Safe Early Output Logic

1. Safe Early Output Logic An Improved QDI Logic System Charlie Brej

2. Overview • DIMS • What we have now • Early Output • What we want • Guarding • How we get it • “Backward safe” guarding • How we make it (even) better Charlie Brej

3. Petri nets An event Discrete movement Occupies a single place Async Four Phase Area between events Specially & Temporally Rising & falling edges Separated What is a Token Req POP Ack Charlie Brej

4. C C C C DIMS vs Early Output Logic Size:48 transistors Delay:4 inversions Size:12 transistors Delay:2 inversions Charlie Brej

5. Early Output cases Average:80% 1 in 5 times down to 60% Charlie Brej

6. No output until all inputs present Output not released until all inputs released DIMS C C C C Charlie Brej

7. Output generated as early as possible State of inputs cannot be determined by the state of the output Early Output Charlie Brej

8. Input latches generate validities Result only acknowledged once valid Validity C Charlie Brej

9. State of stage inputs can be determined from the Validity Input low and Outputs low does not mean stage is reset Not sensing internal wires Not QDI Simple Guarding C Charlie Brej

10. Ensure all wire pairs are cycled up and down QDI Forward Safe Guarding C Charlie Brej

11. Viewpoint of a single output Many inputs Behaviour Charlie Brej

12. All or nothing Synchronises inputs together Behaviour Charlie Brej

13. High throughput Few stalls Short Tokens Stall! Charlie Brej

14. Block stages Spreads Induces other long tokens Long Tokens Charlie Brej

15. Ensure all wire pairs are cycled up and down QDI Acknowledge parts Backward Safe Guarding C Charlie Brej

16. Acknowledge ASAP Behaviour Charlie Brej

17. Late Islands Behaviour Charlie Brej

18. Late Arrival Behaviour Charlie Brej

19. Trimming Behaviour Charlie Brej

20. Conclusion • Early output QDI system • Improved guarding • Localised handshakes • Static Anti-Tokens • Desynchronisation of inputs • Unnecessary speculative should not operations ruin performance Charlie Brej