CS252/Patterson Lec 1.1 1/17/01 Pipelining: Its Natural! Laundry Example Ann, Brian, Cathy, Dave each have one load of clothes to wash, dry, and fold Washer takes 30 minutes Dryer takes 40 minutes “Folder” takes 20 minutes ABCD

CS252/Patterson Lec 1.2 1/17/01 Sequential Laundry Sequential laundry takes 6 hours for 4 loads If they learned pipelining, how long would laundry take? ABCD PM Midnight TaskOrderTaskOrder Time

CS252/Patterson Lec 1.3 1/17/01 Pipelined Laundry Start work ASAP Pipelined laundry takes 3.5 hours for 4 loads ABCD 6 PM Midnight TaskOrderTaskOrder Time

CS252/Patterson Lec 1.4 1/17/01 Pipelining Lessons Pipelining doesn’t help latency of single task, it helps throughput of entire workload Pipeline rate limited by slowest pipeline stage Multiple tasks operating simultaneously Potential speedup = Number pipe stages Unbalanced lengths of pipe stages reduces speedup Time to “fill” pipeline and time to “drain” it reduces speedup ABCD 6 PM 789 TaskOrderTaskOrder Time

CS252/Patterson Lec 1.5 1/17/01 Computer Pipelines Execute billions of instructions, so throughput is what matters What is desirable in instruction sets for pipelining? –Variable length instructions vs. all instructions same length? –Memory operands part of any operation vs. memory operands only in loads or stores? –Register operand many places in instruction format vs. registers located in same place?

CS252/Patterson Lec 1.6 1/17/01 A "Typical" RISC 32-bit fixed format instruction (3 formats) Memory access only via load/store instrutions bit GPR (R0 contains zero, DP take pair) 3-address, reg-reg arithmetic instruction; registers in same place Single address mode for load/store: base + displacement –no indirection Simple branch conditions Delayed branch see: SPARC, MIPS, HP PA-Risc, DEC Alpha, IBM PowerPC, CDC 6600, CDC 7600, Cray-1, Cray-2, Cray-3

CS252/Patterson Lec 1.7 1/17/01 Example: MIPS (Note register location) Op Rs1Rd immediate Op Op Rs1Rs2 target RdOpx Register-Register Register-Immediate Op Rs1Rs2/Opx immediate Branch Jump / Call

CS252/Patterson Lec 1.8 1/17/01 5 Steps of MIPS Datapath Figure 3.1, Page 130, CA:AQA 2e Memory Access Write Back Instruction Fetch Instr. Decode Reg. Fetch Execute Addr. Calc LMDLMD ALU MUX Memory Reg File MUX Data Memory MUX Sign Extend 4 Adder Zero? Next SEQ PC Address Next PC WB Data Inst RD RS1 RS2 Imm

CS252/Patterson Lec 1.9 1/17/01 5 Steps of MIPS Datapath Figure 3.4, Page 134, CA:AQA 2e Memory Access Write Back Instruction Fetch Instr. Decode Reg. Fetch Execute Addr. Calc ALU Memory Reg File MUX Data Memory MUX Sign Extend Zero? IF/ID ID/EX MEM/WB EX/MEM 4 Adder Next SEQ PC RD WB Data Data stationary control – local decode for each instruction phase / pipeline stage Next PC Address RS1 RS2 Imm MUX

CS252/Patterson Lec /17/01 Visualizing Pipelining Figure 3.3, Page 133, CA:AQA 2e I n s t r. O r d e r Time (clock cycles) Reg ALU DMemIfetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg Cycle 1Cycle 2Cycle 3Cycle 4Cycle 6Cycle 7Cycle 5

CS252/Patterson Lec /17/01 Its Not That Easy for Computers Limits to pipelining: Hazards prevent next instruction from executing during its designated clock cycle –Structural hazards: HW cannot support this combination of instructions (single person to fold and put clothes away) –Data hazards: Instruction depends on result of prior instruction still in the pipeline (missing sock) –Control hazards: Caused by delay between the fetching of instructions and decisions about changes in control flow (branches and jumps).

CS252/Patterson Lec /17/01 One Memory Port/Structural Hazards Figure 3.6, Page 142, CA:AQA 2e I n s t r. O r d e r Time (clock cycles) Load Instr 1 Instr 2 Instr 3 Instr 4 Reg ALU DMem Ifetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg ALU DMem Ifetch Reg Cycle 1Cycle 2Cycle 3Cycle 4Cycle 6Cycle 7Cycle 5 Reg ALU DMemIfetch Reg

CS252/Patterson Lec /17/01 One Memory Port/Structural Hazards Figure 3.7, Page 143, CA:AQA 2e I n s t r. O r d e r Time (clock cycles) Load Instr 1 Instr 2 Stall Instr 3 Reg ALU DMem Ifetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg Cycle 1Cycle 2Cycle 3Cycle 4Cycle 6Cycle 7Cycle 5 Reg ALU DMemIfetch Reg Bubble

CS252/Patterson Lec /17/01 I n s t r. O r d e r add r1,r2,r3 sub r4,r1,r3 and r6,r1,r7 or r8,r1,r9 xor r10,r1,r11 Reg ALU DMemIfetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg ALU DMemIfetch Reg Data Hazard on R1 Figure 3.9, page 147, CA:AQA 2e Time (clock cycles) IFID/RF EX MEM WB

CS252/Patterson Lec /17/01 Read After Write (RAW) Instr J tries to read operand before Instr I writes it Caused by a “Dependence” (in compiler nomenclature). This hazard results from an actual need for communication. Three Generic Data Hazards I: add r1,r2,r3 J: sub r4,r1,r3

CS252/Patterson Lec /17/01 Write After Read (WAR) Instr J writes operand before Instr I reads it Called an “anti-dependence” by compiler writers. This results from reuse of the name “r1”. Can’t happen in MIPS 5 stage pipeline because: – All instructions take 5 stages, and – Reads are always in stage 2, and – Writes are always in stage 5 I: sub r4,r1,r3 J: add r1,r2,r3 K: mul r6,r1,r7 Three Generic Data Hazards

CS252/Patterson Lec /17/01 Three Generic Data Hazards Write After Write (WAW) Instr J writes operand before Instr I writes it. Called an “output dependence” by compiler writers This also results from the reuse of name “r1”. Can’t happen in MIPS 5 stage pipeline because: – All instructions take 5 stages, and – Writes are always in stage 5 Will see WAR and WAW in later more complicated pipes I: sub r1,r4,r3 J: add r1,r2,r3 K: mul r6,r1,r7