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.
• ALUOp11
• 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.
• ALUOp01
• 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.

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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)