Resizing your rom

This guide gives you a generously sized ROM with room to grow, and a number of useful features. The engine is custom-built to support this rom, but there are some tweaks we can make if the game isn’t meeting your needs. This chapter will walk you through doubling the size of your game, as well as the basics of the config files.

If you make the changes described in this chapter, you will no longer be able to use the 128kb InfiniteNesLives
cartridge linked in the “Playing your game on a real NES” chapter! (Powerpak will continue to work) The 256kb version will work with these tweaks, but you will have to look to make sure that the cartridge lines up with the features you have added. (Especially if you do something custom)

This chapter does assume you understand the concept of prg banks - the first chapter in section 2 covers this. If you are not, you may want to go there first.

Doubling the size of your cartridge

So you’ve got a game going, and you are running out of space for your code. What can you do? Well, you can increase the size avaialble, of course! From a code perspective this is somewhat simple - the only added costs would be in buying hardware to make a physical cartridge. (And honestly, the cost there isn’t that high either). Luckily, our compiler knows how to do this, so we can get by with a configuration tweak alongside a small engine change.

First, we need to figure out what we want for space. (Okay, I know the title gives that away, but bear with me!) We currently have 128kb of program memory available to us, broken up into 8 16kb chunks. We won’t go into explicit detail here, but computer memory goes up in powers of two - like: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024. while it is possible to combine two smaller sizes, it is much easier to just go up to the next size. Since we currently have 128kb of prg, we probably want to jump up to 256kb. This will give us 16 8kb banks in place of our 8.

This engine actually ships with a pre-built configuration file for this case. You can use it by renaming some configuration files, as follows:

ca65.cfg -> ca65.cfg.backup
ca65_constants.asm -> ca65_constants.asm.backup
ca65_256k.cfg -> ca65.cfg
ca65_256k_constants.asm -> ca65_constants.asm

That should actually do it. Run a create-nes-game clean to get rid of existing code, then build your game as normal to get it up and running with its new size. The build should print the new number of banks. You can now put code into banks 7-15!

This will also allow you to use the 256kb InfiniteNesLives SKROM board, if you’d like.

If you want to understand this configuration file or make other tweaks, there is a section at the bottom of this file that explains it in detail.

Updating the configuration manually to resize your rom banks

NOTE: If you did the steps above, you do not need to do this. Just skip to the next chapter. This is here for anyone who wants to understand the configuration file better, and make their own custom rom size that we don’t natively support.

The first thing we need to do is update the config file for the game. (If you are not using git, back up your original file now! You will want a working configuration to go back to if things go wrong.) Open up config/ca65.cfg. This is gonna look a bit foreign, but it’s pretty simple. Here’s the parts we care about:

MEMORY {

    ZP: 		start = $0000, size = $0100, type = rw, define = yes;
    HEADER:		start = $0000, size = $0010, file = %O ,fill = yes;
	ROM_00:		start = $8000, size = $4000, file = %O, fill = yes, define = no;
	ROM_01:		start = $8000, size = $4000, file = %O, fill = yes, define = no;
	ROM_02:		start = $8000, size = $4000, file = %O, fill = yes, define = no;
    ROM_03:		start = $8000, size = $4000, file = %O, fill = yes, define = no;
	ROM_04:		start = $8000, size = $4000, file = %O, fill = yes, define = no;
	ROM_05:		start = $8000, size = $4000, file = %O, fill = yes, define = no;
    ROM_06:     start = $8000, size = $4000, file = %O, fill = yes, define = no;
	PRG:		start = $c000, size = $3c00, file = %O, fill = yes, define = no;
    DMC: 		start = $fc00, size = $03f0, file = %O, fill = yes, define = yes;
    PRG_STUB:   start = $fff0, size = $0010, file = %O, fill = yes, define = no;
    CHR_00:		start = $0000, size = $1000, file = %O, fill = yes;

# ... a lot more CHR_XX entries, some other stuff...

}

SEGMENTS {

    HEADER:   load = HEADER,         type = ro;
    STARTUP:  load = PRG,            type = ro,  define = yes;
    LOWCODE:  load = PRG,            type = ro,                optional = yes;
    INIT:     load = PRG,            type = ro,  define = yes, optional = yes;
    ROM_00:		load = ROM_00,	type = ro, define = no;
	ROM_01:		load = ROM_01,	type = ro, define = no;
	ROM_02:		load = ROM_02,	type = ro, define = no;
    ROM_03:		load = ROM_03,	type = ro, define = no;
	ROM_04:		load = ROM_04,	type = ro, define = no;
	ROM_05:		load = ROM_05,	type = ro, define = no;
    ROM_06:		load = ROM_06,	type = ro, define = no;
    # Stubs that contain reset code to put ourselves into a known fixed-c000 state.
	STUB_00:   load = ROM_00, type = ro, start = $BFF0;
	STUB_01:   load = ROM_01, type = ro, start = $BFF0;
	STUB_02:   load = ROM_02, type = ro, start = $BFF0;
    STUB_03:   load = ROM_03, type = ro, start = $BFF0;
	STUB_04:   load = ROM_04, type = ro, start = $BFF0;
	STUB_05:   load = ROM_05, type = ro, start = $BFF0;
	STUB_06:   load = ROM_06, type = ro, start = $BFF0;

    CODE:     load = PRG,            type = ro,  define = yes;
    RODATA:   load = PRG,            type = ro,  define = yes;
    DATA:     load = PRG, run = RAM, type = rw,  define = yes;
    STUB_PRG: load = PRG_STUB,    type = ro, start = $FFF0;
    CHR_00:   load = CHR_00,            type = ro;
# More CHR_XX entries again, some other stuff
}

# More "feature" stuff ommitted here

SYMBOLS {
    __STACKSIZE__: value = $0500, type = weak; # 5 pages stack
    
    # WARNING: If you change this next variable, you also need to update its sibling _contants.asm file, in the same
    #          folder. The value of SYS_PRG_BANKS must equal the value of NES_PRG_BANKS
    NES_PRG_BANKS: value = 8, type = weak;   # number of 16K PRG banks, change to 2 for NROM256
    NES_CHR_BANKS: value = 16, type = weak;  # number of 8K CHR banks (If using 4k, divide by 2!)
}

Hopefully this doesn’t look too scary. Anyway, we need to update threee things here. First, we need to update the MEMORY section to have all 16 of our banks. We have 8 right now - ROM_00 through ROM_06, then PRG is the last one. Copy the ROM_06 line over 8 more times, and number them ROM_07 through ROM_0E. The other parameters on each line stay the same.

Next, we need to update the SEGMENTS section - we need to do the same type of copying to point our new ROM_07 through ROM_0E to the right places. (Note that there are two places to update in this section.) We also need to do this a 3rd time to create the STUB_07 through STUB_0E entries. These are used to put some code into the rom that is required by the mapper.

After this, in the SYMBOLS section at the bottom, we have to change NES_PRG_BANKS to 16 to reflect our higher bank count.

Finally, there is a file named config/ca65_constants.asm - the value SYS_PRG_BANKS has to be changed to 16 to match the value we set above. (The .asm file explains why this is)

This should do it; you have manually resized your rom. There is more detail on the syntax of the .cfg file here: ld65 documentation