Advantages of
Dynamic Scheduling
• Handles cases when dependences unknown at
compile time
– (e.g., because they may involve a memory reference)
• It simplifies the compiler
• Allows code that compiled for one pipeline
to run efficiently on a different pipeline
• Hardware speculation, a technique with
significant performance advantages, that
builds on dynamic scheduling
3/12/02
CS252/Culler
Lec 15.1
HW Schemes: Instruction Parallelism
• Key idea: Allow instructions behind stall to proceed
DIVD
ADDD
SUBD
F0,F2,F4
F10,F0,F8
F12,F8,F14
• Enables out-of-order execution
and allows out-of-order completion
• Will distinguish when an instruction begins
execution and when it completes execution;
between 2 times, the instruction is in execution
• In a dynamically scheduled pipeline, all instructions
pass through issue stage in order (in-order issue)
3/12/02
CS252/Culler
Lec 15.2
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,
structural hazards
check
for
• Read operands—Wait until no data hazards,
then read operands
3/12/02
CS252/Culler
Lec 15.3
A Dynamic Algorithm:
Tomasulo’s Algorithm
• For IBM 360/91 (before caches!)
• Goal: High Performance without special compilers
• Small number of floating point registers (4 in 360)
prevented interesting compiler scheduling of operations
– This led Tomasulo to try to figure out how to get more effective
registers — renaming in hardware!
• Why Study 1966 Computer?
• The descendants of this have flourished!
– Alpha 21264, HP 8000, MIPS 10000, Pentium III, PowerPC 604, …
3/12/02
CS252/Culler
Lec 15.4
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);
– form of register renaming ;
– avoids WAR, WAW hazards
– More reservation stations than registers, so can do
optimizations compilers can’t
• 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
3/12/02
CS252/Culler
Lec 15.5
Tomasulo Organization
FP Registers
From Mem
FP Op
Queue
Load Buffers
Load1
Load2
Load3
Load4
Load5
Load6
Store
Buffers
Add1
Add2
Add3
Mult1
Mult2
FP adders
Reservation
Stations
To Mem
FP multipliers
Common Data Bus (CDB)
3/12/02
CS252/Culler
Lec 15.6
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: Qj,Qk=0 => ready
– Store buffers only have Qi for RS producing result
Busy: Indicates reservation station or FU is busy
Register result status—Indicates which functional
unit will write each register, if one exists. Blank
when no pending instructions that will write that
register.
3/12/02
CS252/Culler
Lec 15.7
Three Stages of Tomasulo Algorithm
1. Issue—get instruction from FP Op Queue
If reservation station free (no structural hazard),
control issues instr & sends operands (renames registers).
2. Execute—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
• Example speed:
3 clocks for Fl .pt. +,-; 10 for * ; 40 clks for /
3/12/02
CS252/Culler
Lec 15.8
Tomasulo Example
Instruction stream
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
Load1
Load2
Load3
Register result status:
Clock
0
No
No
No
3 Load/Buffers
Reservation Stations:
Time Name Busy
Add1
No
Add2
No
FU count
Add3
No
down
Mult1 No
Mult2 No
Busy Address
Op
S1
Vj
S2
Vk
RS
Qj
RS
Qk
3 FP Adder R.S.
2 FP Mult R.S.
F0
F2
F4
F6
F8
F10
F12
...
F30
FU
Clock cycle
counter
3/12/02
CS252/Culler
Lec 15.9
Tomasulo Example Cycle 1
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
Reservation Stations:
Time Name Busy
Add1
No
Add2
No
Add3
No
Mult1 No
Mult2 No
Register result status:
Clock
1
3/12/02
FU
Busy Address
Load1
Load2
Load3
Op
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F0
F2
F4
F6
F8
Yes
No
No
34+R2
F10
F12
...
F30
Load1
CS252/Culler
Lec 15.10
Tomasulo Example Cycle 2
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
Reservation Stations:
Time Name Busy
Add1
No
Add2
No
Add3
No
Mult1 No
Mult2 No
Register result status:
Clock
2
FU
Busy Address
Load1
Load2
Load3
Op
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F0
F2
F4
F6
F8
Load2
Yes
Yes
No
34+R2
45+R3
F10
F12
...
