Question: BIOS calls like PLAYER_START?

[city41]

I am brand new to Neo Geo dev.

I've been reading the examples over at https://github.com/freem/freemlib-neogeo/tree/master/examples

I'm curious how to deal with things like PLAYER_START, DEMO_END, COIN_SOUND and BIOS_USER_REQUEST in ngdevkit?

I see if the game defines a function named rom_callback_VBlank then it will get called on vblank. Is it possible to do a similar callback approach for the above subroutines?

Thanks!

[city41]

I think I figured it out.

It looks like runtime/crt0.s is the program header file for the ROM.

neogeodev has one here: https://wiki.neogeodev.org/index.php?title=68k_program_header

Where they do

    jmp        USER
    jmp        PLAYER_START
    jmp        DEMO_END
    jmp        COIN_SOUND

ngdevkit's crt0.S does

	/* ROM Entry Point */
	jmp     GAME_BOOT.l
	jmp     .LStart2.l
	jmp     .LStart3.l

where GAME_BOOT calls C's main(), and the others are noops.

So it looks like if I needed support for say COIN_SOUND, I could edit crt0.S and add it.

[dciabrin]

I need a bit of time to figure out that one.

The whole idea behind the devkit is to have a ld linkscript that woudl automatically map your user-provided functions in the program header, like it's done for the VBlank interrupt handler, and revert to default no-op otherwise.

As a proof-of-concept, I did the vblank and hblank interrupt handlers, but I didn't map anything else yet in the ld linkscript.

I suspect though that this jump table is meant to be called by the original BIOS, so if we want a similar support in ngdevkit, we'd have to implement input polling and jmp in nullbios.

Until I come back with an example in the devkit, I think you don't have any other choice than to poll the inputs yourself in your own vblank handler.

[city41]

I'm currently exploring this. My first goal is to get the eye catcher to run then to move onto the game.
I've edited crt0.S and attempted to get it to follow what freem did in his neo geo lib.

So far the rom just boots to the cross hatch in gngeo.

I'm not exploring it by trying to get user defined functions in the linkscript, just editing the assembly directly. If I can get this working, I'll see about the linkscript next.

[city41]

I'm slowly but surely learning 68k asm :)

This hack allows the eyecatcher to run, then boots into the game proper:

https://github.com/city41/ngdevkit/commit/2f3395ee5208c5bc5746f95434628bba75b53293

Not good enough to make official, but if anyone reading this has any suggestions, I'd love to hear it.

(oh and you need the real neo geo bios instead of null bios)

[dciabrin]

OK so I finally came up with what I believe is what you asked for in the first place. I fixed how crt0 behaves when the BIOS calls it, so it can distinguish between request type 0 (StartupInit) and request type 2 (Game).

Previously, crt0 would jump into your C main as soon as USER subroutine is called.
On MVS though, the first time you plug the cartridge into the board, the BIOS detects it and calls USER subroutine with request 0 before running the Eye Catcher, so crt0 pre-empts the Eye Catcher. It does it only once though, so on next soft or hard reset, my understanding is that the Eye Catcher would be run. (you can experience that with MAME by re-running the emulator several times).

Now crt0 correctly honours what the BIOS is requesting, and calls default functions for all the 4 request types. At link time, the linker checks whether you provided your own functions for each request type, and if so it will use it, otherwise it will default to what crt0 provides currently: initializing the C runtime and calling the C main.

Also, all those behaviour happens at the right time, so the Eye Catcher will play if configured to. If needed, you can disable the Eye Catcher as explained in #19.