3 ms·

It's been a while since I've looked into it so I hope I won't be too far off the mark: - Regarding high impedance it might be relevant if you want to explain

by simias 8y ago

It's been a while since I've looked into it so I hope I won't be too far off the mark: - Regarding high impedance it might be relevant if you want to explain how the controller and memory card interface works, and why you read full 1s when nothing is connected (high impedance line with a pull-up resistor). More generally the issue will crop up every time you have an interface that might be floating, I think people who have no background in electronic engineering might assume that unconnected wire == 0 on the line. - Regarding metastability it crops up in the timers. The PSX timers can either run from the CPU clock or from signals coming from the GPU (pixelclock and horizontal blanking). IIRC this is used in some lightgun games to be able to synchronize the signal coming from the gun ("I saw the lightbeam") with the current position of the pixel in the frame currently being sent to the TV set. The problem with that is that the signals coming from the GPU are not synchronous with the CPU and apparently are not resynchronized. That creates metastability issues when the value from these timers are read from the CPU and you may end up reading bogus values from time to time. The software workaround is to read the register values in a loop until you get twice the same value in row. Now, you probably don't need to emulate that but if you want to be thorough it's probably worth pointing out. So in summary you're right, you don't really need to know that in order to write a proper PSX emulator but if you really want to get into all the details you'll probably want to brush the subject. At least these are concepts that anybody is sure to encounter eventually is they spend time in bare-metal land...

vardump 8y ago
It's conceivable that metastability issues could be used as a part of copy protection. Well, protection against emulation more like. Of course it's not the smartest choice for that purpose, because there might be chip-to-chip differences. And environmental factors like temperature could affect it as well. But who knows what people wrote after Bleem PS1 emulator was public? So have metastability issues (or other HW bugs) ever been used for copy/emulation protection?
- hcs 8y ago
  There are copy protection schemes on magnetic disks that mess with the clock bits on the disk to produce an unstable value: https://retrocomputing.stackexchange.com/a/7853 https://retrocomputing.stackexchange.com/a/7853
- simias 8y ago
  Most (all?) copy-protection schemes on the PSX had to do with the CD subsystem. I think they were mainly attempting to defeat hardware "modchips" used to play non-original copies of games. Obviously those are trivial to bypass in an emulator. I'm not aware of any copy-protection scheme on the console that would target specifically emulators. I guess Bleem was not big enough a threat to warrant specific protections? Besides I expect that Bleem, in order to run full speed on the underpowered hardware of the time, must have had terrible accuracy and therefore must have employed a wealth of game-specific hacks to get around issues. As such I expect that if they had decided to emulate your game they wouldn't have had too many issues reverse-engineering any trivial metastability-detecting code to implement an ad-hoc hack.
anyfoo 8y ago
Ah, thanks. Now that you mention it, high impedance data lines ("open bus") do play a role in other platforms as well. Even more so, on the C64 for example some undocumented instructions cause conflicting drivers to drive the same lines and produce randomish/unstable outcomes. And nice to know that there are indeed metastability issues that crop up very visibly in the PlayStation!

by simias 8y ago

vardump 8y ago

It's conceivable that metastability issues could be used as a part of copy protection. Well, protection against emulation more like. Of course it's not the smartest choice for that purpose, because there might be chip-to-chip differences. And environmental factors like temperature could affect it as well. But who knows what people wrote after Bleem PS1 emulator was public? So have metastability issues (or other HW bugs) ever been used for copy/emulation protection?

hcs 8y ago
There are copy protection schemes on magnetic disks that mess with the clock bits on the disk to produce an unstable value: https://retrocomputing.stackexchange.com/a/7853 https://retrocomputing.stackexchange.com/a/7853
simias 8y ago
Most (all?) copy-protection schemes on the PSX had to do with the CD subsystem. I think they were mainly attempting to defeat hardware "modchips" used to play non-original copies of games. Obviously those are trivial to bypass in an emulator. I'm not aware of any copy-protection scheme on the console that would target specifically emulators. I guess Bleem was not big enough a threat to warrant specific protections? Besides I expect that Bleem, in order to run full speed on the underpowered hardware of the time, must have had terrible accuracy and therefore must have employed a wealth of game-specific hacks to get around issues. As such I expect that if they had decided to emulate your game they wouldn't have had too many issues reverse-engineering any trivial metastability-detecting code to implement an ad-hoc hack.

anyfoo 8y ago

Ah, thanks. Now that you mention it, high impedance data lines ("open bus") do play a role in other platforms as well. Even more so, on the C64 for example some undocumented instructions cause conflicting drivers to drive the same lines and produce randomish/unstable outcomes. And nice to know that there are indeed metastability issues that crop up very visibly in the PlayStation!