ee524/cpts561 advanced computer architecture dynamic scheduling a scheme to overcome data hazards

EE524/CptS561 Advanced Computer Architecture

Dynamic Scheduling

A scheme to overcome data hazards


Advantages of Dynamic Scheduling• Dynamic scheduling - hardware rearranges the

instruction execution to reduce stalls while maintaining data flow and exception behavior

• It handles cases when dependences unknown at compile time

– it allows the processor to tolerate unpredictable delays such as cache misses, by executing other code while waiting for the miss to resolve

• It allows code that compiled for one pipeline to run efficiently on a different pipeline

• It simplifies the compiler • Hardware speculation, a technique with significant

performance advantages, builds on dynamic scheduling


HW Schemes: Instruction Parallelism• Key idea: Allow instructions behind stall to proceed

DIVD F0,F2,F4ADDD F10,F0,F8SUBD F12,F8,F14

• Enables out-of-order execution and allows out-of-order completion (e.g., SUBD)

– In a dynamically scheduled pipeline, all instructions still pass through issue stage in order (in-order issue)

• Will distinguish when an instruction begins execution and when it completes execution; between 2 times, the instruction is in execution

• Note: Dynamic execution creates WAR and WAW hazards and makes exceptions harder


Dynamic Scheduling Step 1

• Simple pipeline had 1 stage to check both structural and data hazards: Instruction Decode (ID), also called Instruction Issue

• Split the ID pipe stage of simple 5-stage pipeline into 2 stages:

• Issue—Decode instructions, check for structural hazards

• Read operands—Wait until no data hazards, then read operands


Tomasulo Algorithm• Control & buffers distributed with Function Units (FU)

– FU buffers called “reservation stations”; have pending operands

• Registers in instructions replaced by values or pointers to reservation stations(RS); called register renaming; – Avoids: WAR

WAW hazards– More reservation stations than registers, so can do optimizations

compilers cannot

• Results to FU from RS, not through registers, over Common Data Bus that broadcasts results to all FUs

• Load and Stores treated as FUs with RSs as well.• Integer instructions can go past branches, allowing

FP ops beyond basic block in FP queue.

RX inst. i

RX inst. j


6

5

4

3

2

1 3

2

1

3

2

1

2

1

FP addersFP Multipliers

Common data bus (CDB)

Reservation Stations

From memory

Load

buffers

FP

Operation

queue

From instruction unit

FP

Registers

To memory

Store

buffers

Operation bus

Operand buses

Tomasulo scheme


Reservation Station Components

Op—Operation to perform in the unit (e.g., + or –)Vj, Vk—Value of Source operands

– Store buffers has V field, result to be stored

Qj, Qk—Reservation stations producing source registers (value to be written)– Note: No ready flags as in Scoreboard; Qj,Qk=0 => ready– Store buffers only have Qi for RS producing result

Busy—Indicates reservation station or FU is busyRegister result status—Indicates which functional unit will write each register, if one exists. Blank when no pending instructions that will write that register.


Three Stages of Tomasulo Algorithm1. Issue—get instruction from FP Op Queue

If reservation station free (no structural hazard), control issues instr & sends operands (renames registers).

2.Execution—operate on operands (EX) When both operands ready then execute;

if not ready, watch Common Data Bus for result

3.Write result—finish execution (WB) Write on Common Data Bus to all awaiting units;

mark reservation station available

• Normal data bus: data + destination (“go to” bus)• Common data bus: data + source (“come from” bus)

– 64 bits of data + 4 bits of Functional Unit source address– Write if matches expected Functional Unit (produces result)– Does the broadcast


Tomasulo Example Cycle 0

Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 Load1 NoLD F2 45+ R3 Load2 NoMULTDF0 F2 F4 Load3 NoSUBD F8 F6 F2DIVD F10 F0 F6ADDDF6 F8 F2Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 No0 Add2 No0 Add3 No0 Mult1 No0 Mult2 No

Register result status

Clock F0 F2 F4 F6 F8 F10 F12 ... F300 FU


6

5

4

3

2

1 3

2

1

3

2

1

2

1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue

From instruction unit FP Registers

To memory

Store buffers

Operation bus

Operand busesLD F6, 34(R2)