F30
Load1
Note: Can have multiple loads outstanding
3/12/02
CS252/Culler
Lec 15.11
Tomasulo Example Cycle 3
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
Reservation Stations:
Time Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes MULTD
Mult2 No
Register result status:
Clock
3
FU
F0
Busy Address
3
S1
Vj
Load1
Load2
Load3
S2
Vk
RS
Qj
Yes
Yes
No
34+R2
45+R3
F10
F12
RS
Qk
R(F4) Load2
F2
Mult1 Load2
F4
F6
F8
...
F30
Load1
• Note: registers names are removed (“renamed”) in
Reservation Stations; MULT issued
CS252/Culler
3/12/02
• Load1 completing; what is waiting for Load1?
Lec 15.12
Tomasulo Example Cycle 4
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
Reservation Stations:
Busy Address
3
4
4
Load1
Load2
Load3
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
No
Yes
No
45+R3
F10
F12
Time Name Busy Op
Add1 Yes SUBD M(A1)
Load2
Add2
No
Add3
No
Mult1 Yes MULTD
R(F4) Load2
Mult2 No
Register result status:
Clock
4
FU
F0
Mult1 Load2
...
F30
M(A1) Add1
• Load2 completing; what is waiting for Load2?
3/12/02
CS252/Culler
Lec 15.13
Tomasulo Example Cycle 5
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
2 Add1 Yes SUBD M(A1) M(A2)
Add2
No
Add3
No
10 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
5
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
M(A1) Add1 Mult2
• Timer starts down for Add1, Mult1
3/12/02
CS252/Culler
Lec 15.14
Tomasulo Example Cycle 6
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
1 Add1 Yes SUBD M(A1) M(A2)
Add2 Yes ADDD
M(A2) Add1
Add3
No
9 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
6
FU
F0
Mult1 M(A2)
Add2
No
No
No
F10
F12
...
F30
Add1 Mult2
• Issue ADDD here despite name dependency on F6?
3/12/02
CS252/Culler
Lec 15.15
Tomasulo Example Cycle 7
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
3
4
Busy Address
4
5
Load1
Load2
Load3
7
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
0 Add1 Yes SUBD M(A1) M(A2)
Add2 Yes ADDD
M(A2) Add1
Add3
No
8 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
7
FU
F0
No
No
No
Mult1 M(A2)
Add2
F10
F12
...
F30
Add1 Mult2
• Add1 (SUBD) completing; what is waiting for it?
3/12/02
CS252/Culler
Lec 15.16
Tomasulo Example Cycle 8
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
7
8
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
2 Add2 Yes ADDD (M-M) M(A2)
Add3
No
7 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
8
3/12/02
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
Add2 (M-M) Mult2
CS252/Culler
Lec 15.17
Tomasulo Example Cycle 9
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
7
8
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
1 Add2 Yes ADDD (M-M) M(A2)
Add3
No
6 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
9
3/12/02
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
Add2 (M-M) Mult2
CS252/Culler
Lec 15.18
Tomasulo Example Cycle 10
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
3
4
4
5
7
8
Busy Address
Load1
Load2
Load3
10
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
0 Add2 Yes ADDD (M-M) M(A2)
Add3
No
5 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
10
FU
F0
No
No
No
Mult1 M(A2)
F10
F12
...
F30
Add2 (M-M) Mult2
• Add2 (ADDD) completing; what is waiting for it?
3/12/02
CS252/Culler
Lec 15.19
Tomasulo Example Cycle 11
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
7
8
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
4 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
11
FU
F0
Mult1 M(A2)
No
No
No
F10
...
F30
(M-M+M)(M-M) Mult2
• Write result of ADDD here?
• All quick instructions complete in this cycle!
