Why can't I check the flag directly?

[pushead]

Hi all,

I've managed to create an interrupt (level 3) either triggered from the vertical blank or the Copper. So far, so good.

But in the interrupt handler I would like to check the source of the interrupt; was the interrupt generated by the copper or the vertical blank. In order to do that I plan to check if the bit #5 (V. blank) or #4 (Copper) is set in $dff01e (INTREQR).

Now, my plan was to use a btst #4,$dff01e to check if the copper was the originator of the interrupt. But if I do that it does not work as expected. Instead, if I first move the content of $dff01e to d0, and then do a btst #4,d0 it works fine. Why? Isn' this in the same?

So then I use this code, it works fine:


irq_level_3:
movem.l d0-d7/a0-a6,-(a7)

;-------------------------------------------------
; Check source of IRQ LEVEL3: COPPER
;-------------------------------------------------

clr.l d0
move.w $dff01e,d0
btst #4,d0
bne exit

; Do some stuff.....

exit:
move.w #$0030,$dff09c
movem.l (a7)+,d0-d7/a0-a6
rte


But if I check, with btst directly, it does not work? It just skips to exit.

irq_level_3:
movem.l d0-d7/a0-a6,-(a7)

;-------------------------------------------------
; Check source of IRQ LEVEL3: COPPER
;-------------------------------------------------

btst #4,$dff01e ; Checking flag directly @$DFF01e
bne exit

; Do some stuff.....

exit:
move.w #$0030,$dff09c
movem.l (a7)+,d0-d7/a0-a6
rte



Ok, I'm quite new to Amiga programming, but this really twisted my mind here....

What am I missing here?

[a/b]

btst and family operate on a byte (if operand is in memory), and bit 4 is part of the next byte ($dff01f).

[pushead]

Ah! Thanks!