1. Prof. John Nestor ECE Department Lafayette College Easton, Pennsylvania 18042 nestorj@lafayette.edu ECE 313 - Computer Organization Lecture 15 - Multi-Cycle Processor Design 2 Fall 2006 Reading: 5.6 - 5.11, C.4 - C.5, Verilog Handout Section 6-10 Homework: 5.32, 5.34, 5.35, 5.36, 5.49, 5.55 Portions of these slides are derived from: Textbook figures © 1998 Morgan Kaufmann Publishers all rights reserved Tod Amon's COD2e Slides © 1998 Morgan Kaufmann Publishers all rights reserved Dave Patterson’s CS 152 Slides - Fall 1997 © UCB Rob Rutenbar’s 18-347 Slides - Fall 1999 CMU other sources as noted

2. ECE 313 Fall 2006Lecture 15 - Multicycle Design 22 Outline - Multicycle Design  Overview  Datapath Design  Controller Design  Aside: FSM Design in Verilog   Performance Considerations  Extending the Design: An Example  Microprogramming  Exceptions

3. ECE 313 Fall 2006Lecture 15 - Multicycle Design 23 Review State Machine Design  Traditional Approach:  Create State Diagram  Create State Transition Table  Assign State Codes  Write Excitation Equations & Minimize  HDL-Based State Machine Design  Create State Diagram (optional)  Write HDL description of state machine  Synthesize

4. ECE 313 Fall 2006Lecture 15 - Multicycle Design 24 Review - State Transition Table / Diagram  Transition List - lists edges in STD PSConditionNSOutput IDLEARM' + DOOR'IDLE0 IDLEARM*DOORBEEP0 BEEPARMWAIT1 BEEPARM'IDLE1 WAITARMBEEP0 WAITARM'IDLE0 IDLE BEEP Honk=1 WAIT ARMDOOR ARM ARM’ ARM’ + ARMDOOR’ = ARM’ + DOOR’

5. ECE 313 Fall 2006Lecture 15 - Multicycle Design 25 Coding FSMs in Verilog  Clocked always block - state register  Combinational always block -  next state logic  output logic

6. ECE 313 Fall 2006Lecture 15 - Multicycle Design 26 Coding FSMs in Verilog - Code Skeleton  Part 1 - Declarations module fsm(inputs, outputs); input...; reg...; parameter [NBITS-1:0] S0 = 2'b00; S1 = 2'b01; S2 = 2b'10; S3 = 2b'11; reg [NBITS-1 :0] CURRENT_STATE; reg [NBITS-1 :0] NEXT_STATE; State Codes State Variable

7. ECE 313 Fall 2006Lecture 15 - Multicycle Design 27 Coding FSMs in Verilog - Code Skeleton  Part 2 - State Register, Logic Specification always @(posedge clk) begin CURRENT_STATE <= NEXT_STATE; end always @(CURRENT_STATE or xin) begin case (CURRENT_STATE) S0:... determine NEXT_STATE, outputs S1 :... determine NEXT_STATE, outputs end case end // always endmodule

8. ECE 313 Fall 2006Lecture 15 - Multicycle Design 28 FSM Example - Car Alarm  Part 1 - Declarations, State Register module car_alarm (arm, door, reset, clk, honk ); input arm, door, reset, clk; output honk; reg honk; parameter IDLE=0,BEEP=1,HWAIT=2; reg [1:0] current_state, next_state; always @(posedge reset or posedge clk) if (reset) current_state <= IDLE; else current_state <= next_state;

9. ECE 313 Fall 2006Lecture 15 - Multicycle Design 29 FSM Example - Car Alarm  Part 2 - Logic Specification always @(current_state or arm or door) case (current_state) IDLE : begin honk = 0; if (arm && door) next_state = BEEP; else next_state = IDLE; end BEEP: begin honk = 1; if (arm) next_state = HWAIT; else next_state = IDLE; end IDLE BEEP Honk=1 WAIT ARMDOOR ARM ARM’ ARM’ + ARMDOOR’ = ARM’ + DOOR’

10. ECE 313 Fall 2006Lecture 15 - Multicycle Design 210 FSM Example - Car Alarm  Part 3 - Logic Specification (cont’d) HWAIT : begin honk = 0; if (arm) next_state = BEEP; else next_state = IDLE; end default : begin honk = 0; next_state = IDLE; end endcase endmodule IDLE BEEP Honk=1 WAIT ARMDOOR ARM ARM’ ARM’ + ARMDOOR’ = ARM’ + DOOR’

11. ECE 313 Fall 2006Lecture 15 - Multicycle Design 211 FSM Example - Verilog Handout  Divide-by-Three Counter S0 out=0 S1 out=0 S1 out=1 reset

12. ECE 313 Fall 2006Lecture 15 - Multicycle Design 212 Verilog Code - Divide by Three Counter Part 1 module divideby3FSM(clk, reset, out); inputclk; inputreset; outputout; reg[1:0] state; reg[1:0]nextstate; parameterS0 = 2’b00; parameterS1 = 2’b01; parameterS2 = 2’b10; // State Register always @(posedge clk or posedge reset) if (reset) state <= S0; else state <= nextstate;

