Simple ARM/Thumb assy lang question - I think...

Development Boards MicroMod
1

I have a (large) piece of code I am debugging, and am stuck on something that seems fairly simple, but doesn’t work.

The snip of code is as follows:

    1d64:       ff82f7ff        bl      1c6c <mySubrouteen>
    1d68:       1c04            mov     r4, r0          (add r4, r0, #0)
    1d6a:       2800            cmp     r0, #0
    1d6c:       d141            bne     1df2 <errorOut>

When I step through the code, I see the following:

1d64:       ff82f7ff        bl      1c6c <mySubrouteen>

    --     routeen returns with value ZERO in r0
            cpsr is 0xd3

 
1d68:       1c04            mov     r4, r0          (add r4, r0, #0)

     ---- r4 now set to zero 
           cpsr set to 0x400000d3 (Zero bit set)

1d6a:       2800            cmp     r0, #0

     ---- No change - zero would be set, but was already set. 


1d6c:       d141            bne     1df2 <errorOut>

    Now here's where it goes wrong - as zero is set, you wouldn't
     expect it to branch - but it does!

If I jump back to this instruction and manually set cpsr to 0xd3

(i.e. clear the zero bit) - it still branches.

Am I totally missing something???

:x

-BKG