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Update 8/29/2006: Check out this article on the XBOX 360 backward compatability as it explains some of the issues with emulation of another hardware. Update 8/5/2006: Sony just announced on its SCEA PS3 website that they will support the transfer of PS1/PS2 game saves to the PS3. Update 6/7/2006: Sony commented the speculations on the PS2 in a PS3, that it "won't contain the entire PS2 architecture, as some of the online stories have alleged". That leaves some room for speculation as the hybrid approach EE in hardware, GS in software would sill be possible. Update 6/5/2006: just after finishing my note here someone else came up with the hardware idea ;-)  One feature of the PS3 that has been announced since E3 05 is the PS2/PSone compatibility which enables loyal customers to take their investment onto the PS3. Despite the statements that for instance Microsofts Peter Moore just recently made regarding backward compatibility, it is definitly a major advantage to play those PS2 and PSone classics on the PS3. What makes this feature even more valuable is the fact that those games will be finally playable in HD resolution.
What's really surprising though is that not much has been reported so far on how this feature actually works. The only article on the net recently cited a unknown SCEE engineer working on the PS3 backward compatibiliy feature. The gentlemen mentions that Sony is using a software emulation to bring both PS2 and PSone games to the PS3. By the time a whole lot of games are already running on the PS3 with more and more games on the way. This is backed up by the fact that SCEE was actually looking for a software engineer last year to work on a emulation project. PSone Backward Compatibility So, let's look a little closer at this. Backward compatibility is nothing new for PlayStation. The first machine that actually was in need of this was the good old PS2. By the time the PS2 came out there where already a good selection of PC emulators available that where able to actually run original PSone games at higher speed and resolution. The best PSone emulator today is most probably ePSXe. But instead of using software only to emulate the PSone Sony decided to actually build part of the PSone hardware into the PS2, using the PSone MIPS CPU as a device to handle PS2 IO under normal operations. With a combination of hardware & software Sony achieved almost 100% backward compatibility (with only a few exceptions). This time it's not going to be that easy. Microsoft overpromissed and underdelivered Looking at the mess that Microsoft has to deal with on the XBOX 360, you can imagine what it takes to really get the PS2 into PS3. Microsoft managed to get a list of about 200 games up and running at launch day, which has been only slightly extended since then. Not enough to call the XBOX 360 fully backward compatible. The bad thing about the approach Microsoft had to take is, that almost every game needs to be migrated by providing a specific emulator for the game. Why is this? Basically, Microsoft had two major challenges to overcome: moving from Intel to PowerPC and from nVidia to ATI. Even though the XBOX is using a small OS and libraries to actually access the hardware, a lot of games are actually written to the hardware as such making an emulation a hell of a job. The PS2 ArchitectureSo, what about the PS2 then? First you have to understand a little of the internals of the PS2 (a good overview can be found here or as part of the PS2 Linux kit) Basically the PS2 consists of 7 logical units (EE, VU0, VU1, IPU, DMA, IOP, GS) which are by now highly integrated on the latest slimline PS2. Non of those chips is actually very complicated compared to todays hardware.  | | PS2 Emotion Engine (source: arstechnica.com) |
The EE (Emotion Engine) is a MIPS III/IV based chip that also contains the VU0, VU1 vector units for geometry data, the IPU image processing unit for image and video decompression and the DMA unit to handle all direct memory access. The GS (Graphics Synthesizer) on the other hand is a very simple GPU (if you can call it like that) that acts as a framebuffer and has only a set of simple drawing commands. Finally the IOP is the the aforementioned PSone compatible chip doing IO processing in PS2 mode. Looking at the GS it is worth mentioning that there are no such things as pixel or vertex shaders nor any other complex rendering functionality you will find today in modern GPUs. The ChallengeEven though the PS2 is fairly simple compared to the first generation XBOX, its flexible hardware architecture made this console very powerful in the hands of an experienced programmer. With its two vector units and the CPU a programmer has many different possibility to define data path thru those units being able to overcome hardware deficencies thru software. The PS2 is a pure data stream processor build to handle multimedia data. When it comes to emulate the PS2 the real challenge is not the emulation of the mentioned 7 functional units. Those are very simple from a functional point of view and in some cases can be mapped really nicely to the Cell chip with its SPUs. What really makes this task a challenge is the amount of timing issues that need to be solved. As the PS2 hardware is highly parallel in terms of EE and the both vector units, game developers optimized their code to the max squeezing every friggen cycle out of those units. Even though the PS2 is a real challenge for an emulator developer (so far no full fledged PS2 emulator is available) the task is for sure not as complicated as with the XBOX. If Sony is able to solve the timeing issues, the PS2 will be back for good on the PS3 and even better then in the old days. Hardware or Software?Even though a software emulation is the most likely scenario Sony is using on the PS3, it is still possible that Sony actually puts the Emotion Engine on a chip into the PS3 as it has been done before on the PS2. Given the integration that is today possible, it wouldn't be a major price addon but would definitly give almost 100% compatibility. The Graphics Synthesizer in this scenario would still be emulated in software. This would enable the rendering of the games in HD thru the PS3 RSX chip. Btw, Sony itself made a little disclaimer itself when talking about the backward compatibility, as only games will be supported that are compliant to their TRC (Technical requirements Checklist) In other words, we will not know for sure until November, as more information will be provided. So, lets cross fingers and hope that the original announcement will become reality.
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