3/12/02
F12
CS252/Culler
Lec 15.20
Tomasulo Example Cycle 12
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
7
8
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
3 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
12
3/12/02
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Mult2
CS252/Culler
Lec 15.21
Tomasulo Example Cycle 13
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
7
8
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
2 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
13
3/12/02
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Mult2
CS252/Culler
Lec 15.22
Tomasulo Example Cycle 14
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
4
5
Load1
Load2
Load3
7
8
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
1 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
14
3/12/02
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Mult2
CS252/Culler
Lec 15.23
Tomasulo Example Cycle 15
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
Busy Address
3
4
15
7
4
5
Load1
Load2
Load3
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
0 Mult1 Yes MULTD M(A2) R(F4)
Mult2 Yes DIVD
M(A1) Mult1
Register result status:
Clock
15
FU
F0
Mult1 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Mult2
• Mult1 (MULTD) completing; what is waiting for it?
3/12/02
CS252/Culler
Lec 15.24
Tomasulo Example Cycle 16
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
3
4
15
7
4
5
16
8
Load1
Load2
Load3
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 No
40 Mult2 Yes DIVD M*F4 M(A1)
Register result status:
Clock
16
FU
F0
Busy Address
M*F4 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Mult2
• Just waiting for Mult2 (DIVD) to complete
3/12/02
CS252/Culler
Lec 15.25
Faster than light computation
(skip a couple of cycles)
3/12/02
CS252/Culler
Lec 15.26
Tomasulo Example Cycle 55
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
3
4
15
7
4
5
16
8
Load1
Load2
Load3
10
11
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 No
1 Mult2 Yes DIVD M*F4 M(A1)
Register result status:
Clock
55
3/12/02
FU
F0
Busy Address
M*F4 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Mult2
CS252/Culler
Lec 15.27
Tomasulo Example Cycle 56
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
3
4
15
7
56
10
4
5
16
8
Load1
Load2
Load3
S1
Vj
S2
Vk
RS
Qj
RS
Qk
56
FU
F0
F2
F4
F6
F8
M*F4 M(A2)
No
No
No
11
Time Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 No
0 Mult2 Yes DIVD M*F4 M(A1)
Register result status:
Clock
Busy Address
F10
F12
...
F30
(M-M+M)(M-M) Mult2
• Mult2 (DIVD) is completing; what is waiting for it?
3/12/02
CS252/Culler
Lec 15.28
Tomasulo Example Cycle 57
Instruction status:
Instruction
LD
F6
LD
F2
MULTD F0
SUBD
F8
DIVD
F10
ADDD
F6
j
34+
45+
F2
F6
F0
F8
k
R2
R3
F4
F2
F6
F2
Exec Write
Issue Comp Result
1
2
3
4
5
6
Reservation Stations:
3
4
15
7
56
10
4
5
16
8
57
11
Load1
Load2
Load3
S1
Vj
S2
Vk
RS
Qj
RS
Qk
F2
F4
F6
F8
Time Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 No
Mult2 Yes DIVD M*F4 M(A1)
Register result status:
Clock
56
FU
F0
Busy Address
M*F4 M(A2)
No
No
No
F10
F12
...
F30
(M-M+M)(M-M) Result
• Once again: In-order issue, out-of-order execution
and out-of-order completion.
CS252/Culler
3/12/02
Lec 15.29
Tomasulo Drawbacks
• Complexity
– delays of 360/91, MIPS 10000, Alpha 21264,
IBM PPC 620 in CA:AQA 2/e, but not in silicon!
• Many associative stores (CDB) at high speed
• Performance limited by Common Data Bus
– Each CDB must go to multiple functional units
high capacitance, high wiring density
– Number of functional units that can complete per cycle
limited to one!
» Multiple CDBs  more FU logic for parallel assoc stores
• Non-precise interrupts!
