NES power-on self-test (POST)

[AWal]

I really like the look of this mock-up. The concept of an aftermarket BIOS is always an interesting topic.

I could see a serial loader handy for an ISP upgrade on a supported game or to test code using a RAM Cart (or just the onboard WRAM), but those of us with access to CopyNES hardware, or the capital to invest in one most likely already have a tool (or several ) for those purposes.

Speaking of CopyNES, I remember the technical description explaining that CopyNES sits in what is normally mirrored memory space. I could see it benefitial to have the code here do a similar thing upon startup (point reset vector, run code in mirrored memory gone ROM). Once booting to cartridge, the ROM would be disabled and could remain compatible with existing carts, or could check for a 'compatible' ROM header and stay resident.

The onboard VRAM could probably be further mirrored for the CHR-RAM if economical (a la Magic Floor). Again this would be disabled on cartridge start, but could be handy for cart free WRAM loading.


If the POST detects a cartridge, showing a (FamicomBox) game title would be nice (if available).

[alphamule]

Technically speaking, something like 90% of POST happens before there's even a video on PCs. I mean, if you were trying to simulate a PC BIOS-stlye Power-On Self-Test.
Essentially, all you need if doing the POST part is to run through some stuff specific to NES such as VRAM/WRAM test, showing checksum of BIOS (if used, or just the ROM bank permanently banked), etc. You'd want to do some tests early on before you even try video, like say the opcode test since if the documented instructions aren't correct, you might never even get video.

Nothing keeps one of us from making a more basic "panic on any error with a number/color on screen" ROM of our own. Good luck with this!

[Joe]

Technically speaking, something like 90% of POST happens before there's even a video on PCs.

Perhaps with modern consumer-grade hardware. Back in the eighties and early nineties, it wasn't uncommon for PCs to take a while running a memory test after the video card had been initialized. Depending on the speed of the hardware, you could end up waiting for upwards of 30 seconds (unless you got impatient and skipped it, of course).

If you want a more modern example, pick any of SuperMicro's server motherboards. They all take forever to boot.

[alphamule]

All the BIOSes I saw did the extensive memory test after they showed video. Identifying installed RAM is a different issue. You're essentially detecting mirroring or inconsistent (open bus, different value than written) reads. In theory, this could be done quite quick.

Edit: Oh heck, thought you were saying that the did it before showing video. :/

[tepples]

Can you reliably measure the CPU frequency with code or will you display wrong information to the people who have overclocked their consoles?

See if this overclock test ROM answers your question. Once I get other pieces made (like controller ID) then I can work on the POST ROM.