needs to get the bold entrees correct. Since in this chapter
those are the ones we are emphasizing. lw, sw,add,sub,and,or,slt,beq, and j.
a. With regwrite stuck at 0, we never write into a register.
Will not work: add, addi, addu, and, andi, jal,lbu, lui,lw, nor, or, ori, slt, slti, sltiu, sltu,
sll, srl, sub and subu. Looking at the greensheet, these instruction all need to make an
assignment to a register.
The rest will work
b. If ALUOp0 is stuck at zero then from figure 1.18, beq cannot work. Everything else
can work. The aluop pair is used to send information to the ALU control about the
instruction. beq is the only instruction that requires a one in the low order bit.
c. If ALUOp1 is stuck at zero then from figure 1.18, the R format instructions cannot
work. Everything else will work. If the high order bit of aluop cannot be one then the
correct signal cannot be sent to the alu control for R format instructions.
d. If Branch is stuck at zero then the beq cannot work. From fig 3.17 if the branch
control is zero, then the control input to the branching MUX is 0 and PC + 4 becomes the
new PC value so the branch can never be taken. Actually this is OK if the branch is not
supposed to be taken ie if the output of the ALU is not zero. So it would still work
correctly sometimes. All other instructions still work.
e. If MemRead is stuck at zero then memory never performs a read operations, thus the
load operations will not work including lw. The other instructions will still work.
f. If MemWrite is stuck at zero then we can never do a store operation and sw will not
work. All operations that do not store in memory will work.
5.8
For the jr instruction we need to get the data from the read data 1 line to be stored in the
PC. One way this can be accomplished by inserting a new MUX into the input line for
the PC after any MUXs that might already be there. Insert a new control signal called rj
and require that when rj is zero the previous value is passed through to the PC and when
rj is one the value from read data 1 is passed through.