– We will address this later
3/12/02
CS252/Culler
Lec 15.30
Tomasulo Loop Example
Loop:LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
• This time assume Multiply takes 4 clocks
• Assume 1st load takes 8 clocks
(L1 cache miss), 2nd load takes 1 clock (hit)
• To be clear, will show clocks for SUBI, BNEZ
– Reality: integer instructions ahead of Fl. Pt. Instructions
• Show 2 iterations
3/12/02
CS252/Culler
Lec 15.31
Loop Example
Instruction status:
ITER Instruction
1
1
1
Iter2
ation 2
Count 2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
Name Busy
Add1
No
Add2
No
Add3
No
Mult1 No
Mult2 No
Op
Vj
Exec Write
Issue CompResult
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
S1
Vk
S2
Qj
RS
Qk
Code:
LD
MULTD
SD
SUBI
BNEZ
No
No
No
No
No
No
Added Store Buffers
F0
F4
F4
R1
R1
Register result status
Clock
0
F0
R1
80
F2
F4
F6
F8
Fu
F10 F12
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Instruction Loop
Fu
Value of Register used for address, iteration control
3/12/02
CS252/Culler
Lec 15.32
Loop Example Cycle 1
Instruction status:
ITER Instruction
1
LD
F0
j
k
0
R1
1
Vj
S1
Vk
Reservation Stations:
Time
Name Busy
Add1
No
Add2
No
Add3
No
Mult1 No
Mult2 No
Exec Write
Issue CompResult
Op
S2
Qj
RS
Qk
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
No
No
No
No
No
80
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
1
3/12/02
R1
80
F0
F2
F4
F6
F8
F10 F12
Fu Load1
CS252/Culler
Lec 15.33
Loop Example Cycle 2
Instruction status:
ITER Instruction
1
1
LD
MULTD
F0
F4
j
k
0
F0
R1
F2
1
2
Vj
S1
Vk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
S2
Qj
RS
Qk
R(F2) Load1
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
No
No
No
No
No
80
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
2
3/12/02
R1
80
F0
Fu Load1
F2
F4
F6
F8
F10 F12
Mult1
CS252/Culler
Lec 15.34
Loop Example Cycle 3
Instruction status:
ITER Instruction
1
1
1
LD
MULTD
SD
F0
F4
F4
j
k
0
F0
0
R1
F2
R1
Reservation Stations:
Time
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
Vj
Exec Write
Issue CompResult
1
2
3
S1
Vk
S2
Qj
RS
Qk
R(F2) Load1
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
No
No
Yes
No
No
80
80
Mult1
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
3
R1
80
F0
Fu Load1
F2
F4
F6
F8
F10 F12
Mult1
• Implicit renaming sets up data flow graph
3/12/02
CS252/Culler
Lec 15.35
Loop Example Cycle 4
Instruction status:
ITER Instruction
1
1
1
LD
MULTD
SD
F0
F4
F4
j
k
0
F0
0
R1
F2
R1
Reservation Stations:
Time
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
Vj
Exec Write
Issue CompResult
1
2
3
S1
Vk
S2
Qj
RS
Qk
R(F2) Load1
Fu
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
Yes
No
No
Yes
No
No
80
80
Mult1
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
4
R1
80
F0
Fu Load1
F2
F4
F6
F8
F10 F12
Mult1
• Dispatching SUBI Instruction (not in FP queue)
3/12/02
CS252/Culler
Lec 15.36
Loop Example Cycle 5
Instruction status:
ITER Instruction
1
1
1
LD
MULTD
SD
F0
F4
F4
j
k
0
F0
0
R1
F2
R1
Reservation Stations:
Time
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
Vj
Exec Write
Issue CompResult
1
2
3
S1
Vk
S2
Qj
RS
Qk
R(F2) Load1
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
No
No
Yes
No
No
80
80
Mult1
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
5
R1
72
F0
Fu Load1
F2
F4
F6
F8
F10 F12
Mult1
• And, BNEZ instruction (not in FP queue)
3/12/02
CS252/Culler
Lec 15.37
Loop Example Cycle 6
Instruction status:
ITER Instruction
1
1
1
2
LD
MULTD
SD
LD
F0
F4
F4
F0
j
k
0
F0
0
0
R1
F2
R1
R1
1
2
3
6
Vj
S1
Vk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
S2
Qj
RS
Qk
R(F2) Load1
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
Yes
No
Yes
No
No
80
72
80
Mult1
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
