The Alternate Display Mode is a small firmware routine that can be activated
from the Desk Accessory menu. It makes the Apple IIGS compatible with
applications that display text or graphics on Page 2 (memory $800-$BFF).
The Apple IIGS does not shadow this range of memory to the Mega II normal
RAM. However, some programs depend on this shadowing in order to display Text
or Low-Res Graphics from Page 2. These programs will not work because the
hardware is not updating the slow screen memory with the appropriate memory
range from the Fast RAM.
Enabling the Alternate Display Mode will install a small routine into the
Heartbeat Manager routine queue. This fast routine copies the appropriate
values every 1/60 of a second, assuming that VBL interrupts have been turned
on and the Heartbeat Manager has been initialized. While this slows overall
execution speed of any programs on the Apple IIGS, they still run faster than
on an Apple IIe or Apple IIc.
For more information on the Apple IIGS Heartbeat Manager, see the Apple IIGS
Firmware Reference Manual or the appropriate database article.