Re: Untagging by subtraction instead of masking on USE_LSB_TAG

[Miles Bader]

Thien-Thi Nguyen <address@hidden> writes:
> For masking, i see offsets (lwz) of 4,0,0 (lines A,B,C).
> For subtraction, -1,-5,-5.
>
> It's very possible that the machine can handle 4,0,0 more
> efficiently; those all are even (0, modulo 2) and in two cases
> "nothing"!  Furthermore, the maximum absolute offset for the
> subtraction method is 5, which is larger (faaarther away) than 4.
>
> Anyway, here is an excerpt from p.532 of "PowerPC 405, Embedded
> Processor Core, User's Manual":

Note that the important value here is the EA, not the offset.  Typically
in a LSB tag scheme, the tags are arranged so that after adding these
"funny" offsets, the resulting EA is aligned properly.

-Miles

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