Den Thu, 12 Oct 2017 14:21:24 +0200 skrev Michał Pleban <lists@michau.name>: > > Baltissen, GJPAA (Ruud) wrote: > > > Then what about hardware that redirects any data written to MDA/CGA > > memory directly to the screen memory of the CBM II? > > I don't think this would be possible, for several reasons: > > * The 8088 accesses the DRAM directly, bypassing the 6509 bus. It > can't see anything that is in bank 15. To make it write there, it > would be necessary to modify the board (and probably also the > mainboard) significantly. That is the show stopper. And if you anyway need to add hardware, it might aswell be an ISA interface. > * The character sets of the two machines are different. The hardware > would have to encode the characters when writing and decode when > reading. Another solution would be to add a character rom twice as big and use some unused i/o pin (or add i/o hardware) to select between PETSCII and PC8 font. > * The screen memory layout is different. Specifically, every other > byte on PC is an attribute byte, and "reverse" bit of this attribute > would have to be written into the 7th bit of character byte in the > 6509 memory, making it very difficult to do one-byte accesses. That would make it necessary to shift all adress bits one step when accessing this memory, and ignoring the attribute bit. In fact AFAIK you cannot emulate PC8 complete with inverted video e.t.c. on a CBM machine (except on C128). -- (\_/) Copy the bunny to your mails to help (O.o) him achieve world domination. (> <) Come join the dark side. /_|_\ We have cookies. Message was sent through the cbm-hackers mailing listReceived on 2017-10-13 02:00:02
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