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Cosc 2150

Cosc 2150. Chapter 9 a Instruction Level Parallelism and Superscalar Processors. Introduction. Before we can look at different methods that are used to increase the speed of processors We need to take a closer look at the fetch/execute cycle. Micro-Operations. A computer executes a program

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Cosc 2150

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  1. Cosc 2150 Chapter 9 a Instruction Level Parallelism and Superscalar Processors

  2. Introduction • Before we can look at different methods that are used to increase the speed of processors • We need to take a closer look at the fetch/execute cycle

  3. Micro-Operations • A computer executes a program • Fetch/Execute cycle • Each cycle has a number of steps • see pipelining • Called micro-operations • Each step does very little • Atomic operation of CPU

  4. Constituent Elements of Program Execution

  5. Fetch - 4 Registers • Memory Address Register (MAR) • Connected to address bus • Specifies address for read or write op • Memory Buffer Register (MBR) • Connected to data bus • Holds data to write or last data read • Program Counter (PC) • Holds address of next instruction to be fetched • Instruction Register (IR) • Holds last instruction fetched

  6. Fetch Sequence • Address of next instruction is in PC • Address (MAR) is placed on address bus • Control unit issues READ command • Result (data from memory) appears on data bus • Data from data bus copied into MBR • PC incremented by 1 (in parallel with data fetch from memory) • Data (instruction) moved from MBR to IR • MBR is now free for further data fetches

  7. Fetch Sequence (symbolic) (tx = time unit/clock cycle) • t1: MAR <- (PC) • t2: MBR <- (memory) • PC <- (PC) +1 • t3: IR <- (MBR) or • t1: MAR <- (PC) • t2: MBR <- (memory) • t3: PC <- (PC) +1 • IR <- (MBR)

  8. Rules for Clock Cycle Grouping • Proper sequence must be followed • MAR <- (PC) must precede MBR <- (memory) • Conflicts must be avoided • Must not read & write same register at same time • MBR <- (memory) & IR <- (MBR) must not be in same cycle • Also: PC <- (PC) +1 involves addition • Use ALU • May need additional micro-operations

  9. Indirect Cycle • MAR <- (IRaddress) address field of IR • MBR <- (memory) • IRaddress <- (MBRaddress) • MBR contains an address • IR is now in same state as if direct addressing had been used

  10. Interrupt Cycle • t1: MBR <-(PC) • t2: MAR <- save-address • PC <- routine-address • t3: memory <- (MBR) • This is a minimum • May be additional micro-ops to get addresses • N.B. saving context is done by interrupt handler routine, not micro-ops

  11. Execute Cycle • Different for each instruction • In general, complete the task of the instruction • Example: • ADD R1,X - add the contents of location X to Register 1 , result in R1 • t1: MAR <- (IRaddress) • t2: MBR <- (memory) • t3: R1 <- R1 + (MBR)

  12. Execute Cycle (BSA) • BSA X - Branch and save address • Address of instruction following BSA is saved in X • Execution continues from X+1 • t1: MAR <- (IRaddress) • MBR <- (PC) • t2: PC <- (IRaddress) • memory <- (MBR) • t3: PC <- (PC) + 1

  13. Instruction Cycle • Each phase decomposed into sequence of elementary micro-operations • E.g. fetch, indirect, and interrupt cycles • Execute cycle • One sequence of micro-operations for each opcode • Need to tie sequences together • Assume new 2-bit register • Instruction cycle code (ICC) designates which part of cycle processor is in • 00: Fetch • 01: Indirect • 10: Execute • 11: Interrupt

  14. What is Superscalar? • Common instructions (arithmetic, load/store, conditional branch) can be initiated and executed independently • Equally applicable to RISC & CISC • In practice usually RISC

  15. General Superscalar Organization

  16. Superpipelined • Many pipeline stages need less than half a clock cycle • Double internal clock speed gets two tasks per external clock cycle • Superscalar allows parallel fetch and execute

  17. Superscalar vSuperpipeline

  18. Limitations • Instruction level parallelism • Compiler based optimisation • Hardware techniques • Limited by • True data dependency • Procedural dependency • Resource conflicts • Output dependency • Antidependency

  19. True Data Dependency • ADD r1, r2 (r1 := r1+r2;) • MOVE r3,r1 (r3 := r1;) • Can fetch and decode second instruction in parallel with first • Can NOT execute second instruction until first is finished

  20. Procedural Dependency • Can not execute instructions after a branch in parallel with instructions before a branch • Also, if instruction length is not fixed, instructions have to be decoded to find out how many fetches are needed • This prevents simultaneous fetches

  21. Resource Conflict • Two or more instructions requiring access to the same resource at the same time • e.g. two arithmetic instructions • Can duplicate resources • e.g. have two arithmetic units

  22. Effect of Dependencies

  23. Design Issues • Instruction level parallelism • Instructions in a sequence are independent • Execution can be overlapped • Governed by data and procedural dependency • Machine Parallelism • Ability to take advantage of instruction level parallelism • Governed by number of parallel pipelines

  24. Instruction Issue Policy • Order in which instructions are fetched • Order in which instructions are executed • Order in which instructions change registers and memory

  25. In-Order Issue In-Order Completion • Issue instructions in the order they occur • Not very efficient • May fetch >1 instruction • Instructions must stall if necessary

  26. In-Order Issue In-Order Completion (Diagram)

  27. In-Order Issue Out-of-Order Completion • Output dependency • R3:= R3 + R5; (I1) • R4:= R3 + 1; (I2) • R3:= R5 + 1; (I3) • I2 depends on result of I1 - data dependency • If I3 completes before I1, the result from I1 will be wrong - output (read-write) dependency

  28. In-Order Issue Out-of-Order Completion (Diagram)

  29. Out-of-Order IssueOut-of-Order Completion • Decouple decode pipeline from execution pipeline • Can continue to fetch and decode until this pipeline is full • When a functional unit becomes available an instruction can be executed • Since instructions have been decoded, processor can look ahead

  30. Out-of-Order Issue Out-of-Order Completion (Diagram)

  31. Antidependency • Write-write dependency • R3:=R3 + R5; (I1) • R4:=R3 + 1; (I2) • R3:=R5 + 1; (I3) • R7:=R3 + R4; (I4) • I3 can not complete before I2 starts as I2 needs a value in R3 and I3 changes R3

  32. Register Renaming • Output and antidependencies occur because register contents may not reflect the correct ordering from the program • May result in a pipeline stall • Registers allocated dynamically • i.e. registers are not specifically named

  33. Register Renaming example • R3b:=R3a + R5a (I1) • R4b:=R3b + 1 (I2) • R3c:=R5a + 1 (I3) • R7b:=R3c + R4b (I4) • Without subscript refers to logical register in instruction • With a subscript then hardware register allocated • Note R3a R3b R3c

  34. Speedups of Machine Organizations Without Procedural Dependencies

  35. Machine Parallelism • Duplication of Resources • Out of order issue • Renaming • Not worth duplication functions without register renaming • Need instruction window large enough (more than 8)

  36. Superscalar Implementation • Simultaneously fetch multiple instructions • Logic to determine true dependencies involving register values • Mechanisms to communicate these values • Mechanisms to initiate multiple instructions in parallel • Resources for parallel execution of multiple instructions • Mechanisms for committing process state in correct order

  37. Q A &

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