pushbx wiki

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.

====== Alignment calculation in three instructions ======

Found [[https://hg.pushbx.org/ecm/warplink/rev/319bf908e2e5#l2.101|in a 2025-08-29 changeset]] of the ecm fork of WarpLink:

<code>    mov bx, [alignment_table + bx]	; bx = alignment mask
	and ax, bx
	neg ax
	and ax, bx</code>

''and'' ''neg'' ''and''. That's odd. I assume it works because I would have worked it out at the time but it's not intuitive.

===== =====

The first ''and'' is obvious enough: Calculate how far past the aligned boundary we are. Zero means exactly aligned, 1 means 1 byte past alignment, etc.

Then ''neg'': Negative value of how far past the aligned boundary. Zero if exactly aligned.

Then the second ''and'': -1 (0FFFFh) results in ''ax'' = alignment mask. -2 (0FFFEh) results in ''mask clr 1'' (one below the mask). -3 (0FFFDh) results in ''mask clr 2'' (two below the mask).

{{tag>warplink codespam}}


~~DISCUSSION~~