Yes, it is possible to disable the built-in video of the Macintosh IIci:
1) Leave the RAM SIMM bank A empty.
2) Don't connect a monitor to the built-in video connector.
The built-in video circuitry disables itself if there is no RAM in bank A
or if it fails to recognize a compatible monitor attached to the
connector. The Macintosh IIci and the new version of the Macintosh II
Video Card identify monitors by checking the states of three ID lines in
the cable between the computer and the monitor. If no monitor is
connected, it disables itself.
Using a NuBus video card can increase or decrease "performance",
depending on what work is being done on the Macintosh IIci. The effect of
built-in video's cycle-stealing on RAM bank A can reduce the speed of
anything that accesses data in that bank by 6 to 51%, depending on the
monitor and bit depth being used. On the other hand, moving large amounts
of video data to the built-in video is faster than moving it over NuBus to
a video card because there is no 10MHz NuBus limitation with built-in
video.
What all this means is that a low-computation, high-graphic program tends
to run faster on the built-in video, while a high-computation, low-graphic
program tends to run faster with a NuBus video card. Optimal system
configuration depends on what the Macintosh IIci is used for.
DayStar Digital's FastCache Card can help offset the cycle-stealing
performance penalty.
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