FINALLY! I got a MacBook Pro, installed XCode, svn'd down MESS source code and managed to make a functional change.
Originally Posted by Ensjo
MC-1000 doesn't provide the external ROM that MC6847 may use to get the characters bytes when it is in "External Alphanumerics" mode (AG=0, AS=0, INTEXT=1). Currently the emulation doesn't provide a cfg.get_char_rom at VIDEO_START, and thus it works as if a byte 0x00 is read. In the actual machine, though, MC6847 puts the character code byte into the wires trying to reach the external character ROM, and then reads the wires again for the pattern... but it gets the very same byte that it put there, i.e., the ASCII code of the character.

I think you could add something like...

UINT8 mc1000_get_char_rom(running_machine *machine, UINT8 ch,int line)
   return ch;
cfg.get_char_rom = mc1000_get_char_rom;
The result is not usefull at all, as you can see by accessing this MC-1000 Java emulator and typing
POKE 245,33
but that's how it works, so here you have it.
In fact, there was no need for VIDEO_START. I found another machine that used external character ROM and found a solution. Under MACHINE_CONFIG_START I added:
And voil�. smile
I'll see if "svn diff" can really send my changes for approval now...