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69 lines
2.6 KiB
Markdown
69 lines
2.6 KiB
Markdown
[< back to index](../doc_index.md)
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# Famicom/NES programming guide
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## Program lifecycle
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The default Famicom vectors are defined as following:
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* on reset, the predefined `on_reset` routine is called, which in turn calls `main`.
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The `main` routine is not allowed to return, or the program will crash.
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* on NMI, the default interrupt handler calls the `nmi` routine.
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It should not be defined as `interrupt`, the handler is, so your routine shouldn't.
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* on IRQ, the default interrupt handler calls the `irq` routine.
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It should not be defined as `interrupt`, the handler is, so your routine shouldn't.
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The minimal Famicom program thus looks like this:
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void main() {
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init_rw_memory()
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// initialize things
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while(true) { }
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}
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void irq() {
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// do things
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}
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void nmi() {
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// do things
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}
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## Mappers
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To use a mapper of your choice, create a new `.ini` file with the definitions you need.
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The most important ones are `[output]format` and `[allocation]segments`.
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Currently, its a bit inconvenient to create programs using mappers that change the bank containing the interrupt vectors.
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Therefore, it's recommended to stick to mappers that have a fixed bank at the end of the address space.
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Mappers that should be fine: NROM (0), CNROM (1), UxROM(2), MMC2 (9), MMC3 (4), MMC4 (10), MMC6 (4).
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Mappers that can have arbitrary bank at the end are therefore not recommended: MMC1 (1), MMC5 (5).
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You should define at least three segments:
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* `default` – from $200 to $7FF, it will represent the physical RAM of the console.
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* `chrrom` (sample name) – from $0000 to $1FFF, it will represent the CHRROM
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(if you need more, you can make it bigger, up to $ffff, or even add another segment of CHRROM).
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Put there only arrays with pattern tables. Don't read from them directly, it won't work.
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* `prgrom` (sample name) – it will contain the code of your program and read-only data.
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Each segment should be defined in a range it is going to be switched into.
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You should set the `default_code_segment` to the segment that contains the $FFxx addresses.
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If your mapper supports it, you can add more CHRROM or PRGROM segments,
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just specify them correctly in the `[output]format` tag.
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The `[output]format` tag should contain a valid iNES or NES 2.0 header of the mapper of your choice
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and then all the segments in proper order (first PRGROM, then CHRROM).
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See [the MMC4 example](../../include/nes_mmc4.ini) to see how it can be done.
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See [the NesDev wiki](https://wiki.nesdev.com/w/index.php/NES_2.0) for more info about the NES 2.0 file format.
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