Cycle: 0


6

5

4

3

2

1 34+R2 3

2

1

3

2

1

2

1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

LD F2, 45(R3)

Cycle: 1

LD F6, 34(R2)

F6 : load1


6

5

4

3

2 45+R3

1 34+R2 3

2

1

3

2

1

2

1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

MULTD F0,F2,F4

Cycle: 2

LD F2, 45(R3)

LD F6, 34(R2)

F6 : load1

F2 : load2


6

5

4

3

2 45+R3

1 Mem[34+R2] 3

2

1

3

2

1

2

M load2 “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

SUB F8,F6,F2

Cycle: 3

MULTD F0,F2,F4

LD F2, 45(R3)

LD F6, 34(R2)

F6 : load1

F2 : load2

F0 : mult1


FP Registers

F2 : load2

F6 : load1F6 Mem[34+R2]

F0 : mult1

L1: Mem[34+R2]

6

5

4

3

2 Mem[45+R3]

1 3

2

1

3

2

1 S load1 load2

2

M load2 “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue

From instruction unit

To memory

Store buffers

Operation bus

Operand buses

DIVD F10,F0,F6

L1: Mem[34+R2]

Mem[34+R2]

Cycle: 4

SUB F8,F6,F2

MULTD F0,F2,F4

LD F2, 45(R3)

LD F6, 34(R2)

F8: add1

L1: Mem[34+R2]


6

5

4

3

2

1 3

2

1

3

2

1 S Mem[R2] load2

D Mult1 2

M load2 “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

ADD F6,F8,F2

L2: Mem[45+R3]

Mem[45+R3]

Mem[45+R3]

Mem[45+R3]

Cycle: 5

DIVD F10,F0,F6

SUB F8,F6,F2

MULTD F0,F2,F4

LD F2, 45(R3)

F2 : load2F2 Mem[45+R3]


F8: add1

F0 : mult1

F10: mult2

L2: Mem[45+R3]L2: Mem[45+R3]


6

5

4

3

2

1 3

2

1

3

2 A add1 M[R3]

1 S Mem[R2] M[R3]

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 6

ADD F6,F8,F2

DIVD F10,F0,F6

SUB F8,F6,F2

MULTD F0,F2,F4

F6: add2

F0 : mult1

F10: mult2

F8: add1


6

5

4

3

2

1 3

2

1

3

2 A add1 M[R3]

1 S Mem[R2] M[R3]

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 7

ADD F6,F8,F2

DIVD F10,F0,F6

SUB F8,F6,F2

MULTD F0,F2,F4

F6: add2

F0 : mult1

F10: mult2

F8: add1


6

5

4

3

2

1 3

2

1

3

2 A add1 M[R3]

1 S Mem[R2] M[R3]

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 8

ADD F6,F8,F2

DIVD F10,F0,F6

SUB F8,F6,F2

MULTD F0,F2,F4

Add1: M()-M()

F6: add2

F0 : mult1

F10: mult2

F8: add1

M()-M()

F8 M()-M()


6

5

4

3

2

1 3

2

1

3

2 A M()-M() M[R3]

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 9

ADD F6,F8,F2

DIVD F10,F0,F6

MULTD F0,F2,F4

F6: add2

F0 : mult1

F10: mult2


6

5

4

3

2

1 3

2

1

3

2 A M()-M() M[R3]

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 10

ADD F6,F8,F2

DIVD F10,F0,F6

MULTD F0,F2,F4

F6: add2

F0 : mult1

F10: mult2


6

5

4

3

2

1 3

2

1

3

2 A M()-M() M[R3]

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 11

ADD F6,F8,F2

DIVD F10,F0,F6

MULTD F0,F2,F4

F6: add2

F0 : mult1

F10: mult2

Add2: (M()-M())+M()

F6 (M()-m())+M()


