Re: Strange 8255 behavior

From: Michał Pleban <*lists_at_michau.name*> Date: Sun, 10 Jun 2018 19:43:59 +0200 Message-ID: <5B1D635F.3050308@michau.name> · lists_at_michau.name

Gerrit Heitsch wrote:

> It is... It can even produce IRQs when writing to $3FE / $3FF (depending
> on side), alerting the other side that data is waiting.

This gives me the idea to create a simple circuit emulating the MDA
hardware: get 4 of these chips and place them at B000:0000 on the 8088
side, and somewhere in bank 15 on the 6509 side. The 8088 would write
character data there as on every MDA, and at the same time a routine on
the 6509 side would continually read the bytes, perform CP437 to PETSCII
conversion, and copy them to the video memory. This would allow running
applicatins which access the video memory directly - it's the major
compatibility obstacle.

Regards,
Michau.