Open source IPL3 alternative

[parasyte]

cargo-n64 does not ship with an IPL ROM for copyright reasons, even though this code is required to cold boot a Nintendo 64. We should add a default open source IPL3, preferably one with MIT license (or equivalent).

I know of two open source IPL3s:

1. https://github.com/PeterLemon/N64/tree/master/BOOTCODE It is unlicensed and looks like a direct copy of 6102.bin with some minor adjustments.
2. https://github.com/pseudophpt/pseultra/blob/master/n64/boot/src/boot.sx Minimal implementation. Appears to be incomplete.

The biggest challenge is that IPL2 authenticates IPL3 with the PIF. Exact details of PIF authentication are out of scope for cargo-n64, but there is more on the topic in the links below:

• https://n64brew.dev/wiki/Peripheral_Interface#Console_startup
• Implementation of the IPL2 checksum algorithm: https://github.com/pseudophpt/pseultra/blob/master/tools/bootcsum/src/bootcsum.c
• Implementation of the CIC-side (which sends the precomputed checksum to the PIF through its side-channel ... i.e. this data is inaccessible from software): https://github.com/jago85/UltraCIC_C/blob/3450b4403a1df190b9abb2dbe071ce07a546179b/cic_c.c#L44

This means that the easiest way to workaround the IPL2 checksum is including a custom CIC implementation with the IPL3 code that sends the appropriate precomputed checksum to the PIF.

Note that the IPL2 checksum is not a problem on emulators or on carts like 64Drive and EverDrive64. These carts include an UltraCIC implementation and boot into a menu (using CIC-NUS-6102).

The 64Drive has a special "USB mode" that makes the cart act like a bare N64 cart with a selectable CIC when powered on with a USB cable plugged in. When running a ROM created with a custom IPL3 in USB mode, it is necessary to reprogram the CIC emulation on the 64drive with the appropriate precomputed checksum. Unfortunately, this is not currently supported by 64drive.

[moparisthebest]

Does https://github.com/hcs64/boot_stub meet the need? It doesn't have a license, but the comment says it's dedicated to the public domain.

[parasyte]

Unfortunately, that uses the RDRAM and cache init code from the official IPL3. But it's another one to add to the list. Thanks for the heads up.