Baron already knows what goes there.

B000-B7FF is used for 1bit per pixel graphics memory for graphics modes on hercules/MDA adapters.

B800-BFFF is used for text modes to contain text mode diaplay data.

Their contents will change, based on what is on the screen. They are used to display what is on the screen. 😁

Typically, VGA cards do not fully emulate MDA adapters. The only region they populate is the B800-BFFF region, which is used to store text display mode character data. As such, 'there is nothing connected' at B000-B7FF on most systems with a VGA card, and it is safe to use this area for UMBs.

For whatever reason, (I suspect the ET4000, since it has register-level MDA/CGA emulation) this area is contested on your hardware, and trying to use it as UMB causes a hang. (Which is why I'd like you to try without the ET4000)

The Baron rightly points out that the B800-BFFF region is not going to be safe to use: YOU NEED IT TO DISPLAY TEXT. He mentioned a fringe case where a loaded driver can change the behavior to use the VGA adapter region (A000) instead, but that way lies dragons.

wierd_w wrote on Today, 20:03:

... that the B800-BFFF region is not going to be safe to use: YOU NEED IT TO DISPLAY TEXT. He mentioned a fringe case where a loaded driver can change the behavior to use the VGA adapter region (A000) instead, but that way lies dragons.

It's perhaps a lot easier to fix. Just set a graphics mode BEFORE running EMM386. If the VGA BIOS works correctly, any text that is output via DOS or BIOS should then be seen even if region B000-BFFF is no longer under VGA control.

I attached such a tool - it sets graphics mode 0x12. I guess It cannot be tested in DosBox, since it needs to be run in CONFIG.SYS:

1DEVICE=C:\SETM12.SYS
2DEVICE=C:\DOS\EMM386.EXE FRAME=E000 RAM=B000-BFFF RAM=D100-D7FF X=D000-D0FF X=D800-D9FF RAM=C800-CFFF RAM=DD00-DFFF