Using AOROM to make a 2 in 1 multicart of NROM games

[Sivak]

Hey all. I've been wondering if something like this is possible. Basically, I was thinking of making a 2 in 1 cart having a menu screen so you can pick your game and then having it load the game using the roms.

I currently have this situation:
-Both NROM roms without their ines header nor their CHR data embedded within them. Just the PRG data and each PRG is stored in a different bank of the AOROM game.
-In the "master" rom, I have it grab the appropriate CHR data and write that all to RAM and then I try to bank switch out everything into the appropriate game.

This actually kinda worked, but the games don't play. One game simply loads the music and the other loads nothing.

I tried clearing zero page with 0's, but that didn't seem to work either.

I don't know if there's a way to call the reset routine of the games or what, but is what I want to ultimately do possible? Thanks.

[Dwedit]

Stick a little piece of code in RAM that selects the bank, then jumps to the reset vector.

Note that AOROM has single screen mirroring, so that means no Super Mario Bros.

[Bregalad]

Yes it's possible but you'll have to hack your games so that they work fine with single screen mirroring, which is likely hard to do. You can modify AOROM to get a fixed mirroring (aka BNROM) easily tough.

[kyuusaku]

GNROM or MMC* would work better.

[tepples]

I made an SNROM-based multicart engine. I could probably have done it in BNROM as easily, but the theme of the compilation included an existing game that was on SNROM.

But yes, GNROM looks like it was almost made for multis. In fact, the mapper of SMB1 + Duck Hunt is just GNROM on a different board layout.

[Sivak]

Well, it doesn't have to be AOROM. I just figured it was ideal as you can swap out everything. Neither of my games use scrolling either, so that's no loss.

I could probably do MMC1 if that's a better solution. The only thing I'm not sure of how to do is "Jumping to the reset vector in RAM".

[tepples]

The source code for my Forbidden Four multicart engine has an example of how to make a RAM-based trampoline. It involves copying a piece of code to RAM that does the following:

1. Set up the mapper to copy CHR.
2. Copy or decompress CHR to CHR RAM. (Omit this step for CHR ROM boards such as GNROM or some MMC1 configurations.)
3. Set up the mapper to run PRG.
4. JMP ($FFFC) to start the game from its reset handler.

[Sivak]

Well, I've got the CHR RAM part of things. The only problem I guess lies with doing the bank swap.

I know that if I bank switch, I'm immediately in the new bank, so in theory I could be anywhere in the program.

The way I'm doing the switch is like this:

Code: Select all

    LDA #1  ;Bank 1 is game 1.  Bank 2 is game 2.  Bank 0 is the menu.  Bank 3 is unused.
    TAX
    STA bankID, X
   ;I think JMP $FFFC would need to go here...
 
;----------This appears later in the program----------
;Things used for bank switching
bankID:
	.db $00, $01, $02, $03

The only thing is once I'm swapped, what do I do to JMP to $FFFC? Obviously JMP'ing to there before swapping will just constantly reset the game...

I guess I'm lost on the whole "copy to RAM and execute code" part of things. Do I just put code similar to what I have above somewhere in, say, $0500 and JSR there or is there more to it?

[tepples]

I guess I'm lost on the whole "copy to RAM and execute code" part of things. Do I just put code similar to what I have above somewhere in, say, $0500 and JSR there

Pretty much. Except you made one mistake: it's JMP ($FFFC), using indirect addressing, not JMP $FFFC.

[Sivak]

Ok. I got a menu and everything so you can pick a game and it switches to the appropriate bank and then performs the reset routine.

My final question is this: Whenever I use the emulator reset button, it doesn't reset back to the multicart's menu, but rather stays in the current bank as if you reset just that game. The power button of course works. Is there any good way around this or is it going to be power all the way?

[tepples]

You could hack each game's reset code to jump back to the multicart's menu. (This was done in SMB+Duck Hunt.) But then you wouldn't be able to use an automated tool to build multicarts because you'd have to find some space in the ROM to put the trampoline code.

[Dwedit]

Who needs rom space when you can add an extra 8k of SRAM in there?

[tepples]

Three things:

1. A*ROM and B*ROM don't support SRAM without adding an extra 74HC10 chip and a bit of analog circuitry to the board.
2. This extra SRAM, the support circuitry, and the revised PCB cost money to replicate.
3. Multicarts including one battery-backed game need to preserve the data in SRAM when switching games.

[Sivak]

Well, the multicart is of the first two games I made, so I have the sources and all, but what exactly would I need to do? Just some normal bank switch or is there more to it?

Also, one guy mentioned that it's possible for AOROM to start in a random bank and not always bank 0.

[tepples]

If you know that none of the NROM games use the last few bytes of PRG ROM for anything, you could just put identical entry point code in $FFF2-$FFFF of all banks:

Code: Select all

reset:
  ldx #$00       ; you don't get a bus conflict if you
  stx reset + 1  ; overwrite the immediate operand
  jmp $8000
  .addr nmi_entry
  .addr reset
  .addr irq_entry

In this case, your menu starts at $8000 of bank 0:

Code: Select all

entry:
  sei
  ; X is already 0 from the code in all banks
  stx PPUCTRL  ; disable NMI
  stx PPUMASK  ; disable rendering
  dex
  txs  ; set stack pointer to $01FF

  ; etc.

But then you'd have to copy the actual start of your game to RAM and JMP through that instead of ($FFFC).