Processor Structure and Function Chapter 12

1. Processor Structure and FunctionChapter 12 Computer Organization and Architecture Designing For Performance William Stallings Seventh Edition

2. Team #5 • - 12.1 Processor Organization • - 12.2 Register Organization • - 12.3 Instruction Cycle • - 12.4 Instruction Pipelining/ Dealing with Branches

3. 12.1 Processor Organization • Fetch Instruction • Interpret instruction • Fetch Data • Process Data • Write Data

4. Remember…. • The processor needs to store some data temporally. • It must remember the location of the last instruction so that it can know where to get the next instruction. • It needs to store instructions and data temporally while an instruction is being executed.

5. CPU with System Bus

6. CPU Internal Structure

7. Register Organization User-visible registers Control and status registers

8. User Visible Registers • General Purpose • Data • Address • Condition Codes

9. General Purpose Registers • Data • Accumulator • Addressing • Segment pointers • Index registers • Stack Pointer

10. Control & Status Registers • Program Counter • Instruction Decoding Register • Memory Address Register • Memory Buffer Register

11. Program Status Word • Sign • Zero • Carry • Equal • Overflow • Interrupt enable/disable • Supervisor

12. Example Register Org.

13. Instruction Cycle • It is the time in which a single instruction is fetched from memory, decoded, and executed • An Instruction Cycle requires the following subcycle:

14. Instruction Cycle • Fetch Read next instruction from memory into the processor • Indirect Cycle (Decode Cycle) May require memory access to fetch operands, therefore more memory accesses. • Interrupt Save current instruction and service the interrupt • Execute Interpret the opcode and perform the indicated operation

15. Fetch Fetch Fetch Fetch Interrupt Interrupt Indirect Indirect Execute Execute Execute Execute Instruction Cycle

17. PC PC MAR MAR Memory Memory Memory Control Unit Control Unit IR MBR MBR Data Flow (Fetch Diagram)

18. MAR MAR Memory Memory Memory Control Unit MBR MBR Data Flow (Indirect Diagram)

19. Data Flow (Execute) • May take many forms • Depends on instruction being executed • May include • Memory read/write • Input/Output • Register transfers • ALU operations

20. PC PC PC MAR MAR Memory Memory Memory Control Unit Control Unit Control Unit Control Unit MBR MBR Data Flow (Interrupt Diagram)

21. 12.4 Instruction Pipelining • Instruction processing is subdivided: - Fetch/ Execute instruction • Pipeline has two independent stages: 1st Stage – Fetch an instruction and buffers it. 2nd Stage – Temporarily free until first stage passes it the buffered instruction. While the second stage is executing the instruction, the first stage fetches and buffers the next instruction. • Instruction prefetch or fetch overlap. - Purpose?  To speed up instruction execution.

22. Two-Stage Instruction Pipeline

23. Instruction Processing • Fetch instruction (FI) • Decode instruction (DI) • Calculate operands (CO) • Fetch operands (FO) • Execute instruction (EI) • Write operand (WO) • Successive instructions in a program sequence will overlap in execution.

24. Timing Diagram for Instruction Pipeline Operation

25. Six-Stage CPU Instruction Pipeline The logic needed for pipelining to account for branches, interrupts, and arising problems.

26. Alternative Pipeline Depiction

27. RISC Pipeline Instruction fetch Instruction decode and register fetch Execute Memory Access Register write back

28. Branches • Branch- group of instructions • Branch Instructions – (Jump Instruction) One of it’s operands is the address of the next instruction to be executed.

29. Branches • Two Types of Branch Instructions • Unconditional – Branch always happens • Conditional – Branch only happens if certain condition is met. • The PC is updated to the address specified in the operand of the conditional branch instruction. • A conditional branch instruction is similar to an if statement.

30. Conditional Branch Instructions • Condition Codes • BRP X • Branch to location X if result is positive • BRZ X • Branch to location X if result is zero • BRE R1,R2,X • Branch to location X if contents of R1 = R2

31. Conditional Branch Instructions

32. Dealing with Branches • A major problem in designing an instruction pipeline is assuring a steady flow of instructions to the initial stages of the pipeline. • Since conditional branches alter the steady flow of instructions, we must come up with ways to execute them efficiently.

33. Dealing with Branches • 5 Approaches to Dealing with Conditional Branches • Multiple Streams • Delayed Branch • Prefetch Branch target • Loop Buffer • Branch Prediction

34. Dealing with Branches • Multiple Streams (IBM 370/168 and IBM 3033) • Pipeline fetches both instructions. • Leads to contention delays, and branches can lead to too many streams. • Delayed Branch • Branch Instruction occurs later than desired. • Prefetch Branch Target (360/91 IBM) • The target of the branch is prefetched, along with the instruction following the branch, so if the branch is taken this will speed up performance.

35. Dealing with Branches • Loop buffer ( Motorola 68010) • Memory containing the n most recently fetched instructions. • Useful with if-then and if-then-else statements, as well as loops • Branch Prediction • Different techniques are used to predict whether the branch will be taken or not • If the prediction is correct this will speed up performance

36. Dealing with Branches

37. Intel Pentium Branch The prediction of whether a jump will occur or not, is based on the branch’s previous behavior. There are four possible states that depict a branch’s disposition to jump: Stage 0: Very unlikely a jump will occurStage 1: Unlikely a jump will occurStage 2: Likely a jump will occurStage 3: Very likely a jump will occur

38. Intel Pentium Branch It is actually believed that Pentium’s original algorithm for branch prediction was incorrect. (Left)

39. Research • http://www.it.jcu.edu.au/Subjects/cp1300/resources/lectnotes/system/fde.html • http://dr-pisit.com/csc331/Lec10-CPU&Pipeline.pdf • http://en.wikipedia.org/wiki/Instruction_pipelining • http://www.itreviews.co.uk/hardware/h738.htm

40. Review Questions 1. What are the major components of a processor? 2. What is the function of the ALU? 3. What is the function of the control unit? 4. What are the two roles that registers in the processor perform? 5. What are bits set by the processor hardware as a result of operations? 6. What is an instruction cycle? 7. What are the four subcycle of an instruction cycle?

41. 8. Is the fetch or execute cycle the same for all CPU? 9. What is the sequence of an interrupt cycle? 10. What is the main purpose for instruction pipelining? 11. How can you make the pipelining more efficient? 12. What is a condition code? 13. What is another name for a branch instruction?

42. Answers • Arithmetic and Logic Unit (ALU) and the Control Unit (CU). • The ALU does the actual computation or processing of data. • The control unit controls the movement of data and instructions into and out of the processor and controls the operations of the ALU. • User-visible registers, and control and status registers. • Condition codes. • It is the time in which a single instruction is fetched from memory, decoded, and executed. • Fetch, Indirect (if any) , execute, and interrupt (if any).

43. No, it depends on the CPU’s design. • PC  MBR Address of Stack MAR MAR  Memory PC  Memory Control Unit request memory write via Control Bus PC is loaded with address of Interrupt handler • To speed up the instruction execution rate. • To gain further speedup, the pipeline must have more stages for decomposition. • A statement that if true will allow the branch to be executed. • A jump instruction.