1. Design of the Control Unit for Single-Cycle Instruction Execution
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2. The simple datapath with the control unit.
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3. The function of each of the seven control signals
The function of each of the seven control signals. When the 1-bit control to a two-way multiplexor is asserted, the multiplexor selects the input corresponding to 1. Otherwise, if the control is deserted, the multiplexor selects the 0 input. Remember that the state elements all have the clock as an implicit input and that the clock is used in controlling writes.
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4. Instruction Execution
R-type: add $x, $y, $z 4 steps
I-type: lw $x, offset ($y) 5 steps
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5. ALU Instruction Execution
add $x, $y, $z
1. Instruction Fetch (IF): An instruction is fetched from the instruction memory and the PC is incremented.
2. Instruction Decode (ID): Two registers, $y and $z, are read from the register file.
3. Execution (EX): The ALU operates on the data read from the register file, using the function code (bits 5-0 of the instruction) to generate the ALU function.
4. Write Back (WB): The result from the ALU is written into the register file using bits of the instruction to select the destination register ($x).
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6. Some conventions for the next slides
Inputs are available at the beginning of the step and output is available at the end of the step.
Inputs are available at the end of the step and output is not available.
Input A is selected by the blue control signal.
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7. The first step of an R-type instruction performs a fetch
from instruction memory and increments the PC.
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8. The second phase in the execution of R-type instruction reads
the two source registers from the register file.
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9. The third phase of execution for R-type instruction involves
the ALU operating on the register data operands.
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10. The final step in an R-type instruction, writing the result.
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11. Memory Instruction Execution
lw $x, offset ($y)
1. Instruction Fetch (IF): An instruction is fetched from the instruction memory and the PC is incremented.
2. Instruction Decode (ID): A register ($y) value is read from the register file.
3. Address Calculation (EX): The ALU computes the sum of the value read from the register file and the sign-extended lower 16 bits of the instruction (offset).
4. Memory Operation (MEM): The sum from the ALU is used as the address for the data memory.
5. Write Back (WB): The data from the memory unit is written into the register file; the register destination is given by bits of the instruction ($x).
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12. The operation of a load instruction with the simple
datapath control scheme.
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13. Branch Instruction Execution
beq $x, $y, offset
1. Instruction Fetch (IF): An instruction is fetched from the instruction memory and the PC is incremented.
2. Instruction Decode (ID): Two registers, $x and $y, are read from the register file.
3. Branch Address calculation (EX): The ALU performs a subtract on the data values read from the register file. The value of PC + 4 is added to the sign-extended lower 16 bits of the instruction (offset); the result is the branch target address.
4. Branch Decision (BD): The Zero result from the ALU is used to decide which adder result to store into the PC.
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14. The datapath in operation for a branch equal instruction.
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15. Control Unit Design Control 0p5 0p0 RegDst ALUSrc ALU0p1 ALU0p0
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16. The control function for the simple one-cycle implementation is completely specified by this truth table. The top half of the table gives the combinations of input signals that correspond to the four opcodes that determine the control output setting. (Remember that Op (5-0) corresponds to bits of the instruction, which is the opcode field.) The bottom portion of the table gives the outputs.
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17. The simple control and datapath are extended to
handle the jump instruction.
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18. A Multiple Clock Cycle Instruction Implementation
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19. Comments on Single-Cycle Implementation
Machine instructions may have different critical path length
Load instruction
FP instructions
Different addressing mode
The cycle time will be determined by the worst critical path
FU duplication may be costly
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20. Multi-Cycle Implementation
Each instruction is divided into a sequence of steps
Each step takes one clock cycle
A function unit can be used by the same instruction at different steps.
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