Mystery Solved: Apple II MouseCard's VBL-Synced Interrupts

This post delves into the synchronization mechanism between the Apple II MouseCard's IRQ interrupts and vertical blanking (VBL). The author initially observed flickering in MAME emulation but smooth rendering on real hardware. Through community interaction and analysis of the MouseCard firmware, the mystery was solved: the MouseCard doesn't directly receive the VBL signal but uses software on a 68705 microprocessor to precisely calculate and trigger interrupts, achieving VBL synchronization. This involves manipulating 6821 PIA registers and configuring the 68705's internal timer, ultimately achieving highly accurate VBL synchronization, consistent with Apple's official documentation.