EECE476 Lectures 10: Multi-cycle CPU Control Chapter 5: Section 5.5 The University of British ColumbiaEECE 476© 2005 Guy Lemieux

2 Multi-cycle BEQ Instruction 1. Fetch Instruction InstructionRegister ← Mem[PC]; PC ← PC Read Registers, Precompute Target A ← Registers[Rs] ; B ← Registers[Rt] ; ALUOut ← PC + {SignExt{Imm16},b’00’} 3. Compare Registers, Conditional Branch if( (A – B) ==0 ) PC ← ALUOut Green shows PC calculation flow (in parallel with other operations) HOMEWORK FOR TOMORROW Print out datapath diagram & ensure RTL is Valid ! Determine control signal value for each cycle !!

3 Multi-cycle CPU Datapath + Control Signals Instr[25:21] Instr[20:16] Instr[15:0] Instruction[5:0] In[15:11] Instr[25:0] PC[31..28] Jump address [31..0] PCWrite IorD MemRead MemWrite MemtoReg IRWrite PCSrc ALUOp ALUSrcA ALUSrcB RegWrite RegDst ALU Control

4 Multi-cycle CPU Datapath + Controller Instr. [31:26] Instr[31:26] Instr[25:21] Instr[20:16] Instr[15:0] Instruction[5:0] In[15:11] Instr[25:0] PC[31..28] Jump address [31..0]

5 Multi-cycle CPU Control: Overview General approach: Finite State Machine (FSM) –Need details in each branch of control… Precise outputs for each state (Mealy depends on inputs, Moore does not) Precise “next state” for each state (can depend on inputs) Control Signal Outputs Control Signal Outputs

6 How to Implement FSM ? Manually with logic gates + FFs –Bubble diagram, next-state table, state assignment –Karnaugh map for each state bit, each output bit (painful!) High-level language description (eg, Verilog, VHDL) –Describe FSM bubble diagram (next-states, output values) –Automatically synthesized into gates + FFs Microcode (µ-code) description –Sequence through many µ-ops for each CPU instruction One µ-op (µ-instruction) sends correct control signal for 1 cycle µ-op similar to one bubble in FSM –Acts like a mini-CPU within a CPU µPC: microcode program counter Microcode storage memory contains µ-ops –Can look similar to RTL or some new “assembly language”

7 FSM Specification: Bubble Diagram Can build this by examining RTL It is possible to automatically convert RTL into this form !

8 FSM: Gates + FFs Implementation FSM High-level Organization

9 FSM: Microcode Implementation Adder 1 Datapath control outputs Sequencing control Inputs from instruction register opcode field Microcode Storage (memory) Inputs Outputs Microprogram Counter Address Select Logic

10 Multi-cycle CPU: Datapath + Control FSM Instr. [31:26] Instr[31:26] Instr[25:21] Instr[20:16] Instr[15:0] Instruction[5:0] In[15:11] Instr[25:0] PC[31..28] Jump address [31..0] FSM Control Outputs Conditional Branch

11 Control FSM: Overview General approach: Finite State Machine (FSM) Need details in each branch of control…

12 Detailed FSM

13 Detailed FSM Instruction Fetch Memory Reference Branch Jump R-Type

14 Detailed FSM: Instruction Fetch Figure 5.32

15 Detailed FSM: Memory Reference Figure 5.33 LW SW

16 Detailed FSM: R-Type Instruction Figure 5.34

17 Detailed FSM: Branch Instruction Figure 5.35

18 Detailed FSM: Jump Instruction Figure 5.36

High-level Performance Comparison Single-cycle CPU vs Multi-cycle CPU

20 Simple Comparison Single-cycle CPU 1 clock cycle 5 clock cycles Multi-cycle CPU 4 clock cycles Multi-cycle CPU 3 clock cycles Multi-cycle CPU SW, R-type BEQ, J LW All

21 What’s really happening? Single-cycle CPU Multi-cycle CPU ( Load Word Instruction ) FetchDecodeMemoryWrite Calc Addr Ideally:

22 In practise, steps differ in speeds… Single-cycle CPU Multi-cycle CPU FetchDecodeMemory Calc Addr FetchDecodeMemory Calc Addr Write Violation! Wasted time! Load Word Instruction

23 Single-cycle vs Multi-cycle Single-cycle CPU LW instruction faster for single-cycle FetchDecodeMemory Calc Addr FetchDecodeMemory Calc Addr Write Violation fixed! Multi-cycle CPU Now wasted time is larger!

24 Single-cycle vs Multi-cycle Single-cycle CPU SW instruction ~ same speed FetchDecodeMemory Calc Addr FetchDecodeMemory Calc Addr Multi-cycle CPU Wasted time! Speed diff

25 Single-cycle vs Multi-cycle Single-cycle CPU BEQ, J instruction faster for multi-cycle FetchDecode Calc Addr FetchDecode Calc Addr Wasted time! Speed diff Multi-cycle CPU

26 Performance Summary Which CPU implementation is faster? –LW  single-cycle is faster –SW,R-type  about the same –BEQ,J  multi-cycle is faster Real programs use a mix of these instructions Overall performance depends instruction frequency !

27 Implementation Summary Single-cycle CPU –1 instruction per cycle (eg, 1MHz  1 MIPS) –No “wasted time” on most complex instruction –Large wasted time on simpler instructions –Simple controller (just a lookup table or memory) –Simple instructions Multi-cycle CPU –<< 1 instruction per cycle (eg, 1MHz  0.2 MIPS) –Small time wasted on most complex instruction Hence, this instruction always slower than single-cycle CPU –Small time wasted on simple instructions Eliminates “large wasted time” by using fewer clock cycles –Complex controller (FSM) –Potential to create complex instructions