1 Datapath and Control CDA 3101 Discussion Section10

2 Questions 5.3 Describe the effect that a single stuck-at-1 fault (the signal is always 1) would have for the signals shown below, in the single-cycle datapath in Figure 5.17 on page 307. Which instructions, if any will not work correctly? Explain why. a.RegWrite b.ALUOp0 c.ALUOp1 d.Branch e.MemRead f.MemWrite

3 ALU Control Figure 5.12 How the ALU control bits are set depends on the ALUOp control bits and the different function codes for the R-type instruction. Figure 5.18 The setting of the control lines is completely determined by the opcode fields of the instruction.

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6 Question 5.3 RegWrite=1 sw and beq should not write results to the register file. sw(beq) will overwrite a random register with either the store address (branch target) or random data from the memory data read port. ALUOp0=1 When ALUOp0 is stuck at 1. beq will work fine. lw and sw will not work fine because a subtraction will be done instead of addition. Some R-Type instructions will work, others will not. (The previous slide is the logic for the ALU Control unit. Operation bit3 is always 0, not shown in the circuit). Only the R-Type instructions which need Operation 2 bit as 1 will work, others wont. ALUOp1=1 When ALUOp1 is stuck at 1. The R-Type instructions will work fine. Lw and sw will not work if the bits 0-5 of the instruction representation is anything other than the function code for add. beq will not work unless the bits 0-5 of the instruction representation is the function code for subtract. 0

7 Question 5.3 Branch=1 Instructions other than branches (beq) will not work correctly if the ALU Zero signal is raised. An R-format instruction that produces zero output will branch to a random address determined by its least significant 16 bits. MemRead=1 All instructions but sw will work correctly. (Data memory is always read, but memory data is never written to the register file except in the case of lw.) sw will not work because both MemRead and MemWrite should not be set to 1 at the same time. MemWrite=1 Only sw will work correctly. The rest of instructions will store their results in the data memory, while they should not.

8 Questions 5.8 Add the instruction jr to the single-cycle datapath described in this chapter. Add any necessary datapaths and control signals to the single-cycle datapath of Figure 5.17 on page 307 and show the necessary additions to Figure 5.18 on page 308. Jump Register – jr rs PC=R[rs] R-type, Op/Fun code: 0/0x08

9 Question 5.8 0MUX10MUX1 X JumpReg Instruction[5-0]

10 Control Signals (Figure 5.18) InstructionRlwswbeqJr RegDstX ALUSrcX MemtoRegX RegWriteSAME0 MemRead0 MemWrite0 Branch0 ALUop[1:0][x x] JumpReg00001