mirror of
https://github.com/KarolS/millfork.git
synced 2024-05-29 04:41:30 +00:00
69 lines
2.6 KiB
Markdown
69 lines
2.6 KiB
Markdown
[< back to index](../doc_index.md)
|
||
|
||
# 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](https://github.com/KarolS/millfork/blob/master/include/nes_mmc4.ini) to see how it can be done.
|
||
|
||
See [the NesDev wiki](https://wiki.nesdev.com/w/index.php/NES_2.0) for more info about the NES 2.0 file format.
|
||
|
||
|
||
|