6

5

4

3

2

1 3

2

1

3

2

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 12

DIVD F10,F0,F6

MULTD F0,F2,F4

F0 : mult1

F10: mult2


6

5

4

3

2

1 3

2

1

3

2

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 13

DIVD F10,F0,F6

MULTD F0,F2,F4

F0 : mult1

F10: mult2


6

5

4

3

2

1 3

2

1

3

2

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 14

DIVD F10,F0,F6

MULTD F0,F2,F4

F0 : mult1

F10: mult2


6

5

4

3

2

1 3

2

1

3

2

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 15

DIVD F10,F0,F6

MULTD F0,F2,F4

F0 : mult1

F10: mult2


6

5

4

3

2

1 3

2

1

3

2

1

D Mult1 M[R3] 2

M M[R3] “F4” 1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 16

DIVD F10,F0,F6

MULTD F0,F2,F4

F0 : mult1

F10: mult2

Mult1: M()*F4

F0 M()*F4

M()*F4


6

5

4

3

2

1 3

2

1

3

2

1

D M()*F4 M[R3] 2

1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 17

DIVD F10,F0,F6

F10: mult2


6

5

4

3

2

1 3

2

1

3

2

1

D M()*F4 M[R3] 2

1



Reservation

Stations

From memory

Lo

ad b

uff

ers

FP

op

erat

ion

qu

eue


To memory

Store buffers

Operation bus

Operand buses

Cycle: 57

DIVD F10,F0,F6

F10: mult2

Mult2: M()*F4 / M()

F10 M()*F4 / M()



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 Load1 No 34+R2LD F2 45+ R3 Load2 NoMULTDF0 F2 F4 Load3 NoSUBD F8 F6 F2DIVD F10 F0 F6ADDDF6 F8 F2Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 No0 Add2 No

Add3 No0 Mult1 No0 Mult2 No


Clock F0 F2 F4 F6 F8 F10 F12 ... F301 FU Load1

Yes



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 Load1 Yes 34+R2LD F2 45+ R3 2 Load2 Yes 45+R3MULTDF0 F2 F4 Load3 NoSUBD F8 F6 F2DIVD F10 F0 F6ADDDF6 F8 F2Reservation Stations S1 S2 RS for j RS for k




Clock F0 F2 F4 F6 F8 F10 F12 ... F302 FU Load2 Load1

Note: Unlike 6600, can have multiple loads outstanding



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 Load1 Yes 34+R2LD F2 45+ R3 2 Load2 Yes 45+R3MULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2DIVD F10 F0 F6ADDDF6 F8 F2Reservation Stations S1 S2 RS for j RS for k


Add3 No0 Mult1 Yes MULTD R(F4) Load20 Mult2 No


Clock F0 F2 F4 F6 F8 F10 F12 ... F303 FU Mult1 Load2 Load1

• Note: registers names are removed (“renamed”) in Reservation Stations; MULT issued vs. scoreboard

• Load1 completing; what is waiting for Load1?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 Load2 Yes 45+R3MULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4DIVD F10 F0 F6ADDDF6 F8 F2Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 Yes SUBD M(34+R2) Load20 Add2 No

Add3 No0 Mult1 Yes MULTD R(F4) Load20 Mult2 No


Clock F0 F2 F4 F6 F8 F10 F12 ... F304 FU Mult1 Load2 M(34+R2) Add1

• Load2 completing; what is waiting for it?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4DIVD F10 F0 F6 5ADDDF6 F8 F2Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk2 Add1 Yes SUBD M(34+R2) M(45+R3)0 Add2 No

Add3 No10 Mult1 Yes MULTD M(45+R3) R(F4)

0 Mult2 Yes DIVD M(34+R2) Mult1Register result status

Clock F0 F2 F4 F6 F8 F10 F12 ... F305 FU Mult1 M(45+R3) M(34+R2) Add1 Mult2



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4DIVD F10 F0 F6 5ADDDF6 F8 F2 6Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk1 Add1 Yes SUBD M(34+R2) M(45+R3)0 Add2 Yes ADDD M(45+R3) Add1

