COMP 206: Computer Architecture and Implementation

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COMP 206Computer Architecture and Implementation

• Montek Singh
• Mon., Sep 30, 2002
• Topic Instruction-Level Parallelism
• (Dynamic Scheduling Tomasulos Algorithm)

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Dynamic Scheduling Tomasulos Algorithm

• For IBM 360/91 (about three years after CDC 6600)
• Goal High performance without special compilers
• Differences between IBM 360 and CDC 6600 ISA
• IBM has only 2 register specifiers/instruction
  versus 3 in CDC 6600
• IBM has 4 FP registers versus 8 in CDC 6600
• Differences between Tomasulo Algorithm and
  Scoreboard
• Control and buffers distributed with Function
  Units versus centralized in scoreboard called
  reservation stations
• Registers in instructions replaced by pointers to
  reservation station buffer
• Hardware renaming of registers to avoid WAR and
  WAW hazards
• Common Data Bus broadcasts results to all FUs
  (forwarding)
• Load and Stores treated as FUs as well

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Tomasulo Organization
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More Details of Tomasulo Organization

• Entities that produce values are assigned 4-bit
  tags
• 1, 2, 3, 4, 5, 6 for load buffers
• 8, 9 for multiplier reservation stations
• 10, 11, 12 for adder reservation stations
• Tag 0 indicates presence of valid data
• FP registers have busy bits
• 0 means that register holds valid data
• 1 means that it is waiting to receive value from
  source identified by its tag field

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Tomasulo Representing Data Dependences

• Inputs
• Operand is a register with busy bit 0
• Data copied immediately (through register bus)
  into reservation station
• Tag field of RS set to 0
• Operand is a register with busy bit 1
• Tag field of RS receives a copy of the register
  tag field
• Operand is a load buffer that contains valid data
• Data copied into RS
• Operand is a load buffer that is awaiting data
• Tag field of RS receives tag of load buffer
• Outputs
• Output is a register
• Busy bit set to 1, tag set to RS tag
• Output is a store buffer
• Tag set to RS tag, destination address set

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Three Stages of Tomasulo Algorithm

• Issue get instruction from FP operation queue
• If reservation station free, the scoreboard
  issues instruction and sends operands (renames
  registers)
• Execution operate on operands (EX)
• When both operands ready then executeif not
  ready, watch CDB for result
• Write Result finish execution (WB)
• Write on Common Data Bus to all awaiting units
  mark reservation station available

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Tomasulo State Transitions
Load Buffer
Register

• In case of a CDB conflict, earlier instruction
  has priority
• If more than one instruction is enabled in the
  reservation stations of
• adder or multiplier in same cycle, top entry has
  priority
• If CDB transfer and issue occur in same cycle,
  CDB transfer is assumed to
• occur first
• Every instruction should spend at least one cycle
  in R stage
• If an instruction being issued both reads and
  writes the same register, and the
• source operand is actually in the register (busy
  bit 0), then first the register is
• read, and then its busy bit is turned to 1,
  making it unreadable

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Tomasulo Example
100 F0 ? A 101 F0 ? F0 F1 102 F0 ? F0
B 103 F2 ? F2 F3 104 F1 ? F1 F2 105 C ?
F1 106 F0 ? F1 / F0
I Issued R In reservation station X In
execution W Writing result through CDB
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Tomasulo Example Cycle 0

• System is quiescent

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Tomasulo Example Cycle 1

• (A) will arrive at tag 4
• (F0) will come from tag 4
• F0 is set to busy

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Tomasulo Example Cycle 2

• (F0) will be produced at tag 10
• Right input of adder came from register (tag bit
  0)
• Left input of adder will come from tag 4
• Forwarding tag of F0 has been changed from 4 to 10

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Tomasulo Example Cycle 3

• (F0) will be produced at tag 11
• (B) will arrive at tag 3
• Right input of adder will come from tag 3
• Left input of adder will come from tag 10
• (A) arrives from memory
• Forwarding tag of F0 has been changed from 10 to
  11

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Tomasulo Example Cycle 4

• (F2) will be produced at tag 12
• Right input of adder came from register (tag bit
  0)
• Left input of adder came from register (tag bit
  0)
• (A) with tag 4 is broadcast on CDB
• Adder (at tag 10) picks it up, and is thereby
  enabled
• The instruction that will write F2 has already
  read the old contents of F2

