[thought_alarm]

This game achieves its smoothness by synchronizing to the video refresh. That's not a particularly challenging thing to do, but it wasn't a viable option for commercial games back in the day due to uncertainty of how to do it properly across the different Apple II models.

Apple added VBL polling with the IIe in 1983, and then broke it a year later with the IIc. And an unfortunate typo in the IIc technical reference manual meant that developers couldn't figure out how to do it properly.

The only official, cross-platform way to do it was through the mouse firmware, but most users didn't have the mouse hardware installed.

This game uses the mouse firmware, if available (II+ w/ mouse card, IIc, IIgs), to generate VBL interrupts. On the IIe without a mouse card, it polls for VBL.

By the way, if you're running this game on the Virtual ][ emulator for MacOS, be sure to disable the mouse card. Its emulated mouse card only generates VBL interrupts at 30 Hz, so the game runs at half speed.