Add3 No9 Mult1 Yes MULTD M(45+R3) R(F4)0 Mult2 Yes DIVD M(34+R2) Mult1


Clock F0 F2 F4 F6 F8 F10 F12 ... F306 FU Mult1 M(45+R3) Add2 Add1 Mult2

• Issue ADDD here vs. scoreboard?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4 7DIVD F10 F0 F6 5ADDDF6 F8 F2 6Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 Yes SUBD M(34+R2) M(45+R3)0 Add2 Yes ADDD M(45+R3) Add1



Clock F0 F2 F4 F6 F8 F10 F12 ... F307 FU Mult1 M(45+R3) Add2 Add1 Mult2

• Add1 completing; what is waiting for it?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDD F6 F8 F2 6Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 No 2 Add2 Yes ADDD M()-M() M(45+R3)0 Add3 No7 Mult1 Yes MULTD M(45+R3) R(F4)0 Mult2 Yes DIVD M(34+R2) Mult1


Clock F0 F2 F4 F6 F8 F10 F12 ... F308 FU Mult1 M(45+R3) Add2 M()-M() Mult2



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDDF6 F8 F2 6Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 No1 Add2 Yes ADDD M()–M() M(45+R3)0 Add3 No6 Mult1 Yes MULTD M(45+R3) R(F4)0 Mult2 Yes DIVD M(34+R2) Mult1


Clock F0 F2 F4 F6 F8 F10 F12 ... F309 FU Mult1 M(45+R3) Add2 M()–M() Mult2


Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDDF6 F8 F2 6 10Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 No0 Add2 Yes ADDD M()–M() M(45+R3)0 Add3 No5 Mult1 Yes MULTD M(45+R3) R(F4)0 Mult2 Yes DIVD M(34+R2) Mult1


Clock F0 F2 F4 F6 F8 F10 F12 ... F3010 FU Mult1 M(45+R3) Add2 M()–M() Mult2


• Add2 completing; what is waiting for it?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDD F6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k

Time Name Busy Op Vj Vk Qj Qk0 Add1 No0 Add2 No0 Add3 No4 Mult1 Yes MULTD M(45+R3) R(F4)0 Mult2 Yes DIVD M(34+R2) Mult1


Clock F0 F2 F4 F6 F8 F10 F12 ... F3011 FU Mult1 M(45+R3) (M-M)+M() M()ĞM() Mult2

• Write result of ADDD here vs. scoreboard?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 Load3 NoSUBD F8 F6 F2 4 6 7DIVD F10 F0 F6 5ADDDF6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k



Clock F0 F2 F4 F6 F8 F10 F12 ... F3012 FU Mult1 M(45+R3) (M-M)+M() M()–M() Mult2

• Note: all quick instructions complete already







Clock F0 F2 F4 F6 F8 F10 F12 ... F3013 FU Mult1 M(45+R3) (M–M)+M() M()–M() Mult2



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 15 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDDF6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k





• Mult1 completing; what is waiting for it?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 15 16 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDDF6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k


Add3 No0 Mult1 No

40 Mult2 Yes DIVD M*F4 M(34+R2)Register result status

Clock F0 F2 F4 F6 F8 F10 F12 ... F3016 FU M*F4 M(45+R3) (M–M)+M() M()–M() Mult2

• Note: Just waiting for divide



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 15 16 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5ADDDF6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k


Add3 No0 Mult1 No1 Mult2 Yes DIVD M*F4 M(34+R2)





Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 15 16 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5 56ADDDF6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k


Add3 No0 Mult1 No0 Mult2 Yes DIVD M*F4 M(34+R2)



• Mult 2 completing; what is waiting for it?



Instruction status Execution WriteInstruction j k Issue complete Result Busy AddressLD F6 34+ R2 1 3 4 Load1 NoLD F2 45+ R3 2 4 5 Load2 NoMULTDF0 F2 F4 3 15 16 Load3 NoSUBD F8 F6 F2 4 7 8DIVD F10 F0 F6 5 56 57ADDDF6 F8 F2 6 10 11Reservation Stations S1 S2 RS for j RS for k




Clock F0 F2 F4 F6 F8 F10 F12 ... F3057 FU M*F4 M(45+R3) (M–M)+M() M()–M() M*F4/M

• Again, in-order issue, out-of-order execution, completion


Tomasulo Drawbacks

• Complexity– delays of 360/91, MIPS 10000, IBM 620?

