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What are hints?

Juha Aaltonen

What does it mean that an instruction is a hint instruction, like NOP, YIELD and WFE?

I haven't found any explanations in ARM Architecture Reference Manual, ARMv7-A and ARMv7-R edition, Issue C.

[EDIT]

Aha, this has already been answered in:

Are processors really only "taking a hint" for WFE,WFI,SEV?

It would have been nice to know what instructions like NOP and YIELD affect the HW, or are the hint instructions more like partially/fully implementation dependent?

[/EDIT]

  • 0

    And in the same manual:

    B9.3.10    MSR (Banked register)

    Encoding A1    ARMv7VE

    MSR<c> <banked_reg>, <Rn>

    cond 0 0 0 1 0 R 1 0       M1 (1) (1) (1) (1) (0) (0) 1 M 0 0 0 0 Rn

    Is ARMv7VE a version/profile/variant of ARMv7-A or ARMv7-R?

    Is this instruction there in ARMv7-A?

    What are M1 and M?

    [EDIT]

    Aha, The M1 and M are explained in ARMv8 ARM.

    Is it so, that the mask is 5-bit field and the M is the highest order bit that's got separated from the rest?

    [/EDIT]

  • 0

    Hello,

    the behavior (or side effect) of NOP or YIELD is implementation dependent.

    As far as I know, NOP or YIELD would affect nothing to the hardware.

    A silicon vendor which has the architecture license can add a side effect to those instructions for their convenience.

    For example, at the x86 architecture YIELD is the same as a very slow NOP (taking about several cycles for execution).

    Best regards,

    Yasuhiko Koumoto.


  • 0 in reply to Yasuhiko Koumoto

    Thanks. I suspected something like that.