PSX compatibility

[Blyss Sarania]

Oh also, PCSX2 is much closer to booting PSX games than you think.

We were tinkering last night since this thread, and it's like this. Right now there is a part in the CDVD reader that outright rejects PSX games. But if you remove that and use boot full it will boot about 3/4 way through PSX BIOS(which btw a version of PSX BIOS is embedded in PS2 BIOS so you don't need a separate one) before hanging. It even will play the PSX BIOS sound before hanging.

[Blyss Sarania]

Triple post, sorry.

Here is a real video of PSX game running in PCSX2. Well, trying to run. It mostly executes the PSX BIOS, plays the sound, and hangs for unimplemented opcodes. To get this far you have to remove the CDVD code that rejects PSX as I said above.

[Marge Simpson]

Oh also, PCSX2 is much closer to booting PSX games than you think.

We were tinkering last night since this thread, and it's like this. Right now there is a part in the CDVD reader that outright rejects PSX games. But if you remove that and use boot full it will boot about 3/4 way through PSX BIOS(which btw a version of PSX BIOS is embedded in PS2 BIOS so you don't need a separate one) before hanging. It even will play the PSX BIOS sound before hanging.

I know the PS2 has a custom PS1 BIOS embedded into it.

Anyways, what exactly is contained with the PS1 BIOS as far as its contents in the PS2 BIOS? Is it the full BIOS actually put into it, or is it the more probable concept of containing enough BIOS that links it to the PS2 BIOS and boots into PS1 mode?

[Blyss Sarania]

It doesn't contain enough stuff to like be usable in PSX emulators(we've done tests before extracting it and trying that). But it should be usable in PCSX2 since PCSX2 is actually trying to replicate PS2 and not PSX behavior.

When you put a PSX disc in the PS2, the PS2 BIOS transfers main system control to the IOP(which is the R3000, e.g. the PSX main CPU. In PS2 mode it's used for input/output management, hence "IOP") and the EE goes into a kind of low powerish, not doing much mode. The SPU in PS2 is literally just two chips of the PSX SPU, so in PSX mode one is disabled, and the other one used per normal. The PS2 practically has an entire PSX in it, and more importantly, those chips aren't just for backwards compatibility - they are used in PS2 mode as well. As a result of that, we are already emulating them. It's just that, we have no timing on the IOP(PS2 games don't really care. PSX games obviously do since that's their main CPU) and a few other accuracy issues that stop us from running PSX games.

[Marge Simpson]

It doesn't contain enough stuff to like be usable in PSX emulators(we've done tests before extracting it and trying that). But it should be usable in PCSX2 since PCSX2 is actually trying to replicate PS2 and not PSX behavior.

When you put a PSX disc in the PS2, the PS2 BIOS transfers main system control to the IOP(which is the R3000, e.g. the PSX main CPU. In PS2 mode it's used for input/output management, hence "IOP") and the EE goes into a kind of low powerish, not doing much mode. The SPU in PS2 is literally just two chips of the PSX SPU, so in PSX mode one is disabled, and the other one used per normal. The PS2 practically has an entire PSX in it, and more importantly, those chips aren't just for backwards compatibility - they are used in PS2 mode as well. As a result of that, we are already emulating them. It's just that, we have no timing on the IOP(PS2 games don't really care. PSX games obviously do since that's their main CPU) and a few other accuracy issues that stop us from running PSX games.

I thought that was how the PS2's PS1 mode worked!

Oh and AFAIK the PlayStation logo is actually a model on the PS1 disc, normally stored in its executable, so getting the logo to load is getting the disc to load too.

[Nobbs66]

The BIOS is failing on GPU initialization.


Also, the PS2 is missing both the PSX GPU and GTE. We really need a capable PSX dev who is willing to help out.

[Blyss Sarania]

Yeah I actually don't know how the PSX GPU works in the PS2?

Is it soft emulated on the EE? Or done part EE part GS?

[Nobbs66]

I don't know.

[Marge Simpson]

Yeah I actually don't know how the PSX GPU works in the PS2?

Is it soft emulated on the EE? Or done part EE part GS?

To be honest it's likely EE-based. I know that PS1 games under the PS2 run at their native resolution (prolly 320x240/288 or possibly 640x480), likely outputted at the PS2's native 640x448/640x512 (?) resolution then fit into a 720x480/720x576 image.

[Blyss Sarania]

They run and are output at their native resolution.

GSdx is also a PS1 emulator GPU plugin, so in theory the proper code for that is there, although I am not sure if it's exposed to PCSX2 or not.

So that basically leaves the IOP timing and GTE as issues.