ATI EGA wonder emulatie - HOW does it work

[Chuck Forsberg WA7KGX]

Boards such as the EGA Wonder switch emulation modes by generating an
interrupt to a routine in their on board BOIS chip when the hardware
detects a change of mode that requires some software (firmware) help
to grok.

This causes problems for true operating systems such as Xenix (and I
suppose even OS/2).

First, a spare vector must be found for this interrupt.
This is a no-go because the PC bus doesn't have enough available
interrupt vectors as it is. It would help if boards supported the AT's
second set of vectors, but few bother.

Second, the entity that catches these interrupts must be recoded
to run in protected mode, etc.

So, if you can get commented source code for ATI's BIOS and scrounge
an unused interrupt vector, you could write a device driver to make the
EGA Wonder and do its thing under Xenix or whatever.