Datapath and Control

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Datapath and Control
CDA 3101 Discussion Section10
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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.
• RegWrite
• ALUOp0
• ALUOp1
• Branch
• MemRead
• MemWrite

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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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Question 5.3
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• RegWrite1
• 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.
• ALUOp01
• 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.
• ALUOp11
• 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.

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Question 5.3

• Branch1
• 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.
• MemRead1
• 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.
• MemWrite1
• Only sw will work correctly. The rest of
  instructions will store their results in the data
  memory, while they should not.

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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
• PCRrs
• R-type, Op/Fun code 0/0x08

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Question 5.8
X
0 M U X 1
JumpReg
Instruction5-0
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Control Signals (Figure 5.18)
Instruction R lw sw beq Jr
RegDst X
ALUSrc X
MemtoReg X
RegWrite S A M E 0
MemRead 0
MemWrite 0
Branch 0
ALUop10 x x
JumpReg 0 0 0 0 1