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Tomasulo Example Cycle 5

• (F1) will be produced at tag 8
• Right input of multiplier will come from tag 12
• Left input of multiplier came from register (tag
  bit 0)
• Adder (at tag 10) starts computing
• (B) arrives from memory

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Tomasulo Example Cycle 6

• Memory address of destination is C
• Data will come from tag 8
• Adder (at tag 10) finishes computing
• (B) with tag 3 is broadcast on CDB
• Adder (at tag 11) picks it up
• Adder (at tag 12) starts computing

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Tomasulo Example Cycle 7

• (F1) will be produced at tag 9
• Right input of divider will come from tag 11
• Left input of divider will come from tag 8
• Result of adder (with tag 10) is broadcast on CDB
• Adder (at tag 11) picks it up and is thereby
  enabled
• Adder (at tag 12) finishes computing

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Tomasulo Example Cycle 8

• Result of adder (at tag 12) is broadcast on CDB
• Multiplier (at tag 12) picks it up and is thereby
  enabled

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Tomasulo Example Cycle 9

• Multiplier (at tag 8) starts computing
• Adder (at tag 11) finishes computing

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Tomasulo Example Cycle 10

• Result of adder (with tag 11) is broadcast on CDB
• Divider (at tag 9) picks it up
• Register F0 picks it up

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Tomasulo Example Cycle 11

• Multiplier (at tag 8) finishes computing

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Tomasulo Example Cycle 12

• Result of multiplier (at tag 8) is broadcast on
  CDB
• Divider (at tag 9) picks it up, and is thereby
  enabled
• Store buffer (at tag 1) picks it up, and is
  thereby enabled

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Observations on Tomasulos Algorithm

• Instructions move from decoder to reservation
  stations
• in program order
• dependences can be correctly recorded
• Data Flow Graph The graph of pointers
  connecting the RS, registers, and memory buffers
• helps accomplish out-of-order sequencing of
  instructions
• Chief cost of this scheme high-speed
  associative hardware
• RS hardware has to search for tags when CDB
  broadcasts some value with its tag
• Full load bypassing is supported
• load and store buffers are treated just like
  functional units
• additional hardware on 360/91 also supported load
  forwarding

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Tomasulo Example of Load Bypassing

• Instruction 202 depends on instructions 200 and
  201, so instruction 203 will start executing much
  before 202 (assuming C and D are found to be
  different memory addresses)
• Work out details off-line

200 F0 ? A 201 F0 ? F0 / F1 202 C ? F0 203 F0
? D 204 F0 ? F0 F2
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Tomasulo Loop Unrolling in Hardware

• 360/91 supported limited kind of speculation
• Small loops could be held in a loop buffer
• Loop closing branches were predicted as taken
• This has the effect of loop unrolling at run-time
• Given the small number of FP registers in
  machine, software loop unrolling was not a viable
  option

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Tomasulo Loop Example

• Loop L.D F0 0 R1
• MULT.D F4 F0 F2
• S.D F4 0 R1
• SUBI R1 R1 8
• BNEZ R1 Loop
• Multiply takes 4 clocks
• Loads have cache misses

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Loop Example Cycle 0
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Loop Example Cycle 1
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Loop Example Cycle 2
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Loop Example Cycle 3
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Loop Example Cycle 4
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Loop Example Cycle 5
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Loop Example Cycle 6
Load2
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Loop Example Cycle 7
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Loop Example Cycle 8
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Loop Example Cycle 9
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Loop Example Cycle 10
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Loop Example Cycle 11
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Loop Example Cycle 12
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Loop Example Cycle 13
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Loop Example Cycle 14
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Loop Example Cycle 15
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Loop Example Cycle 16
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Loop Example Cycle 17
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Loop Example Cycle 18
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Loop Example Cycle 19
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Loop Example Cycle 20
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Loop Example Cycle 21
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Summary of Tomasulos Algorithm

• Prevents registers as bottleneck
• Avoids WAR and WAW hazards of scoreboard
• Allows loop unrolling in hardware
• Not limited to basic blocks (provided we have
  branch prediction)
• Lasting contributions
• Dynamic scheduling
• Register renaming
• Load/store disambiguation
• Next Dynamic branch prediction