Alignment

[Mark_Larson]

  The bigger the data size the worse the penalty for mis-aligned data.  I did some code to show the alignment problems with an MMX version of a string copy routine.

http://www.masmforum.com/simple/index.php?topic=1589.45  - search for "alignment".  The orignal code ran in 87 cycles accessing 8 bytes at a time ( MMX registers).  The misaligned code caused it to run in about 250-280 cycles depending on how unaligned it was.  Other than the misaligned data, there are no other changes to the code.

[Randall Hyde]

BYTE data can be any single byte.

WORD data should be "01" or "23"

DWORD data should only be "0123".

Intel's documentation (for the PIV) claims that word access of the form "12" are also fine. Also, most of the time you only get a big hit if the data object crosses a cache line.
Cheers,
Randy Hyde

[James Ladd]

ok, so now I know how to align my data and procedures.
But am I right in thinking I can align the code within a procedure using an align statement as well ?
If I use this align keyword, with masm put NOPs in the code to make it pad out ?

[MichaelW]

If I use this align keyword, with masm put NOPs in the code to make it pad out ?

We beat that subject to death here:

http://www.masmforum.com/simple/index.php?topic=1622.0

[James Ladd]

Michael, Thanks for beating it one more time :)