At the moment, only the RetroArch CPC emulator has a proper rewind feature: [https://youtu.be/YwilYlSe5LQ Demonstration of rewind in RetroArch]
To reverse the game, users press and hold the rewind button. And at any point, releasing the button makes time goes forward again.
A very simple approach would be to make a SNA for each frame and limit the RAM snapshot to 128KB, which is ok for 99% of CPC software.
(But way more efficient methods certainly exist as the GBA emulator [https://youtu.be/Sfc_1NKbiKg SkyEmu demonstrated super long rewind] while needing very few RAM)
To reverse the game at 1x speed, users press and hold the rewind button. And at any point, releasing the button makes time goes forward again. For debugging, this method lets you do things like go going back to the previous instruction (is easily done if the emulator stores the time information is kept) and in when that instruction was executed. In the same way , emulators can provide access to the previous HBL, IRQ, VBL, etc.
= Emulator accuracy checking tools =