FAQ: Difference between revisions

 

Homebrew developers should anyway carefully consider going past 64 MiB with their ROMs. In fact, most emulators do not support ROMs beyond that limit (even though the modification is expected to be quick once they are made aware that it is not a hardware limit), and flashcarts commonly used to play home-brew productions on hardware (such as EverDrive 64 or SummerCart 64) only have about 64 MiB of SDRAM to keep ROM contents, so they do not support larger ROMs as well.

 

=== Would it be possible to create a larger RAM expansion for N64, to go beyond the total 8 MiB we can get today with the Expansion Pak? ===

 

The RDRAM initialization is performed by [[IPL3]], a piece of the Nintendo 64 secure boot code (there are a few variants to its contents but the differences are not related to RAM management). IPL3 does the RDRAM chip initialization using a process called "current calibration", and then map them into the (phyisical) address space, by giving to each chip its own address. Nintendo's IPL3 can correctly handle up to 4 2-MiB chips (it also has buggy support for 1 MiB chips, which probably were never released by Rambus, so the code was never tested).

For a long time, it was then believed that changing IPL3 would be enough to allow more chips to be mapped, assuming somebody built an expansion pak card with more chips in it. Instead, late work on [https://github.com/DragonMinded/libdragon/tree/preview/boot Libdragon's open source IPL3]brought more reverse engineering of the RI chip, which eventually proven that RI *internally* runs a state machine that tracks RDRAM bank status, and only has enough room for 8 1-MiB banks (each 2-MiB chip is logically made of 2 1-MiB bank). More details can be found in the [[RDRAM Interface#Bank Status Tracking]] wiki section.

 

On top of this, Rasky (who authored Libdragon's IPL3) also got the chance to test a custom made expansion pak with 8 MiB of RAM in it, designed by LambBraink. This pak uses donor chips from official expansion paks, and is correctly recognized by Nintendo IPL3, though just as a 4 MiB pak. Anyway, by hacking Libdragon's open source IPL3, it can be seen that the extra banks were indeed present on the bus, but it turned out to be inaccessible after bus mapping. This is further proof that RI is not actually able to handle more than 8 MiB of RAM.