13. ECE 313 Fall 2006Lecture 15 - Multicycle Design 213 Verilog Code - Divide by Three Counter Part 2 // Next State Logic always @(state) case (state) S0: nextstate = S1; S1: nextstate = S2; S2: nextstate = S0; default: nextstate = S0; endcase // Output Logic assign out = (state == S2); endmodule

14. ECE 313 Fall 2006Lecture 15 - Multicycle Design 214 Verilog Example: MIPS Control Unit

15. ECE 313 Fall 2006Lecture 15 - Multicycle Design 215 Review: Full Multicycle Implementation

16. ECE 313 Fall 2006Lecture 15 - Multicycle Design 216 MIPS Control Unit “Skeleton” - Part 1 module mips_control( clk, reset, Op, PCWrite, PCWriteCond, IorD, MemRead, MemWrite, MemtoReg, IRWrite, PCSource, ALUOp ALUSrcB, ALUSrcA, RegWrite, RegDst ); input clk; input reset; input [5:0] Op; output PCWrite; output PCWriteCond; output IorD; output MemRead; output MemWrite; output MemtoReg; output IRWrite; output [1:0] PCSource; output [1:0] ALUOp; output ALUSrcA; output [1:0] ALUSrcB; output RegWrite; output RegDst; port declarations

17. ECE 313 Fall 2006Lecture 15 - Multicycle Design 217 MIPS Control Unit “Skeleton” - Part 2 reg PCWrite; reg PCWriteCond; reg IorD; reg MemRead; reg MemWrite; reg MemtoReg; reg IRWrite; reg [1:0] PCSource; reg [1:0] ALUOp; reg ALUSrcA; reg [1:0] ALUSrcB; reg RegWrite; reg RegDst; parameter R_FORMAT = 6'd0; parameter LW = 6'd35; parameter SW = 6'd43; parameter BEQ = 6'd4; parameter J = 6’d2; parameter S0=4'd0, S1=4'd1, S2=4'd2, S3=4'd3, S4=4'd4, S5=4'd5, S6=4'd6, S7=4'D7, S8=4'd8, S9=4'd9; Symbolic Constants - opcodes Symbolic Constants - state codes reg declarations for output ports

18. ECE 313 Fall 2006Lecture 15 - Multicycle Design 218 MIPS Control Unit “Skeleton” - Part 3 reg [3:0] current_state, next_state; always @(negedge clk) begin if (reset) current_state <= S0; else current_state <= next_state; end always @(current_state or Op) begin // default values PCWrite = 1'b0; PCWriteCond = 1'b0; IorD = 1'bx; MemRead = 1'b0; MemWrite = 1'b0; MemtoReg = 1'bx; IRWrite = 1'b0; PCSource = 2'bxx; ALUOp = 2'bxx; ALUSrcA = 1'bx; ALUSrcB = 2'bxx; RegWrite = 1'b0; RegDst = 1'bx; case (current_state) S0: begin MemRead = 1'b1; ALUSrcA = 1'b0; IorD = 1'b0; IRWrite = 1'b1; ALUSrcB = 2'b01; ALUOp = 2'b00; PCWrite = 1'b1; PCSource = 2'b00; next_state = S1; end … endcase end endmodule Add code here! Default Values More Default Values

19. ECE 313 Fall 2006Lecture 15 - Multicycle Design 219 Controller Implementation  Typical Implementation: Figure 5-37, p. 338  Variations  Random logic  PLA  ROM address lines = inputs data lines = outputs contents = “truth table” Datapath control outputs Inputs from Instr. Reg (opcode) Combinational Control Logic State Next State

20. ECE 313 Fall 2006Lecture 15 - Multicycle Design 220 Outline - Multicycle Design  Overview  Datapath Design  Controller Design  Aside: FSM Design in Verilog  Performance Considerations   Extending the Design: An Example  Microprogramming  Exceptions

21. ECE 313 Fall 2006Lecture 15 - Multicycle Design 221 Performance of a Multicycle Implementation  What is the CPI of the Multicycle Implementation?  Using measured instruction mix from SPECINT2000 lw5 cycles25% sw4 cycles10% R-type4 cycles52% branch3 cycles11% jump3 cycles2%  What is the CPI?  CPI = (5 cycles * 0.25) + (4 cycles * 0.10) + (4 cycles * 0.53) + (3 cycles * 0.11) + (3 cycles * 0.02)  CPI = 4.12 cycles per instruction

22. ECE 313 Fall 2006Lecture 15 - Multicycle Design 222 Performance Continued  Assuming a 200ps clock, what is average execution time/instruction?  Sec/Instr = 4.12 CPI * 200ps/cycle) = 824ps/instr  How does this compare to the Single-Cycle Case?  Sec/Instr = 1 CPI * 600ps/cycle = 600ps/instr  Single-Cycle is 1.38 times faster than Multicycle  Why is Single-Cycle faster than Multicycle?  Branch & jump are the same speed (600ps vs 600ps)  R-type & store are faster (600ps vs 800ps)  Load word is faster (600ps vs 1000ps)

