Specifications

Apple II Technical Notes
6 of 4 #70: Fast Graphics Hints
Clever updates can modify twice as many pixels on the screen by sacrificing some smoothness,
running at 30 frames per second instead of 60. The technique is as follows:
1. Wait for the scan line beam to reach the first scan line.
2. Start updates from the top of the screen, being careful not to pass the scan line
beam.
3. Continue updates while the scan line beam progresses toward the bottom of the
screen, then goes into vertical blanking, then restarts at the top of the screen.
4. Finish the update before the scan line beam catches the update point.
Careful use of this method allows a frame to be updated during two scans of the screen instead of
just one. If you are not sufficiently careful, tearing results.
Note: The Apple IIGS main logic board Mega II-VGC registers and interrupts are not
synchronous to the Apple II Video Overlay Card video and therefore should not
be used for time synchronization with the Apple II Video Overlay Card video
output. However, they can be used for time synchronization with the Apple IIGS
video output. See the Apple II Video Overlay Card Development Kit for more
information.
Interrupts
It is not possible to support interrupts while sustaining a high graphics update rate, unless
jerkiness or tearing is acceptable. Be aware that many system activities such as GS/OS and
AppleTalk depend on interrupts and do not function if interrupts are disabled.
Further Reference
• Apple IIGS Firmware Reference
• Apple IIGS Hardware Reference
• Apple II Video Overlay Card Development Kit
• Apple IIGS Technical Note #39, Mega II Video Counters
• Apple IIGS Technical Note #40, VBL Signal
• Apple IIGS Technical Note #68, Tips for I/O Expansion Slot Card Design