Sergey's Micro 8088 and the IOCHK NMI signal

[Matt Balmer]

Hi, folks --

I've had a fabulous time building up and working with Sergey's micro 8088 board, and using it to help test out all kinds of older hardware, but I've run into one card that seems to consistently get hung up: an ATI EGA Wonder 800.

Every time I put it in the backplane, it goes through the memory count like I expect, and then I get the message:

IOCHK NMI detected. Type 'i' to ignore, 'd' to disable NMIs, or 'r' to reboot.

At this point, the keyboard simply stops functioning and nothing short of pushing RESET will work.

Any other video card (including VGA cards set to work in 8-bit mode) that I own and plug into the system functions correctly and doesn't generate this. I've searched around and seen some messages that seem to indicate that the signal is coming from the video card itself but even blocking the pin physically doesn't seem to stop it from happening. 

I'd love to try out this card with the micro 8088 a bit further, but this is just a bit confusing as to why this card specifically seems to do this and why it's getting hung up. 

Any ideas?

~~ Matt Balmer

[Sergey Kiselev]

The ATI EGA Wonder 800 uses 82C435 EGA controller.

Looking at the datasheet for that controller, it can generate a NMI, called TRAP in the datasheet.

There is a long list of conditions that can trigger NMI on pages 40-41, that generally have to do with emulating Hercules graphics...

I am not sure what the default values for the controller registers are, so I don't know if emulation or TRAP are enabled by default.

The Micro 8088 BIOS shouldn't write the emulation register [3B5h (MDA) / 3D5h (CGA), index 0FFh], unless there is a bug...

You can give GLaBIOS a try... see if that works any better: https://github.com/640-KB/GLaBIOS

-Sergey