Re: Software for MS-DOS 1.25

From: Michał Pleban <*lists_at_michau.name*> Date: Thu, 12 Oct 2017 14:21:24 +0200 Message-ID: <59DF5E44.1080705@michau.name> · lists_at_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.

* The character sets of the two machines are different. The hardware
would have to encode the characters when writing and decode when reading.

* 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.

Regards,
Michau.

       Message was sent through the cbm-hackers mailing list