6
R1
72
F0
Fu Load2
F2
F4
F6
F8
F10 F12
Mult1
• Notice that F0 never sees Load from location 80
3/12/02
CS252/Culler
Lec 15.38
Loop Example Cycle 7
Instruction status:
ITER Instruction
1
1
1
2
2
LD
MULTD
SD
LD
MULTD
F0
F4
F4
F0
F4
j
k
0
F0
0
0
F0
R1
F2
R1
R1
F2
1
2
3
6
7
Vj
S1
Vk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 Yes Multd
S2
Qj
RS
Qk
R(F2) Load1
R(F2) Load2
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
Yes
No
Yes
No
No
80
72
80
Mult1
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
7
R1
72
F0
Fu Load2
F2
F4
F6
F8
F10 F12
Mult2
• Register file completely detached from computation
• First and Second iteration completely overlapped
3/12/02
CS252/Culler
Lec 15.39
Loop Example Cycle 8
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
Vj
S1
Vk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 Yes Multd
S2
Qj
RS
Qk
R(F2) Load1
R(F2) Load2
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
Yes
No
Yes
Yes
No
80
72
80
72
Mult1
Mult2
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
8
3/12/02
R1
72
F0
Fu Load2
F2
F4
F6
F8
F10 F12
Mult2
CS252/Culler
Lec 15.40
Loop Example Cycle 9
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
9
Vj
S1
Vk
S2
Qj
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 Yes Multd
RS
Qk
R(F2) Load1
R(F2) Load2
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
Yes
No
Yes
Yes
No
80
72
80
72
Mult1
Mult2
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
9
R1
72
F0
Fu Load2
F2
F4
F6
F8
F10 F12
Mult2
• Load1 completing: who is waiting?
• Note: Dispatching SUBI
3/12/02
CS252/Culler
Lec 15.41
Loop Example Cycle 10
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
4
Exec Write
Issue CompResult
1
2
3
6
7
8
S1
Vk
9
10
10
S2
Qj
Name Busy Op
Vj
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd M[80] R(F2)
Mult2 Yes Multd
R(F2) Load2
RS
Qk
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
Yes
No
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
Fu
72
80
72
Mult1
Mult2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
10
R1
64
F0
Fu Load2
F2
F4
F6
F8
F10 F12
Mult2
• Load2 completing: who is waiting?
• Note: Dispatching BNEZ
3/12/02
CS252/Culler
Lec 15.42
Loop Example Cycle 11
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
3
4
Exec Write
Issue CompResult
1
2
3
6
7
8
S1
Vk
Name Busy Op
Vj
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd M[80] R(F2)
Mult2 Yes Multd M[72] R(F2)
9
10
10
11
S2
Qj
RS
Qk
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
64
80
72
Fu
Mult1
Mult2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
11
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult2
• Next load in sequence
3/12/02
CS252/Culler
Lec 15.43
Loop Example Cycle 12
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
2
3
Exec Write
Issue CompResult
1
2
3
6
7
8
S1
Vk
Name Busy Op
Vj
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd M[80] R(F2)
Mult2 Yes Multd M[72] R(F2)
9
10
10
11
S2
Qj
RS
Qk
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
64
80
72
Fu
Mult1
Mult2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
12
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult2
• Why not issue third multiply?
3/12/02
CS252/Culler
Lec 15.44
Loop Example Cycle 13
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
1
2
Exec Write
Issue CompResult
1
2
3
6
7
8
S1
Vk
Name Busy Op
Vj
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd M[80] R(F2)
Mult2 Yes Multd M[72] R(F2)
9
10
10
11
S2
Qj
RS
Qk
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
64
80
72
Fu
Mult1
Mult2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
13
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult2
• Why not issue third store?
