23. Advanced mapper MMC1

A mapper is some circuitry on the cartridge that allows you to “map” more than 32k of PRG ROM and/or more than 8k of CHR ROM to the NES. By dividing a larger ROM into smaller “banks” and redirecting read/writes to different banks, you can trick the NES into allowing much larger ROMs. MMC1 was the most common mapper.

MMC1 – 681 games
MMC3 – 600 games
UxROM – 270 games
NROM – 248 games
CNROM – 155 games
AxROM – 76 games
*source BootGod


I borrowed this from Kevtris’s website. The smaller chip on the bottom left says “Nintendo MMC1A”.

MMC1 has the ability to change PRG banks and CHR banks. It can have PRG sizes up to 256k and CHR sizes up to 128k. (some rare variants could go up to 512k in PRG, but that won’t be discussed). It can change the mirroring from Horizontal to Vertical to One Screen. Metroid and Kid Icarus were MMC1, and they switch mirroring to scroll in different directions.

The MMC1 boards frequently had WRAM of 8k ($2000) bytes at $6000-7FFF, which could be battery backed to save a game. When I tried to make an NROM demo with WRAM, several emulators (and my PowerPak) decided that the WRAM didn’t exist because no NROM games ever had WRAM. But you wouldn’t have that problem with MMC1.

(In the most common arrangement…) The last PRG bank is fixed to $C000-FFFF and the $8000-BFFF can be mapped to any of the other banks. PRG banks are 16k ($4000) in size. Graphics can be swapped too. You can either change the entire pattern tables (PPU $0-1FFF) or change each separately (PPU $0-FFF and $1000-1FFF). CHR banks are 4k ($1000) in size. One thing you can do with swappable CHR banks is animate the background like Kirby’s Adventure does (by changing CHR banks every few frames).

I chose a 128k PRG ROM and 128k CHR ROM, and have it set to change each tileset separately.

Behind the scenes, the MMC1 mapper has registers at $8000,$A000,$C000, and $E000. It has to write 5 times to each, because it technically can only send 1 bit at a time. The $8000 register is the MMC1 control, the $A000 register changes the first CHR bank (tileset #0), the $C000 register changes the second CHR bank (tileset #1) (does nothing if CHR are in 8k mode), and the $E000 register changes which PRG bank is mapped to $8000-BFFF.

This is all tricky to program, in general, and more so for cc65. It is important to keep the main C code and all libraries in the fixed bank, including the init code (crt0.s) where the reset code and vectors are and neslib.s where the nmi code is. Level data should go in swapped banks. Infrequently used code should go in swapped banks.

Music is special. You would typically reserve an entire bank for music code and data. And all the music functions have to swap the music code/data in place to use it. You will need to explicitly put the music in a certain bank and change the SOUND_BANK definition to match it (in crt0.s).

Most of this new code was written by cppchriscpp with slight modification by me. Here’s the link to Chris’s code…


Things I changed.

I included all the files in the MMC1 folder. The .c and .h file at the top of the main .c file. The .asm files are included near the bottom of crt0.s.

In the header (crt0.s)…Flag 6 indicates the mapper # = 1 (MMC1). The NES_MAPPER symbol is defined in the .cfg file. Flags 8, indicate 1 PRG RAM (WRAM) bank. At the top of crt0.s the SOUND_BANK bank will need to be correct, and music put in the corresponding segment.

Also in crt0.s, I added the MMC1 reset code, and include the 2 .asm files in the MMC1 folder. I put the music in BANK 6, and now bank 6 is swapped before the music init code is called. All CHR files are put in the CHARS segment, which is 128k in size (it’s not completely filled).

The neslib.s file in the LIB folder has also been changed, specifically the nmi code and the music functions.

Each segment is defined in the .cfg file… MMC1_128_128.cfg. In the asm files, you just have to put a .segment “BANK4” to put everything below that in BANK 4. In the .c and .h files, you have to do this…

#pragma rodata-name (“BANK4”)
#pragma code-name (“BANK4”)

RODATA for Read Only data, like constant arrays.
CODE for code, of course.

Look at the ROM in a hex editor, and you can see how the linker constructed the ROM. I specifically wrote strings called “BANK0” in bank #0 and “BANK1” in bank #1, etc.


What’s new?

Banked calls are necessary, when calling a function in another bank.

banked_call(unsigned char bankId, void (*method)(void));

What this does is push the current PRG bank on an array, swap a new one in place, call the function with a function pointer, return, and pop the old bank back into place, then return. You can even nest banked_calls from one swapped bank to another, but there is a limit of 10 deep before it breaks. In fact, these banked_calls are very slow, so try to stay in one bank as much as possible before switching.

Also, music functions are in their own bank, and it has to do a similar…save bank, swap new one, jump there, return, pop bank, return… thing for any music or sfx call. So, try to minimize how many sfx you call in a frame.

set_prg_bank(unsigned char bank_id);

Use this to read data from a swappable bank (from the fixed bank). It sets a specific bank at $8000, and then you can access the data there.

set_chr_bank_0(unsigned char bank_id);
set_chr_bank_1(unsigned char bank_id);

Use these to change the CHR banks. bank_0 for the first set (which I used for background). bank_1 for the second set (which I used for sprites).

set_mirroring() to change the mirroring from Horizontal to Vertical to Single Screen.

If you are doing a split screen, like with a sprite zero hit, you could set the CHR bank that shows at the top of the screen with this function.


And turn it off with this function.


These have been removed as of October 2022. See below.

