Famicom/NES programming guide

Program lifecycle

The default Famicom vectors are defined as following:

on reset, the predefined on_reset routine is called, which in turn calls main. The main routine is not allowed to return, or the program will crash.
on NMI, the default interrupt handler calls the nmi routine. It should not be defined as interrupt, the handler is, so your routine shouldn't.
on IRQ, the default interrupt handler calls the irq routine. It should not be defined as interrupt, the handler is, so your routine shouldn't.

The minimal Famicom program thus looks like this:

void main() {
    init_rw_memory()
    // initialize things
    while(true) { }
}

void irq() {
    // do things
}

void nmi() {
    // do things
}

Mappers

To use a mapper of your choice, create a new .ini file with the definitions you need. The most important ones are [output]format and [allocation]segments.

Currently, it's a bit inconvenient to create programs using mappers that change the bank containing the interrupt vectors. Therefore, it's recommended to stick to mappers that have a fixed bank at the end of the address space.

Mappers that should be fine: NROM (0), CNROM (1), UxROM(2), MMC2 (9), MMC3 (4), MMC4 (10), MMC6 (4).

Mappers that can have arbitrary bank at the end are therefore not recommended: MMC1 (1), MMC5 (5).

You should define at least three segments:

default – from $200 to $7FF, it will represent the physical RAM of the console.
chrrom (sample name) – from $0000 to $1FFF, it will represent the CHRROM (if you need more, you can make it bigger, up to $ffff, or even add another segment of CHRROM).
Put there only arrays with pattern tables. Don't read from them directly, it won't work.
prgrom (sample name) – it will contain the code of your program and read-only data.
Each segment should be defined in a range it is going to be switched into. You should set the default_code_segment to the segment that contains the $FFxx addresses.

If your mapper supports it, you can add more CHRROM or PRGROM segments, just specify them correctly in the [output]format tag.

The [output]format tag should contain a valid iNES or NES 2.0 header of the mapper of your choice and then all the segments in proper order (first PRGROM, then CHRROM). See the MMC4 example to see how it can be done.

See the NesDev wiki for more info about the NES 2.0 file format.