Gameboy Development Forum

andreai · 2014-08-24 09:39:06

Hi there,

Just trying to remove an interrupt handler using remove_VBL with GBDK 2.95, but whenever I try the game crashes, any idea what I might be doing wrong? just doing this:
remove_VBL(myHandler);

Thanks,
Andrea

qbertreynolds · 2019-11-20 00:10:48

Did you ever solve this? Even on ultra stripped down projects, where all it does is add then remove the interrupt, it crashes.

bbbbbr · 2019-11-22 16:17:47

Disclaimer: I haven't dug much into GBDK for this so the following may not be entirely accurate.

A couple things I noticed

* Do not call wait_vbl_done() if the VBL interrupt is not enabled / running (I believe it will get stuck waiting)

* It's probably best to turn off interrupts before removing a handler

* It looks like GBDK may install it's own VBL interrupt (in crt0.s see: .vbl: ) to service the OAM. For example, even if you don't install your own VBL handler, calling "disable_interrupts()" will cause sprites to no longer refresh, then calling "enable_interrupts()" will cause them to resume. (likewise: set_interrupts(0x00) and then set_interrupts(VBL_IFLAG)

* calling add_VBL() repeatedly with the same entry will append more duplicate entries, it will not replace the existing entry that matches. So avoid calling add_VBL() without first removing the same entry

* add_VBL() appends entries to the end of the VBL interrupt list (at 0xCEDD), but remove_VBL removes the first matching entry from the front of the list. I'm not entirely sure, but it looks like it may also remove (copy over) the preceding entry. This could be part of the problem and also points to a workaround.

Here is a potential (ugly) workaround:

Code:

// If this gets optimized out, then add something to the body
void vbl_shim() {
}
.
.
.

// Call in this sequence to remove a *different* handler previously added with add_VBL()
// DO NOT call if you haven't previously added a handler, otherwise it will crash
// This adds an entry after the most recent entry, and then deletes itself (and via a bug(?), the preceding/most recent entry as well)
disable_interrupts();
add_VBL(vbl_shim);
remove_VBL(vbl_shim);
enable_interrupts();

Last edited by bbbbbr (2019-11-22 17:13:04)

bbbbbr · 2019-11-22 19:46:46

Update- if you want a fix instead of a workaround, I think it would be as follows (seems to work when I patch and rebuild crt0.s). My ASM isn't very good, so there might be a more optimal way to do this.

gbdk/libc/gb/crt0.s

Code:

FROM this:
    ;; Now do a memcpy from here until the end of the list
    LD    D,H
    LD    E,L
    DEC    DE
    INC    HL

TO this:
    ;; Now do a memcpy from here until the end of the list
    ; Fix for _remove_VBL() erasing previous VBL entry
    ; Copy next entry -> current entry, instead of copying current -> previous
    LD  D,H
    LD  E,L
    INC HL
    INC HL

Once built, back up and repalce the crt0.o found under the gbdk/lib/... folder

Edit: Add rebuild instructions below

You should be able to use the tools in gbdk (lcc) to rebuild it.

I'm using gbdk 2.96 on Linux, so location of file paths and command line may vary for you.

Go to where ctr0.s is located and rebuild it:

Code:

cd /opt/gbdk/libc/gb
/opt/gbdk/bin/lcc -c crt0.s

Backup and replace the existing crt0.o binary:

Code:

cp ../../lib/small/asxxxx/gb/crt0.o ../../lib/small/asxxxx/gb/crt0.o.dist
mv crt0.o ../../lib/small/asxxxx/gb

Last edited by bbbbbr (2020-01-20 17:00:36)

Zalo · 2020-04-21 02:32:01

Hi
I am planning to add this to gbdk 3.1. Has it been properly tested already?

bbbbbr · 2020-04-21 15:57:49

Zalo wrote:
Hi
I am planning to add this to gbdk 3.1. Has it been properly tested already?

I tested while investigating the fix, but it would be good to have the fix confirmed by another person. Particularly if one of the posters above could confirm that it resolved their issue.

The process for testing is fairly straightforward if I recall: calling remove_VBL() on your previously installed handler should produce a crash in an unpatched build. In the patched version the crash should go away and the desired VBL handler should get removed.

Last edited by bbbbbr (2020-04-21 15:58:20)

Gameboy Development Forum

#1 2014-08-24 09:39:06

GBDK remove_VBL crashing?

#2 2019-11-20 00:10:48

Re: GBDK remove_VBL crashing?

#3 2019-11-22 16:17:47

Re: GBDK remove_VBL crashing?

Code:

#4 2019-11-22 19:46:46

Re: GBDK remove_VBL crashing?

Code:

Code:

Code:

#5 2020-04-21 02:32:01

Re: GBDK remove_VBL crashing?

#6 2020-04-21 15:57:49

Re: GBDK remove_VBL crashing?

Zalo wrote:

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