23. ECE 313 Fall 2006Lecture 15 - Multicycle Design 223 Outline - Multicycle Design  Overview  Datapath Design  Controller Design  Aside: FSM Design in Verilog  Performance Considerations  Extending the Design: An Example   Microprogramming  Exceptions

24. ECE 313 Fall 2006Lecture 15 - Multicycle Design 224 Multicycle Example Problem  Extend the design to implement the “jr” (jump register) instruction: jr rsPC = Reg[rs]  Format:  Steps: 1.Review instruction requirements (register transfer) 2.Modify datapath 3.Modify control logic 0 rs00 8 0 6 bits5 bits 6 bits

25. ECE 313 Fall 2006Lecture 15 - Multicycle Design 225 Reg[rs] Example Problem: Datapath What needs to be changed? 3 2 1 0

26. ECE 313 Fall 2006Lecture 15 - Multicycle Design 226 Example Problem: Control What needs to be changed? PCWrite PCSource = 11 (OP = ‘JR‘)

27. ECE 313 Fall 2006Lecture 15 - Multicycle Design 227 Outline - Multicycle Design  Overview  Datapath Design  Controller Design  Aside: FSM Design in Verilog  Performance Considerations  Extending the Design: An Example  Microprogramming   Exceptions

28. ECE 313 Fall 2006Lecture 15 - Multicycle Design 228 Control Implementation - Another View  Separate Logic into two pieces  Output Logic (this is a Moore Machine - why?)  Next-State Logic

29. ECE 313 Fall 2006Lecture 15 - Multicycle Design 229 Microprogramming - Motivation  Problems with graphical approach to FSM Design  Unwieldy for large number of states (real processors may have hundreds of instructions -> hundreds of states)  Unwieldy if instruction types vary radically (can you say… x86?)  Most states are sequential (state 4 follows state 3; state 3 follows state 2; state 7 follows state 6; etc.  Idea: expand on ROM implementation of control

30. ECE 313 Fall 2006Lecture 15 - Multicycle Design 230 Consider Output Logic in ROM  ROM Characteristics - "lookup table"  State code for each state is a ROM address  Control outputs for each state are a ROM word

31. ECE 313 Fall 2006Lecture 15 - Multicycle Design 231 Microprogramming - Basic Idea  Idea: expand on ROM control implementation  One state = one ROM word = one microinstruction  State sequences form a microprogram  Each state code becomes a microinstruction address

32. ECE 313 Fall 2006Lecture 15 - Multicycle Design 232 Microprogramming - Sequencer Design

33. ECE 313 Fall 2006Lecture 15 - Multicycle Design 233 Describing Microcode  Each microinstruction is lots of 1's and 0's  To ease understanding:  Break into fields related to different datapath functions  Use mnemonics to describe different field values Datapath Control Signals ALU controlSequencingLabel AddRead PCALUSeqstring Subt Func Code PC A B 4 Extend Extshft Read Write ALU Write MDR Read ALU Write ALU ALUOut-cond Jump address Fetch Dispatch i See also: Figure C.5.1, p. C-28 SRC1Reg. controlMemoryPCWrite control ALUOp ALUSrcA ALUSrcB RegWrite RegDst MemRead SRC2 MemWrite IRWrite PCWrite PCWriteCond IorD PCSource MemtoReg AddrCtl Sequencer Control Signal

34. ECE 313 Fall 2006Lecture 15 - Multicycle Design 234 Microcode for Multicycle Implementation

35. ECE 313 Fall 2006Lecture 15 - Multicycle Design 235 Sequencer Implementation Details

36. ECE 313 Fall 2006Lecture 15 - Multicycle Design 236 Microcoding Tradeoffs +Makes design easier +Flexible  Easy to adapt to changes in organization, timing, technology  Can make changes late in design cycle  Can add more instructions just by adding microcode -Costly to implement -Slow - "extra level" of interpretation

37. ECE 313 Fall 2006Lecture 15 - Multicycle Design 237 Microcoding Perspective  Not used in modern RISC processors  simple instructions -> simple control  hardwired control -> faster execution  pipelining used to enhance performance  Used heavily in CISC processors  Traditional CISC: all instructions microcoded multiple dispatch ROMs to handle different instruction classes, addressing modes, etc.  Current CISC (see Section 5.9) Microinstructions pipelined like RISC instructions! Simple instructions translate to one microinstruction Complex instructions translate to multiple microinstructions

38. ECE 313 Fall 2006Lecture 15 - Multicycle Design 238 Instruction Decoding in the Pentium 4 Source: “The Microarchitecture of the Pentium® 4 Processor”, Intel Technology Journal, First Quarter 2001 http://developer.intel.com/technology/itj/q12001/articles/art_2.htm.

39. ECE 313 Fall 2006Lecture 15 - Multicycle Design 239 Instruction Decoding in the Pentium 4 Source: “The Microarchitecture of the Pentium® 4 Processor”, Intel Technology Journal, First Quarter 2001 http://developer.intel.com/technology/itj/q12001/articles/art_2.htm.

40. ECE 313 Fall 2006Lecture 15 - Multicycle Design 240 Coming Up  Implementing Exceptions  Pipelined Design