FAQ: Difference between revisions
no edit summary
No edit summary |
No edit summary |
||
Line 11:
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? ===
'''Short answer:''' Currently''',''' it is thought to be impossible because of a physical hardware limit of the RDRAM controller (RI) within the RCP chip.
The RDRAM chips are connected to the RCP via a bus called RAMBUS. This bus allows to connect multiple chips to a controller; the controller can then talk to each chip and configure it to reply to a certain range of addresses (that is, "map it" into a memory map).
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. The code in IPL3 is ready to handle 1 MiB and 2 MiB chips, but it does currently map only enough chips until 8 MiB is reached.
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. This is made difficult because IPL3 contents are verified through a hash and are part of the secure boot chain, so even if the hash can now be bruteforced with GPUs (and has been done a few times as proof of concepts), nobody has still written and released an open source IPL3 boot code to tinker with.
This notwithstanding, Mazamars312 has done investigations while implementing his N64-on-FPGA system, and has reported that, even if the chips are mapped to more than 8 MiB, it seems like the RCP chip itself (specifically, the RI subsystem which is in charge of converting VR4300 memory accesses to RAMBUS packets) is physically limited to managing requests in the first 8 MiB range. That is, even if the VR4300 generates accesses beyond 8 MiB, it refuses to generate the corresponding RAMBUS packets to talk to the chips that have been mapped there. In other words, the 8 MiB limit seems hardcoded in the RCP chip. If there is a way to unlock this limit via RI hardware registers, it is not known at this time.
The tests Mazamars312 conducted are currently not reproducible with open source source and tools. Libdragon is planning to eventually release an open source IPL3, at which point it would be easier for others to experiment with the RDRAM initialization sequence.
| |||