3/12/02
CS252/Culler
Lec 15.45
Loop Example Cycle 14
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
0
1
Exec Write
Issue CompResult
1
2
3
6
7
8
9
14
10
11
S1
Vk
S2
Qj
RS
Qk
Name Busy Op
Vj
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd M[80] R(F2)
Mult2 Yes Multd M[72] R(F2)
10
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
64
80
72
Fu
Mult1
Mult2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
14
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult2
• Mult1 completing. Who is waiting?
3/12/02
CS252/Culler
Lec 15.46
Loop Example Cycle 15
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
Reservation Stations:
Time
0
Exec Write
Issue CompResult
1
2
3
6
7
8
9
14
10
15
11
S1
Vk
S2
Qj
RS
Qk
Name Busy Op
Vj
Add1
No
Add2
No
Add3
No
Mult1 No
Mult2 Yes Multd M[72] R(F2)
10
15
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
64
80
72
Fu
[80]*R2
Mult2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
15
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult2
• Mult2 completing. Who is waiting?
3/12/02
CS252/Culler
Lec 15.47
Loop Example Cycle 16
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
9
14
10
15
11
16
Vj
S1
Vk
S2
Qj
RS
Qk
Reservation Stations:
Time
4
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
10
15
R(F2) Load3
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
No
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
64
80
72
Fu
[80]*R2
[72]*R2
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
16
3/12/02
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult1
CS252/Culler
Lec 15.48
Loop Example Cycle 17
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
9
14
10
15
11
16
Vj
S1
Vk
S2
Qj
RS
Qk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
10
15
R(F2) Load3
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
Yes
64
80
72
64
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
[80]*R2
[72]*R2
Mult1
Register result status
Clock
17
3/12/02
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult1
CS252/Culler
Lec 15.49
Loop Example Cycle 18
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
9
14
18
10
15
10
15
Vj
S1
Vk
S2
Qj
RS
Qk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
11
16
R(F2) Load3
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
Yes
Yes
Yes
64
80
72
64
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
[80]*R2
[72]*R2
Mult1
Register result status
Clock
18
3/12/02
R1
64
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult1
CS252/Culler
Lec 15.50
Loop Example Cycle 19
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
9
14
18
10
15
19
10
15
19
11
16
Vj
S1
Vk
S2
Qj
RS
Qk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
R(F2) Load3
Busy Addr
Load1
Load2
Load3
Store1
Store2
Store3
No
No
Yes
No
Yes
Yes
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
Fu
64
72
64
[72]*R2
Mult1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
Register result status
Clock
19
3/12/02
R1
56
F0
Fu Load3
F2
F4
F6
F8
F10 F12
Mult1
CS252/Culler
Lec 15.51
Loop Example Cycle 20
Instruction status:
ITER Instruction
1
1
1
2
2
2
LD
MULTD
SD
LD
MULTD
SD
F0
F4
F4
F0
F4
F4
j
k
0
F0
0
0
F0
0
R1
F2
R1
R1
F2
R1
1
2
3
6
7
8
9
14
18
10
15
19
10
15
19
11
16
20
Vj
S1
Vk
S2
Qj
RS
Qk
Reservation Stations:
Time
Exec Write
Issue CompResult
Name Busy Op
Add1
No
Add2
No
Add3
No
Mult1 Yes Multd
Mult2 No
R(F2) Load3
Busy Addr
Fu
Load1
Load2
Load3
Store1
Store2
Store3
Yes
No
Yes
No
No
Yes
56
64
Mult1
Code:
LD
MULTD
SD
SUBI
BNEZ
F0
F4
F4
R1
R1
0
F0
0
R1
Loop
R1
F2
R1
#8
...
F30
64
Register result status
Clock
20
R1
56
F0
Fu Load1
F2
F4
F6
F8
F10 F12
Mult1
• Once again: In-order issue, out-of-order execution
and out-of-order completion.
CS252/Culler
3/12/02
Lec 15.52
Why can Tomasulo overlap iterations
of loops?
• Register renaming
– Multiple iterations use different physical destinations for
registers (dynamic loop unrolling).
