On Trident isa video card like 8900 NMI signal (ISA A1 - CHCHK#) connected to main videochip, marked pin as OUTPUT.
What purpose ?
It's like a parity check on the video cards RAM, or the card might output the signal to declare and unrecoverable crash. It'll halt the whole system or at least the VGA part.
Normally unused unless the card has a problem.
MikeSG wrote on 2026-09-17, 12:04:It's like a parity check on the video cards RAM, or the card might output the signal to declare and unrecoverable crash. It'll halt the whole system or at least the VGA part.
VideoRAM never has parity, and videochip cannot "crash", it's hardware, not software.
Drivers, memory corruption, etc can all cause them to crash. I used bad generalities.
The Matrox Impression 220 (IS-ATHENA) uses an NMI pin to trap requests written to legacy registers, CGA, Hercules, VGA, and CRTC Mode Switch (resolution changes), read by the Matrox's BIOS for emulation/compatibility. The CPU is halted during this time.
MikeSG wrote on Today, 11:01:The Matrox Impression 220 (IS-ATHENA) uses an NMI pin to trap requests written to legacy registers, CGA, Hercules, VGA, and CRTC Mode Switch (resolution changes), read by the Matrox's BIOS for emulation/compatibility. The CPU is halted during this time.
Trapping NMI is tricky (hi to bugged awe32 midi emulator), also Trident ISA cards has good CGA/Hercules emulaton in hardware, no need to trap anything