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In my opinion, the PS2 remains the best way to experience original PlayStation 1 games, short of an actual PS1 console. The ability of the PS2 to mimic/emulate the PS1 hardware seems really good, despite the modest hardware of the PS2 (by comparison to a modern computer doing emulation). This makes me think there is little in the way of software-based emulation involved in the PS2 case.

As far as I know, the PS1 had its unique CPU, I/O, GPU, video codec, and SPU hardware components. I've read conflicting stories about how much of this hardware is wholesale included in the PS2, and especially in the reduced-cost PS2 Slim models. What is the definitive answer on which PS1 hardware is actually present and what is emulated by PS2 software, if anything? And how does this relate to the PS1 being so well mimicked by the PS2?

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    There is this cuestion where you will find answers. But I don't know whether or not it's a dupe.
    – Omar and Lorraine
    Commented Oct 13, 2022 at 16:13
  • @OmarL That question is useful but I'm looking for a lot more in the way of Playstation specific information.
    – Brian H
    Commented Oct 13, 2022 at 18:03
  • I worked on PS1 and PS2 game development during the period before the PS2 was officially released. It's been a long time, but as I recall the PS2's sound processor was basically a nearly-complete PS1.
    – bmow
    Commented Oct 25, 2022 at 0:06

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Calling it an emulation might be selling it under value. There is way more upward compatibility included than just software.

Essentially the PS2 contains a full PS1 subsystem sans the GPU. It might be better to think of the PS2 as like a Macintosh 630 with a 486 DOS card, having all the hardware of a PC except for the video card. With its new GPU being a superset of the PS1 GPU and a much faster main CPU handing the GPU commands in time, including scaling and improving wasn't rocket science.

For development history, there's a nice write up from the main(?) developer of the compatibility mode software (Here's an English translation).

... really good, despite the modest hardware of the PS2 [...]

It's still about 8-10 times the bare CPU speed, not to mention a more than capable but compatible CPU, but ...

As far as I know, the PS1 had its unique CPU, I/O, GPU, Video codec, and SPU hardware components.

True, and the PS2 included exactly that PS1 CPU as I/O processor, as well as the Sound Processor as SPU#2, both reducing the effort by far.

The I/O Processor was exactly the same design as the PS1, including Geometry Transformation Engine (GTE) and Motion Decoder (MDEC). It also had its own 2 MiB RAM, doubling as PS1 RAM during 'emulation'. To work cycle-accurate the I/O processor was clocked down from 37.5 MHz in PS2 mode to 33.8688 MHz in PS1 mode. The I/O Processor also had his own link to the PS1 compatible SPU#2.

There was no need to create an emulation or even just a translation layer for CPU, GTE, MDEC or SPU related code. That already relieved all constraints of emulating either component, leaving only the graphics complex to be done.

Here, a software layer running on the main PS2 CPU intercepts all commands and data handed from PS1 CPU to PS1 GPU and hands it to the PS2 GPU - possibly with parameter translation and additional data in case of upscaling and/or additional filtering for better graphics.

This leaves only non timing critical components like controller-I/O to be translated. Sure, for a human player a millisecond may be fast and important, for a 300 MHz CPU it's nothing of concern :) Same goes for save cards and other additions.

Here BTW lies the other part where the PS2 can improve PS1 games: Loading speed. The PS2 can read about 12 times faster from CD-ROM than the PS1 (24x vs. 2x). IIRC the PS1 mode had a feature to disable this as some games wouldn't work proper when data came too fast.

(But SCPH-7500x ...)

While the first PS2-Slimline version is 'only' a more compact package of the original model, its later (2005) V14 (SCPH-7500x) incarnation is a new hardware design replacing several hardware components by new implementations or complete different ones. For example the I/O-Processor was replaced by a complete different architecture.

Since the original I/O-CPU was at the core of the emulation it needed to be replaced by one running on the main CPU. Thanks to code compatibility (the PS2's R5900 is mostly upward compatible to the PS1's R3000) and it's way higher speed (300 MHz vs. 34 MHz) a workable emulation in Software was possible.

The hardware (and software) change wasn't a perfect one, resulting in a large number of troubled games. Not just many (>150) PS1, but as well PS2 titles.

Bottom line, the Slimline PS2 models of V14 (SCPH-7500x) and later are a complete different machine. less compatible than the PS2 before.

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  • I've only ever seen the PS2 output 240p and 480i for PS1 games; no upscaling. I think the PS3 does upscale it's PS1 emulation.
    – Brian H
    Commented Oct 13, 2022 at 17:27
  • Also, this sounds like a wriggle, if Wikipedia is to be trusted: "[I/O processor] replaced with the PowerPC-based "Deckard" IOP with 4 MB SDRAM starting with SCPH-7500x."
    – Brian H
    Commented Oct 13, 2022 at 17:31
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    @BrianH Mind to explain what 'wriggle' is supposed to mean? You asked about PS2, not the later models like the Slim. Which is as well a cost reduced version and did in fact do a partiality emulation on the PS2 CPU. Not as hard, as it's for most parts compatible (can run most code direct) as well as fast enough to emulate the missing bits. Except, doing so resulted in a large number of incompatible PS1 games, plus a lot of non working PS2.
    – Raffzahn
    Commented Oct 13, 2022 at 17:44
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    @BrianH In general, I would think that testimonials of people involved weight at least as much some subclause on a Wiki entry.
    – Raffzahn
    Commented Oct 13, 2022 at 17:53
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    @BrianH No, the PPC is not emulating the MIPS 3000 at all. It's a complete new and different emulation running on the main CPU - AFAIK. And yes, I did go for the PS1, not later models for the simple reason as the question would be way to broad as it would ask for two different machines architectures under one label.
    – Raffzahn
    Commented Oct 13, 2022 at 18:11

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