ca65 programming tricks: Trimming and merging tileset assets

Don’t want to copy, paste, trim, split or concatenate binary files (such as tilesets and other rodata, or precompiled programs) manually in your NES project?

if using ca65, you’ll be happy to know that the .incbin directive is rather versatile. You can add an offset and a specified length of bytes you want to use, like so:

.incbin ”myTileset.chr”, 0, 3328
.incbin ”myOtherTileset.chr”, $D0 * $10, 768

Since tilesheets are a common target for the .incbin directive, it may be easier to multiply any parameter by $10 if you’re counting tiles, rather than bytes. (one tile = 16 bytes = $10 bytes in hexadecimal).

Here, it was used to provide a reusable method to paste 3 rows of HUD elements from one level sheet and the rest of the other level sheet, making a new one.

Okay, so the practical example is a bit unwarranted since you can achieve the same in one line in bash on linux or powershell on windows.. But it is still convenient to know how to do this inside a project, and you can of course replace these magic numbers with defined constants, make the cc suite compile assets for you from smaller modules, and so on.

You can download the zip here, containing the .bat, .cfg and .s files that make up the example. The graphics have been replaced with something that’s just there to make it work. Naturally, you need to edit paths to fit your environment/preference.


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