Gameboy Development Forum

Howdy fellas ! I've got a programming issue. Now, before striking the WRONG BOARD hammer, please read the rest. Thank you
Basically, the LCD interrupt is requested when it shouldn't every time I write to STAT. Below is a more precise description of the context in which it happens.


All of the following description has been done using BGB 1.5.3, but the behavior is the same on console.
This happens on a game flagged as GBC-only, and in DMG / SGB mode.

Here's my situation : right after my game's init, I have only the VBlank and LCD bits set in IE, and bits 3-6 of STAT are reset. IME is reset, and only the VBlank bit is set in IF.
So far, so good.
Then, IME is set, but the VBlank handler discards itself because it's not starting on scanline $90 (it's a failsafe).
On the next VBlank, though, the handler runs its full code, which includes setting bit 5 of STAT (this is used for music operations).
First thing I'm confused about is, the bit is set during scanline $93, yet IF bit 1 (LCD interrupt request) gets set immediately. We're still in Mode 1, so why ?

If I continue to trace execution, right as the VBlank handler exits, execution jumps to the STAT interrupt vector. However, the procedure runs a "res 5, [hl]" while hl = $FF41 to clear the Mode 2 interrupt flag. The flag is reset, but for some reason, this also sets the LCD bit in IF !
So the cycle repeats, with the LCD bit getting set each time precisely at the "res 5, [hl]".
[EDIT]
I tried replacing ith "ld a, [hl] / res 5, [hl] / ld [hl], a", I tried replacing the [hl] with [$FF41], I tried using ldh instead, but the bit in IF always gets set right after writing back to $FF41.



So now, why is this in the hardware board ? Because this behavior does NOT occur on the GBC. Someone suggested me it could be a hardware difference between DMG and GBC (I guess it would make sense, the LCD differs between these consoles), so I'm asking to the hardware pros : do you know why this occurs ?
If you are interested, I can attach the ROM of my game where this occurs. Just let me know.


Big thanks to anyone who will answer, this is really puzzling me.

Last edited by ISSOtm (2017-05-08 04:51:07)