• Many associative stores (CDB) at high speed• Performance limited by Common Data Bus

– Multiple CDBs => more FU logic for parallel assoc stores


Tomasulo Loop Example

Loop: LD F0 0 R1 MULTD F4 F0 F2 SD F4 0 R1 SUBI R1 R1 #8 BNEZ R1 Loop

• Assume Multiply takes 4 clocks• Assume first load takes 8 clocks (cache miss?),

second load takes 4 clocks (hit)• To be clear, will show clocks for SUBI, BNEZ• Reality, integer instructions ahead


Loop Example Cycle 0

Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 Load1 NoMULTDF4 F0 F2 1 Load2 NoSD F4 0 R1 1 Load3 No QiLD F0 0 R1 2 Store1 NoMULTDF4 F0 F2 2 Store2 NoSD F4 0 R1 2 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R10 Mult1 No SUBI R1 R1 #80 Mult2 No BNEZ R1 Loop


Clock R1 F0 F2 F4 F6 F8 F10 F12... F300 80 Qi



Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 Load1 Yes 80MULTDF4 F0 F2 1 Load2 NoSD F4 0 R1 1 Load3 No QiLD F0 0 R1 2 Store1 NoMULTDF4 F0 F2 2 Store2 NoSD F4 0 R1 2 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R10 Mult1 No SUBI R1 R1 #80 Mult2 No BNEZ R1 Loop


Clock R1 F0 F2 F4 F6 F8 F10 F12... F301 80 Qi Load1



Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 Load1 Yes 80MULTDF4 F0 F2 1 2 Load2 NoSD F4 0 R1 1 Load3 No QiLD F0 0 R1 2 Store1 NoMULTDF4 F0 F2 2 Store2 NoSD F4 0 R1 2 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R10 Mult1 Yes MULTD R(F2) Load1 SUBI R1 R1 #80 Mult2 No BNEZ R1 Loop


Clock R1 F0 F2 F4 F6 F8 F10 F12... F302 80 Qi Load1 Mult1



Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 Load1 Yes 80MULTDF4 F0 F2 1 2 Load2 NoSD F4 0 R1 1 3 Load3 No QiLD F0 0 R1 2 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 Store2 NoSD F4 0 R1 2 Store3 NoReservation Stations S1 S2 RS for jRS for k




• Note: MULT1 has no registers names in RS



Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 Load1 Yes 80MULTDF4 F0 F2 1 2 Load2 Yes 72SD F4 0 R1 1 3 Load3 No QiLD F0 0 R1 2 6 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 Store2 NoSD F4 0 R1 2 Store3 NoReservation Stations S1 S2 RS for jRS for k




• Note: F0 never sees Load1 result



Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 Load1 Yes 80MULTDF4 F0 F2 1 2 Load2 Yes 72SD F4 0 R1 1 3 Load3 No QiLD F0 0 R1 2 6 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 7 Store2 NoSD F4 0 R1 2 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R10 Mult1 Yes MULTD R(F2) Load1 SUBI R1 R1 #80 Mult2 Yes MULTD R(F2) Load2 BNEZ R1 Loop



• Note: MULT2 has no registers names in RS



Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 Load1 Yes 80MULTDF4 F0 F2 1 2 Load2 Yes 72SD F4 0 R1 1 3 Load3 No QiLD F0 0 R1 2 6 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 7 Store2 Yes 72 Mult2SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 Load1 Yes 80MULTDF4 F0 F2 1 2 Load2 Yes 72SD F4 0 R1 1 3 Load3 No QiLD F0 0 R1 2 6 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 7 Store2 Yes 72 Mult2SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k







Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 Load2 Yes 72SD F4 0 R1 1 3 Load3 No QiLD F0 0 R1 2 6 10 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 7 Store2 Yes 72 Mult2SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R14 Mult1 Yes MULTD M(80) R(F2) SUBI R1 R1 #80 Mult2 Yes MULTD R(F2) Load2 BNEZ R1 Loop






