I'm sure the answer is "you can't decrement like that".

ref

When incrementing multiple-byte values you can do the obvious:

INX:BNE done
INY
.done

But decrementing doesn't work that way. The following does not work:

.loop
; do something with YX
DEX:BNE skip
DEY
.skip
BNE loop

Picture what happens at the boundary case. We'll start at $0402:


Have you seen it? The counter goes 0402, 0401, 0300, 03FF. It "rolls over" when you go from 1 to 0 instead of when going from 0 past the end and back around to FF.

In loop you need to test that you've wrapped. When incrementing you get that from the increment, going from FF to 00 is the wrap, and going from FF to 00 sets the zero flag.

When decrementing you wrap when going from 00 to FF. There is no flag set when going from 00 to FF and only then. M is set, but also for every other decrement until you get to 7F. NE is set, but also for every other decrement until you get back to 0.

You know you are going to wrap when the *existing* value is zero, not the *changed* value. So you need to test *before* you change the value, not afterwards.

What you need to do is:

.loop
; do something
TXA:BNE skip :\ Skip if low byte not about to wrap from &00 to &FF
DEY :\ Decrement high byte
.skip
DEX :\ Decrement low byte
BNE loop
TYA:BNE loop

This gives:

_________________
--
JGH - http://mdfs.net