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Executing an ALU instruction: Part 2 - a register and a constant

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We now examine in detail how the basic pipeline CPU execute instructions with constants as second operand.

• ALU instructions with a constant as source operand 2
  • Example: Add R1, 2, R3 (R3 := R1 + 2)

  • The instruction is encoded as follows:

    		Br Cond
    		------
       LD  ST  BRA  Opcode  Im Cen <-- Dest -> <- Src1 --> <-- Src2 ->
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | 0 | 0 | 1 | 0 | 1 | 0 |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    

    • Notice the Immediate mode bit is set to 1 to indicate that the SRC2 field contains a constant

• Step 1: Fetch

  • At start of the CPU cycle, the IF stage sends out PC
  • At end of the CPU cycle, the IR(ID) register is updated with the instruction fetched (ADD R1, 2, R3)

    

  • The picture above depicts the content of the CPU at end of the first CPU cycle (and the start of the 2nd cycle)

• Step 2: Decode (fetch operands)
  • At start of the CPU cycle, the ID stage sends out selection signal that selects value from R1 (and also R2 !!! - R2 is fetched but will not be used)

    The ID stage also extracts the constant stored inside the instruction "add r1, 2, r3"

  • At end of the CPU cycle, A register is updated with R1, and IR1 register is updated with the value 0000000000000010 (= 2)

    Incidently, the B register is updated with R2 because the ID stage does not know which operand will be used.... You will see in the next step that there is no harm in fetching R2 into B.

    And ofcourse, D register is updated with R3 (but R3 will not be used in this instruction).

  • Also, at the end of the CPU cycle, the instruction (ADD R1, 2, R3) is moved into IR(EX) and a new instruction is fetched into IR(ID)

    

  • The picture above depicts the content of the CPU at end of the second CPU cycle (and the start of the 3rd cycle)

• Step 3: Execute (compute)
  • At start of the CPU cycle, the EX stage selects values from R1 in A register (and not PC1) and IR1 (not R2 in B register, because the Immediate but is set !) for the ALU, use the ALU opcode (ADD) to make ALU add the input values

  • At end of the CPU cycle, ALUo and DMAR registers is updated with the value R1+2 !!!

    Also, SMDR register will be updated with the value R3 (but R3 will not be used - see next step).

    Clearly, fetching R2 into B register was harmless.

  • Also, at the end of the CPU cycle, the instruction (ADD R1, 2, R3) is moved into IR(MEM) and a new instruction is fetched into IR(EX)

    

  • The picture above depicts the content of the CPU at end of the 3rd CPU cycle (and the start of the 4th cycle)

• Step 4: Memory Access or Branch
  • Because the MEM stage of the basic pipelined CPU is used to execute memory access and branch instructions, and "ADD R1, 2, R3" is neither, there is no processing taken place in the MEM stage.

  • At end of the CPU cycle, ALUo1 registers is updated with the value R1+2

  • At the end of the CPU cycle, the value R3 in SMDR register is discarded (SMDR register is only used in a STORE instruction - see later)

  • Also, at the end of the CPU cycle, the instruction (ADD R1, 2, R3) is moved into IR(WB) and a new instruction is fetched into IR(MEM)

    

  • The picture above depicts the content of the CPU at end of the 4rd CPU cycle (and the start of the 5th cycle)

• Step 5: Update register
  • At the start of the CPU cycle, the destination register (R3) is selected by the Dest field of the IR(WB) and the value in ALUo1 (= R1 + 2) is selected for output on the C-bus (because the instruction is an ADD)
  • At end of the CPU cycle, the register R3 register is updated with the value R1+2
  • Also, at the end of the CPU cycle, the instruction (ADD R1, 2, R3) is discarded and a new instruction is fetched into IR(WB)

    

  • The picture above depicts the content of the CPU at end of the 5rd CPU cycle (and the start of the 6th cycle)