Hi, this is a program to probe the behaviour of Map Mask register behaviour when Mode 13h is set.

I have been trying to probe the behaviour of Map Mask register if Mode 13h is set (which implies chain-4 mode usage) for personal reasons. I believe it will help to make VGA emulation more accurate. So far from reading official IBM docs, it seems like the register can in fact affect chain-4 modes, but I need to see which video cards actually follow this behaviour.

When this program is launched, the screen should be filled with dark blue. Press Enter again, and the screen should have a checkerboard pattern. Press Enter again after that to exit.

Results so far:
1. 86Box, dosbox-staging: Screen is filled with full-on black, implying that the map mask register is completely ignored.
2. Intel UHD Graphics 620: Screen is filled with checkerboard pattern.
3. VirtualBox: Screen is filled with checkerboard pattern.

Source code is licensed under BSD 3-Clause. The archive here ships with DOS/32A 9.12 DOS extender (renamed to DOS4GW.EXE).