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Tomasulo Algorithm

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.

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Tomasulo Algorithm

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  1. 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.

  2. Structure of the Tomasulo-Algorithm

  3. 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)

  4. 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.

  5. Reservation table • Reservation station are stored in the reservation table.

  6. 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.

  7. Example

  8. Beispiel Tomasulo-Algorithmus

  9. Beispiel Tomasulo-Algorithmus

  10. Beispiel Tomasulo-Algorithmus Remaining cylces in FU

  11. Beispiel Tomasulo-Algorithmus Source operands are filled in RS2

  12. Beispiel Tomasulo-Algorithmus

  13. Beispiel Tomasulo-Algorithmus

  14. Beispiel Tomasulo-Algorithmus

  15. Beispiel Tomasulo-Algorithmus

  16. Beispiel Tomasulo-Algorithmus

  17. Beispiel Tomasulo-Algorithmus

  18. Beispiel Tomasulo-Algorithmus

  19. Beispiel Tomasulo-Algorithmus

  20. 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.

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