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Cortex A8 Instruction Cycle Timing

barney vardanyan
Note: This was originally posted on 17th March 2011 at http://forums.arm.com

Hi) sorry for bad English

I need to count latency for two instruction, and all I have is the arm cortex A 8 documantation(charter 16) !
but I have no idea how can do this work using that documantation(
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  • 0
    Note: This was originally posted on 29th June 2011 at http://forums.arm.com


    smlal r0, r1, r3, r4
    smlal r0, r1, r3, r4
    smlal takes 3 cycle, destination register is available in E5. So the first instruction releases r0, r1 at the cycle 3 + 5 = 8.


    You're right.
    There were an error into the my cycle table (on the accumulator acces stage).

    Be carreful! you're explanation was not complete.
    r0 is available 8 cycles later but, it could have been possible that the next instruction only need r0 on stage 3.
    The second smlal need r0 in stage 1 so you remark is correct.

    Just replace the second smlal by a add and you could believe that smlal is faster
    http://pulsar.webshaker.net/ccc/sample-4de8ce82

    ...

    The correction is done.
    The excel file hab been updated and the cycle counter too.

    Thank you.
    Your help is very useful.
Reply
  • 0
    Note: This was originally posted on 29th June 2011 at http://forums.arm.com


    smlal r0, r1, r3, r4
    smlal r0, r1, r3, r4
    smlal takes 3 cycle, destination register is available in E5. So the first instruction releases r0, r1 at the cycle 3 + 5 = 8.


    You're right.
    There were an error into the my cycle table (on the accumulator acces stage).

    Be carreful! you're explanation was not complete.
    r0 is available 8 cycles later but, it could have been possible that the next instruction only need r0 on stage 3.
    The second smlal need r0 in stage 1 so you remark is correct.

    Just replace the second smlal by a add and you could believe that smlal is faster
    http://pulsar.webshaker.net/ccc/sample-4de8ce82

    ...

    The correction is done.
    The excel file hab been updated and the cycle counter too.

    Thank you.
    Your help is very useful.
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