Gameboy Development Forum

ISSOtm wrote:

My advice is to not use GBDK for projects larger than 32k.

Also, don't use MBC1. It just adds extra complexity for no reason, use MBC3 or MBC5.

Why? GBDK has lots of issues but I never had any problems with bank switching or the MBC1.

JS7777 wrote:

Hello there,


I'm actually finding how to bankswitch to 0x4000 with GBDK.

I actually know there is a known cmd : SWITCH_ROM_MBC1(1);

but I can't figure it out how to fully switch to 0x4000 to execute something else like a 16kb game.

Thanks by advance

Regards

That's not enough information to really help you. Maybe you should first read through the documents I provided in post #7 of this thread: http://gbdev.gg8.se/forums/viewtopic.php?id=453

In summary: You want to use the bank switching instruction for something it isn't indended for. For such purposes, you should know exactly what you're doing which unfortunately is never the case with GBDK. The best option would in fact be to switch to assembly. If you don't want to do that, here are some suggestions:

First thing to know is that switching a bank with an MBC1 means nothing else but to write the desired bank number to memory address $2000. Therefore, in theory

SWITCH_ROM_MBC1(7);

should translate to

ld a, 7
ld ($2000), a

in assembly.

To trace the problem, you could inspect the assembly output of GBDK and see whether GBDK translates your code that way. If it doesn't, that is probably the cause for the unexpected behaviour.

A solution to solve this in C could be to write to memory address $2000 manually (use a pointer to that address). This may or may not work with GBDK, but it's at least worth a try.

A solution to solve this in C could be to write to memory address $2000 manually (use a pointer to that address). This may or may not work with GBDK, but it's at least worth a try.

How could I do that ? I've haven't find any info on internet about pointers.

Thanks by advance

Great thanks and what would be the code to compile it ?

That's the current code :

..\bin\lcc -Wa-l -Wl-m -Wl-j -DUSE_SFR_FOR_REG -c -o banks.o banks.c

..\bin\lcc -Wa-l -Wl-m -Wl-j -DUSE_SFR_FOR_REG -Wf-bo1 -Wf-ba1 -c -o bank_1.o bank_1.c

..\bin\lcc -Wa-l -Wl-m -Wl-j -DUSE_SFR_FOR_REG -Wl-yt0 -Wl-yo4 -Wl-ya1 -o banks.gb banks.o  bank_1.o

pause

Thanks, i get an error :

bank_1.c(1):error *** parse error at end of of input

I didn't put anything in bank_1.c

Here is the folder if you want to check an eye : https://www.dropbox.com/s/n4m2vfg6vkb6njl/bank.zip?dl=0

Thanks alot for your help

You're going to wrong way. A14 is one of the address pins; what the program does is simply switch to a bank, which changes the address read from. This modifies A14's state, but the MBC doesn't interpret it any differently.
You should go the ASM route anyways, especially if you want to use a custom mapper. GBDK is just a source of headaches.

JS7777 wrote:

You should go the ASM route anyways, especially if you want to use a custom mapper. GBDK is just a source of headaches.

I know nothing about asm and I didn't find any documentations to start with.

In my opinion it's a bit unfair to call GBDK a headache. It has its issues and the code it generates runs slowly, but for a beginner it's much more accessible than a cryptic programming language that will take you a day or so just to put "Hello World" on the screen... In the case at hand, however, assembly would be the better solution beacuse you always know what exactly it does with the hardware. It might turn out that what you want to do cannot be achieved with GBDK.

JS7777 wrote:

Well here what happens :

In BGB debugger rom=1 becomes rom=3 when pressing start but no switching to an adress.

Maybe I got you wrong but isn't that what you wanted? You switched to another bank which means that the content of that bank can now be accessed in the address space from $4000 to $8000.

It's probably not bank 7 (as in your code) because you compiled a ROM with only 4 banks.

JS7777 wrote:

Not the content of that bank but lunching the content of that bank from $4000 to $8000. If I put a 16kb game between those adresses nothing happens. (even changing the bank number)

I'm not sure whether we're talking about the same thing...

Generally bank switching works as follows: In your ROM file you have the banks lined up in a row (0-1-2-3-4-5- etc.). When you start up the Game Boy you have banks 0 and 1 accessible. Bank 0 will be in the address space from $0000 to $3FFF and bank 1 will be in the address space from $4000 to $7FFF. The bank switching instruction will exchange what you can access in the address space from $4000 to $7FFF. For example,

SWITCH_ROM_MBC1(3);

will make the content of bank 3 appear in the address space from $4000 to $7FFF while the content of bank 1 will be switched out and cannot be accessed anymore.

Bank 0 is not switchable, so what is in the first 16 kb of the ROM file will always be accessible in the address space from $0000 to $3FFF.

Oh, I understand, thanks.

But how could I launch a game from 0x4000 to 0x8000 like the menu from Reiner Zeigler ?




Thanks

Last edited by JS7777 (2018-07-29 15:25:05)

Correct but the tools aren't programmed in the rom, they are programmed apart like a game. A 16KB .gb file. To add them you just need to paste it in 4000 to 8000 or 8000 to etc..

I got an issue, it doesn't seems to work. A14 is always high, then goes low when pressing START. It should be the other way round.

So I tried that rom from reinerzeigler and A14 is low at startup then it goes high when I start one of his tools, that's correct.

Last edited by JS7777 (2018-07-29 16:21:43)

Anyway, one more remark: Since your real goal is controlling external devices, why don't you use the link cable (which is made for that purpose)?

Oh, didn't think about that. How can I control it ?

Last edited by JS7777 (2018-07-29 16:54:27)

Thanks.

Well, I can control A14 state but on a MBC1 flash cartridge, not a ROM Only flash cartridge, maybe because of the SWITCH_ROM_MBC1 command wouldn't work on a ROM only cartridge. What about your idea with pointers, what do you think  ?

Thanks alot for your help.