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 Load2 NoSD F4 0 R1 1 3 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 7 Store2 Yes 72 Mult2SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R13 Mult1 Yes MULTD M(80) R(F2) SUBI R1 R1 #84 Mult2 Yes MULTD M(72) R(F2) BNEZ R1 Loop





Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 Load2 NoSD F4 0 R1 1 3 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 Yes 80 Mult1MULTDF4 F0 F2 2 7 Store2 Yes 72 Mult2SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k







Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 Yes 80 M(80)*R(F2)MULTDF4 F0 F2 2 7 15 Store2 Yes 72 Mult2SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k

Time Name Busy Op Vj Vk Qj Qk Code:0 Add1 No LD F0 0 R10 Add2 No MULTDF4 F0 F20 Add3 No SD F4 0 R10 Mult1 No SUBI R1 R1 #80 Mult2 Yes MULTD M(72) R(F2) BNEZ R1 Loop






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 Yes 80 M(80)*R(F2)MULTDF4 F0 F2 2 7 15 16 Store2 Yes 72 M(72)*R(72)SD F4 0 R1 2 8 Store3 NoReservation Stations S1 S2 RS for jRS for k






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 Yes 80 M(80)*R(F2)MULTDF4 F0 F2 2 7 15 16 Store2 Yes 72 M(72)*R(72)SD F4 0 R1 2 8 Store3 Yes 64 Mult1Reservation Stations S1 S2 RS for jRS for k






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 18 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 Yes 80 M(80)*R(F2)MULTDF4 F0 F2 2 7 15 16 Store2 Yes 72 M(72)*R(72)SD F4 0 R1 2 8 Store3 Yes 64 Mult1Reservation Stations S1 S2 RS for jRS for k






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 18 19 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 NoMULTDF4 F0 F2 2 7 15 16 Store2 Yes 72 M(72)*R(72)SD F4 0 R1 2 8 Store3 Yes 64 Mult1Reservation Stations S1 S2 RS for jRS for k






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 18 19 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 NoMULTDF4 F0 F2 2 7 15 16 Store2 Yes 72 M(72)*R(72)SD F4 0 R1 2 8 20 Store3 Yes 64 Mult1Reservation Stations S1 S2 RS for jRS for k






Instruction status ExecutionWriteInstruction j k iteration Issue completeResult Busy AddressLD F0 0 R1 1 1 9 10 Load1 NoMULTDF4 F0 F2 1 2 14 15 Load2 NoSD F4 0 R1 1 3 18 19 Load3 Yes 64 QiLD F0 0 R1 2 6 10 11 Store1 NoMULTDF4 F0 F2 2 7 15 16 Store2 NoSD F4 0 R1 2 8 20 21 Store3 Yes 64 Mult1Reservation Stations S1 S2 RS for jRS for k





Tomasulo Summary• Reservations stations: renaming to larger set of

registers + buffering source operands– Prevents registers as bottleneck– Avoids WAR, WAW hazards of Scoreboard– Allows loop unrolling in HW

• Not limited to basic blocks (integer units gets ahead, beyond branches)

• Helps cache misses as well• Lasting Contributions

– Dynamic scheduling– Register renaming– Load/store disambiguation

• 360/91 descendants are Pentium II; PowerPC 604; MIPS R10000; HP-PA 8000; Alpha 21264


Fetch Unit

Dispatch unit w/ 8-entry

instruction queue

Register nos.

DataCache

InstructionCache

Completionunit w/

reorder buffer

XSU0 XSU1 MCFXU LSU FPUBPU

Instruction dispatch buses

GP operand buses

Instruction Operation buses

Reorder buffer information

Reservation Stations

GP result buses

Result status buses

Register

nos.Register nos.

Register nos.

FP operand buses

FP result buses

Branch correction

ee524/cpts561 advanced computer architecture dynamic scheduling a scheme to overcome data hazards

Documents

instruction execution

dynamic scheduling slide

dynamic execution

data hazards

instruction unit fp

instruction issue

order execution

stage pipeline