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Originally Posted by Megalomaniac
Apologies, the way the Wikipedia article about the Amstrad Plus is written makes it sound 100% compatible, as it defaults to standard CPC mode and the Plus features need specifically accessing in a way that can't be done accidentally. How close would you say it is - better than A500+ / STe / Spectrum +2A? Which games do have issues running on the Plus, and why? Might be worth editing the Wiki article to clarify that?
It certainly is aesthetically beautiful, in part because the design is so close to an A500. Actually, if you asked ChatGPT to create an Amiga with a tape deck, that's probably what it would concoct.
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I'd put it in the same sort of category as the A500+, STe and +2A for sure. In that it's not actually as bad as some made out at the time, but owning one you'd almost certainly run in to at least some compatibility issues.
It's not actually the "Plus" parts of the hardware that are the issue, the locking mechanism on them works pretty well and I've certainly never come across anything that accidentally enables the additional hardware registers.
The number one issue you tend to encounter is keyboard scanning. It's a messy business on the CPC as it's connected to the AY sound chip, which itself is accessed via an 8255 PPI. Amstrad produced documentation for the original CPC which specified the exact process for reading the keyboard (and joysticks), but a lot of developers found that you could cut out certain steps and read the keyboard quicker. With every cycle being squeezed, that kind of code became common. But the Plus emulated the 8255 inside the ASIC chip and that emulation is a lot more fussy - the end result being software that just fails to read key presses.
Lesser issues were timing issues with the lower power disc drives in the 6128+. Didn't cause an issue with AMSDOS, but software that hit the disc controller directly to do faster or fancier track loading could fail under certain circumstances. Then there were minor issues around how the 27 colour palette is almost, but not quite the same 27 colours, or little things like the exact moment a palette change takes effect being just ever so slightly different.
There were also a few inevitable quirks with external hardware devices like memory expansions or multiface style hacking devices, which might work most of the time when using only old software but don't necessarily behave correctly once the new hardware comes is enabled.
Finally there were CRTC differences in edge cases too, although given there were already 4 different variants of CRTC in use, it's inevitable the result wouldn't be compatible with all of them. The ASIC emulation does at least fall into the "good" set of CRTCs and you get the benefit that, when writing Plus only software, you know they all have the same CRTC and thus which tricks will work (though the new features of the hardware mostly make such tricks unnecessary).