Recently [Colin Leroy-Mira] found himself slipping into a bit of a rabbit hole while investigating why only under Apple II MAME emulation there was a lot of flickering when using the (emulated) Apple II MouseCard. This issue could not be reproduced on real (PAL or NTSC) hardware. The answer all comes down to how the card synchronizes with the system’s vertical blanking (VBL) while drawing to the screen.

The Apple II MouseCard is one of the many peripheral cards produced for the system, originally bundled with a version of MacPaint for the Apple II. While not a super popular card at the time, it nevertheless got used by other software despite this Apple system still being based around a command line interface.

According to the card’s documentation the interrupt call (IRQ) can be set to 50 or 60 Hz to match the local standard. Confusingly, certain knowledgeable people told him that the card could not be synced to the VBL as it had no knowledge of this. As covered in the article and associated MAME issue ticket, it turns out that the card is very much synced with the VBL exactly as described in The Friendly Manual, with the card’s firmware being run by the system’s CPU, which informs the card of synchronization events.