• Reservation stations
– Permit instruction issue to advance past integer control flow
operations
– Also buffer old values of registers - totally avoiding the WAR
stall that we saw in the scoreboard.
• Other perspective: Tomasulo building data flow
dependency graph on the fly.
3/12/02
CS252/Culler
Lec 15.53
Tomasulo’s scheme offers 2 major
advantages
(1) the distribution of the hazard detection logic
–
–
–
distributed reservation stations and the CDB
If multiple instructions waiting on single result, & each
instruction has other operand, then instructions can be
released simultaneously by broadcast on CDB
If a centralized register file were used, the units
would have to read their results from the registers
when register buses are available.
(2) the elimination of stalls for WAW and WAR
hazards
3/12/02
CS252/Culler
Lec 15.54
What about Precise Interrupts?
• State of machine looks as if no instruction
beyond faulting instructions has issued
• Tomasulo had:
In-order issue, out-of-order execution, and
out-of-order completion
• Need to “fix” the out-of-order completion
aspect so that we can find precise
breakpoint in instruction stream.
3/12/02
CS252/Culler
Lec 15.55
Relationship between precise
interrupts and specultation:
• Speculation: guess and check
• Important for branch prediction:
– Need to “take our best shot” at predicting branch direction.
• If we speculate and are wrong, need to back up and
restart execution to point at which we predicted
incorrectly:
– This is exactly same as precise exceptions!
• Technique for both precise interrupts/exceptions
and speculation: in-order completion or commit
3/12/02
CS252/Culler
Lec 15.56
HW support for precise interrupts
• Need HW buffer for results
of uncommitted instructions:
reorder buffer
– 3 fields: instr, destination, value
– Use reorder buffer number
instead of reservation station
FP
when execution completes
Op
– Supplies operands between
Queue
execution complete & commit
– (Reorder buffer can be operand
source => more registers like RS)
– Instructions commit
Res Stations
– Once instruction commits,
FP Adder
result is put into register
– As a result, easy to undo
speculated instructions
on mispredicted branches
or exceptions
3/12/02
Reorder
Buffer
FP Regs
Res Stations
FP Adder
CS252/Culler
Lec 15.57
Four Steps of Speculative Tomasulo
Algorithm
1.Issue—get instruction from FP Op Queue
If reservation station and reorder buffer slot free, issue instr &
send operands & reorder buffer no. for destination (this stage
sometimes called “dispatch”)
2.Execution—operate on operands (EX)
When both operands ready then execute; if not ready, watch CDB
for result; when both in reservation station, execute; checks RAW
(sometimes called “issue”)
3.Write result—finish execution (WB)
Write on Common Data Bus to all awaiting FUs
& reorder buffer; mark reservation station available.
4.Commit—update register with reorder result
When instr. at head of reorder buffer & result present, update
register with result (or store to memory) and remove instr from
reorder buffer. Mispredicted branch flushes reorder buffer
(sometimes called “graduation”)
3/12/02
CS252/Culler
Lec 15.58
Program Counter
Valid
Exceptions?
Result
Reorder Table
FP
Op
Queue
Res Stations
FP Adder
Compar network
Dest Reg
What are the hardware complexities with
reorder buffer (ROB)?
Reorder
Buffer
FP Regs
Res Stations
FP Adder
• How do you find the latest version of a register?
– (As specified by Smith paper) need associative comparison network
– Could use future file or just use the register result status buffer to track
which specific reorder buffer has received the value
• Need as many ports on ROB as register file
3/12/02
CS252/Culler
Lec 15.59
Summary
• Reservations stations: implicit register 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)
• Today, helps cache misses as well
– Don’t stall for L1 Data cache miss (insufficient ILP for L2 miss?)
• Lasting Contributions
– Dynamic scheduling
– Register renaming
– Load/store disambiguation
• 360/91 descendants are Pentium III; PowerPC 604;
MIPS R10000; HP-PA 8000; Alpha 21264
3/12/02
CS252/Culler
Lec 15.60