1. Systems Architecture I
September 4, 1997
Systems Architecture I (CS ) Lecture 15: A Multicycle Implementation of MIPS*
Jeremy R. Johnson
Wed. May 23, 2000
*This lecture was derived from material in the text (sec ).
All figures from Computer Organization and Design: The Hardware/Software Approach, Second Edition, by David Patterson and John Hennessy, are copyrighted material (COPYRIGHT 1998 MORGAN KAUFMANN PUBLISHERS, INC. ALL RIGHTS RESERVED).
May 23, 2000
Systems Architecture I

2. Systems Architecture I
September 4, 1997
Introduction
Objective: To re-implement the MIPS instruction set using a multicycle implementation. The benefits are shared hardware and that instructions can take a different number of cycles (reduced computing time).
How: break instructions into steps, each step taking a clock cycle.
What’s New:
control depends on step
Need to store temporary values in internal registers
Topics
Steps in multicycle implementation
Control using finite state machine
Control using microprogramming
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Systems Architecture I

3. Systems Architecture I
Multicycle Approach
Break up the instructions into steps, each step takes a cycle
balance the amount of work to be done
restrict each cycle to use only one major functional unit
Functional units: memory, register file, and ALU
At the end of a cycle
Use internal registers to store results between steps
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Systems Architecture I

4. Systems Architecture I
Execution Steps
Instruction fetch
IR = Memory[PC];
PC = PC + 4;
Instruction decode and register fetch
A = Reg[IR[25-21]];
B = Reg[IR[20-16]];
ALUOut = PC + (sign-extend (IR[15-0]) << 2);
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Systems Architecture I

5. Systems Architecture I
Execution Steps
Execution, memory address computation, or branch completion
Memory reference
ALUOut = A + sign-extend (IR[15-0]);
Arithmetic-logical instruction (R-type)
ALUOut = A op B;
Branch
if (A == B) PC = ALUOut;
Jump
PC = PC [31-28] || (IR[25-0]<<2)
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Systems Architecture I

6. Systems Architecture I
Execution Steps
Memory access or R-type instruction completion
memory reference
MDR = Memory [ALUOut]; or
Memory [ALUOut] = B;
R-type completion
Reg [IR[15-11]] = ALUOut;
Memory read completion
Reg [IR[20-16]] = MDR;
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Systems Architecture I

7. Systems Architecture I
Summary
3 - 5 clock cycles
Uses “optimistic” actions
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Systems Architecture I

8. Systems Architecture I
CPI in a Multicycle CPU
Using the previous multicycle implementation determine the average cycles per instruction (from gcc)
22% loads
11% stores
49% R-format
16% branches
2% jumps
CPI = 0.22      3
= 4.04
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9. Systems Architecture I
Control Signals
1-bit control signals
RegDst
RegWrite
ALUSrcA
MemRead
MemWrite
MemtoReg
IorD
IRWrite
PCWrite
PCWriteCond
2-bit control signals
ALUOp
00 (add)
01 (sub)
10 (funct field)
ALUSrcB
00 (B register)
01 (constant 4)
10 (lower 16 bits from IR)
11 (lower 16 bits from IR
shifted left 2 bits)
PCSource
00 (output of ALU = PC+4)
01 (ALUOut = branch addr)
10 (jump target)
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10. Systems Architecture I
Complete Datapath
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11. Systems Architecture I
Finite State Machine
Set of states
Next function determined by input and current state
Output determined by current state and possibly input
Moore machine (output determined only by current state)
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12. Systems Architecture I
Control Using FSM
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13. Systems Architecture I
Implementation of FSM
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Systems Architecture I

14. Control using Microprogramming
Represent asserted values on control lines symbollically (microinstructions)
Fields: Label, ALU control, SRC1, SRC2, Register control, Memory, PCWrite control, Sequencing
specifies non-overlapping set of control signals
Placed in ROM or PLA (provides address for microinstructions)
Sequencing mechanism
next microinstruction
branch to microinstruction (FETCH) for next MIPS instruction
choose next microinstruction based on control unit input (dispatch). Implemented using a table of addresses (dispatch table).
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Systems Architecture I

15. Microinstruction Fields
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16. Systems Architecture I
Microprogram
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17. Systems Architecture I
Implementation
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Systems Architecture I