Chapter One Introduction to Pipelined Processors

1. Chapter One Introduction to Pipelined Processors

2. Principle of Designing Pipeline Processors (Design Problems of Pipeline Processors)

3. Internal Data Forwarding and Register Tagging

4. Internal Forwarding and Register Tagging • Internal Forwarding: It is replacing unnecessary memory accesses by register-to-register transfers. • Register Tagging: It is the use of tagged registers for exploiting concurrent activities among multiple ALUs.

5. Internal Forwarding • Memory access is slower than register-to-register operations. • Performance can be enhanced by eliminating unnecessary memory accesses

6. Internal Forwarding • This concept can be explored in 3 directions: • Store – Load Forwarding • Load – Load Forwarding • Store – Store Forwarding

7. Store – Load Forwarding

8. Load – Load Forwarding

9. Store – Store Forwarding

10. Example EXAMPLE

11. Example EXAMPLE

12. Register Tagging

13. Example : IBM Model 91 : Floating Point Execution Unit

14. Example : IBM Model 91-FPU • The floating point execution unit consists of : • Data registers • Transfer paths • Floating Point Adder Unit • Multiply-Divide Unit • Reservation stations • Common Data Bus

16. Example : IBM Model 91-FPU • There are 3 reservation stations for adder named A1, A2 and A3 and 2 for multipliers named M1 and M2. • Each station has the source & sink registers and their tag & control fields • The stations hold operands for next execution.

18. Example : IBM Model 91-FPU • 3 store data buffers(SDBs) and 4 floating point registers (FLRs) are tagged • Busy bits in FLR indicates the dependence of instructions in subsequent execution • Common Data Bus(CDB) is to transfer operands

19. Example : IBM Model 91-FPU • There are 11 units to supply information to CDB: 6 FLBs, 3 adders & 2 multiply/divide unit • Tags for these stations are :

20. Example : IBM Model 91-FPU • Internal forwarding can be achieved with tagging scheme on CDB. • Example: • Let F refers to FLR and FLBi stands for ith FLB and their contents be (F) and (FLBi) • Consider instruction sequence ADD F,FLB1 F  (F) + (FLB1) MPY F,FLB2 F  (F) x (FLB2)

21. Example : IBM Model 91-FPU • During addition : • Busy bit of F is set to 1 • Contents of F and FLB1 is sent to adder A1 • Tag of F is set to 1010 (tag of adder) F

22. 6 Storage Bus Instruction Unit 5 4 Control 3 2 1 Decoder Adder Multiplier (Common Data Bus) Floating Point Buffers (FLB)

23. Example : IBM Model 91-FPU • Meantime, the decode of MPY reveals F is busy, then • F should set tag of M1 as 1010 (Tag of adder) • F should change its tag to 1000 (Tag of Multiplier) • Send content of FLB2 to M1 F

24. 6 Storage Bus Instruction Unit 5 4 Control 3 2 1 Decoder Adder Multiplier (Common Data Bus) Before addition Floating Point Buffers (FLB)

25. 6 Storage Bus Instruction Unit 5 4 Control 3 2 1 Decoder Adder Multiplier (Common Data Bus) After addition Floating Point Buffers (FLB)

26. Example : IBM Model 91-FPU • When addition is done, CDB finds that the result should be sent to M1 • Multiplication is done when both operands are available

27. Hazard Detection and Resolution

28. Hazard Detection and Resolution • Hazards are caused by resource usage conflicts among various instructions • They are triggered by inter-instruction dependencies Terminologies: • Resource Objects: set of working registers, memory locations and special flags

29. Hazard Detection and Resolution • Data Objects: Content of resource objects • Each Instruction can be considered as a mapping from a set of data objects to a set of data objects. • Domain D(I) : set of resource of objects whose data objects may affect the execution of instruction I.(e.g.Source Registers)

30. Hazard Detection and Resolution • Range R(I): set of resource objects whose data objects may be modified by the execution of instruction I .(e.g. Destination Register) • Instruction reads from its domain and writes in its range

31. Hazard Detection and Resolution • Consider execution of instructions I and J, and J appears immediately after I. • There are 3 types of data dependent hazards: • RAW (Read After Write) • WAW(Write After Write) • WAR (Write After Read)

32. RAW (Read After Write) • The necessary condition for this hazard is

33. RAW (Read After Write) • Example: I1 : LOAD r1,a I2 : ADD r2,r1 • I2 cannot be correctly executed until r1 is loaded • Thus I2 is RAW dependent on I1

34. WAW(Write After Write) • The necessary condition is

35. WAW(Write After Write) • Example I1 : MUL r1, r2 I2 : ADD r1,r4 • Here I1 and I2 writes to same destination and hence they are said to be WAW dependent.

36. WAR(Write After Read) • The necessary condition is

37. WAR(Write After Read) • Example: • I1 : MUL r1,r2 • I2 : ADD r2,r3 • Here I2 has r2 as destination while I1 uses it as source and hence they are WAR dependent

38. Hazard Detection and Resolution • Hazards can be detected in fetch stage by comparing domain and range. • Once detected, there are two methods: • Generate a warning signal to prevent hazard • Allow incoming instruction through pipe and distribute detection to all pipeline stages.