Tomasulo Algorithm
• Each functional unit has a reservation station
• The issue stage assigns an instruction to a free reservation
station.
• It takes over the control of the execution.
• It stores the operands.
• The results are directly forwarded to other functional
units that require the data.
• For this purpose the result bus is used.
Structure of the Tomasulo-Algorithm
Tomasulo Algorithm
• The reservation stations do an implicit register renaming
and remove WAR and WAW hazards.
• Phases
• Issue:
– Select free reservation station.
– Copy available operands from registers into the reservation station
or store the reservation station that generates a required operand
(resolving RAW and WAR hazards)
– Store in the register which reservation station generates the next
value (handling of WAW hazards)
Tomasulo Algorithm
• Phases (contd)
• Execute
– If operands are missing, wait.
– Insert new operands in the reservation station.
– If all operands are available, start operation.
• Write result
– As soon as the result is available, it is broadcast fo the result bus
(common data bus)
– The target register and the other reservations take the result from
the bus.
Reservation table
• Reservation station are stored in the reservation table.
Tomasulo Algorithm: Register
• Each register stores besides the data
• The valid bit, it indicates whether the value is valid or is
currently computed.
• Number of the reservation station of the operation that
computes the new value.
Example
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Remaining cylces in FU
Beispiel Tomasulo-Algorithmus
Source operands are filled in RS2
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Beispiel Tomasulo-Algorithmus
Tomasulo-Algorithmus Fazit
• Implicit register renaming
• Direct propagation of results
• RAW and WAR are resolved via reservation stations.
• WAW hazards are resolved since last write is always taken.
• Simple scheme does not support speculative
execution.