Look at the code…

in main(), it uses

banked_call(BANK_0, function_bank0);

This function swaps bank #0 into place, then calls function_bank0(), which prints some text, “BANK0”, on the screen.

banked_call(BANK_1, function_bank1);

Does the same, but if you look at function_bank1(), it also calls

banked_call(BANK_2, function_bank2);

to show that you can nest one banked call inside another…up to 10 deep.

And we see that both “BANK1” and “BANK2” printed, so both of those worked.

Next we see that this banked_call() can’t take any extra arguments, so
you would have to pass arguments with global variables…I called them arg1 and arg2.

arg1 = ‘G’; // must pass arguments with globals
arg2 = ‘4’;
banked_call(BANK_3, function_bank3);

function_bank3() prints “BANK3” and “G4”, so we know that worked. Passing arguments by global is error prone, so be careful.

Skipping to banked_call(BANK_5, function_bank5);

function_bank5() also calls function_2_bank5() which is also in the same bank. You would use standard function calls for that, and not banked_call(). It printed “BANK5” and “ALSO THIS” so we know it worked alright. Use regular functions if it’s in the same bank.

Finally, banked_call(BANK_6, function_bank6); reads 2 bytes from the WRAM at $6000-7FFF. Just to have an example of it working. In the .cfg file I stated that there is a BSS segment there called XRAM. At the top of this .c file I declared a large array wram_array[] of 0x2000 bytes. You can read and write to it as needed.

It printed “BANK6” and “AC” (our test values) correctly.

Once we return to the main() function, we know we are in the fixed bank. Without using banked_call() we could swap a bank in place using set_prg_bank(). We could do that to read data in bank… like, for example, level data. You just read from it normally, as if that bank was always there.

I recommend you never use set_prg_bank() and then jumping to it without using banked_call(). The bank isn’t saved to the internal bank variable. If an NMI is triggered, the nmi code swaps the music code in place and then uses the internal bank variable to reset the swapped bank before returning… and that would be the wrong bank, and it would crash. Actually, this scenario might work without crashing. But I still recommend using the banked_call().

There is an infinite loop next, that reads the controller, and processes each button press.

Start button = changes the CHR bank. It calls this function…


Which changes the background tileset. You notice that the sprite (the round guy) never changes. Sprites are using the second tileset. If we wanted to change the second tileset, we would use…





I am also testing the music code. Start calls a DMC sample. Button A calls the song play function music_play(). Button B calls a sound effect sfx_play(). and Select pauses and unpauses the music with music_pause().

I just wanted to make sure that the music code is working correctly, because I rewrote the function code. All the music data and code is in bank #6, and the code swaps in bank #6 and then calls the music function. Then swaps banks back again before returning.

I didn’t show any examples of changing the mirroring, but that is possible too.

Link to the code…


I’m glad I got this working. Now on to actual game code. Oh, also… I was using MMC1_128_128.cfg but you could double the PRG ROM to 256k, by using the MMC1_256_128.cfg (edit the compile.bat linker command line arguments).

You could easily turn the WRAM at $6000-7FFF into save RAM by editing the header. Flags 6, indicate contains battery-backed SRAM, set bit 1… so add 2 to flags 6 in the header in crt0.s.

Maybe next time I will make an MMC3 demo, which can easily use 512k PRG ROM and 256k CHR ROM, and has a scanline counter. Also, there is a homebrew mapper 30, the oversized UNROM 512 board with extra CHR-RAM and (optional) 4-screen mirroring. Either would be easy to adapt the banked call system.


It has been brought to my attention that some of the MMC1 boards do not always reliably boot with the last bank in the fixed position at $c000. Some MMC1 games put reset vectors in every bank, which put the correct bank in place. My example code does not do that. It assumes the last bank is fixed and in place. Every emulator that I know of will boot it with the last bank fixed at $c000. But, this assumption may break if loaded onto a real cartridge. The user might have to hit reset a few times to get it to work. Or, they might get angry and play another game.

Update October 2022

I have made some changes (bug fixes) to the MMC1 example code to make it more stable (less likely to crash). A poorly timed bank switch that was interrupted by NMI would crash the game. This has been fixed. Also, the last bank pop from exiting a banked call wasn’t restoring the default bank (as you might have expected). That has been fixed.

Some other things have been changed. The startup code is different. Multiple instances of resetting the shift register have been added. The NMI code has changed.

set_nmi_chr_tile_bank() and unset_nmi_chr_tile_bank() have been removed. The CHR banks will automatically be rewritten at the top of each frame. If you need to change the CHR bank mid screen (say, with a sprite zero hit) for a split screen effect, use these functions after the sprite zero hit.

split_chr_bank_0() and split_chr_bank_1()

Which will temporarily change a CHR bank until the next NMI is reached.

I hope these changes work for you. I have not been able to test them on a real MMC1 board, but they are working well in all the emulators I’ve tested.


4 thoughts on “23. Advanced mapper MMC1

  1. Follow up question: What are you referring to by Flag 6? I don’t see a variable by that name in either your linker script or your crt0.s.


    1. In crt0.s, in the .segment “HEADER”, I mean the 6th byte (actually the 7th, starting from 0). It currently says “.byte <NES_MIRRORING|(<NES_MAPPER<<4)" and it would need to say ".byte <NES_MIRRORING|(<NES_MAPPER<<4)|2" with a bitwise OR 2 on the end. Flag 6 is the INES header name for this byte (see nesdev wiki).


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