MGA_MCA: Notes on PS/2


- The IBM Model 70, will not work properly with the Microchannel MGA.  
The system will work properly on power-on, but will not reboot when the 
<Ctrl><Alt><Del> keys are used.  We get a planar video error (#2401) that 
occurs during POST.  What happens is that the system is looking for a VGA but 
the VGA on the motherboard has been disabled on power-up. We cannot use the MGA 
VGA at that time because it's initialization hasn't been performed yet, the VGA 
test in POST is done before our BIOS is enabled.

One practical solution is to execute a little program that writes 01 at I/O 
address 03C3h - to enable the on-board VGA - when the <Ctrl><Alt><Del> keys 
are pressed.  This will work properly except for when we go through the system 
configuration utilities, because our BIOS has already been executed the on-
board VGA will already have been disabled.  Since we cannot renable the on-
board VGA at that time, we will get the #2401 error until we turn off the 
computer and turn it back on again.

The MGA Must be mapped at any address other than AC00. You must also remove the 
device=c:\mga\setup\rambios.sys from your config.sys. This will allow you to
run ACAD and Windows correctly.