[Simh] [simh] Scrambled text in VAXStation 3100m38 GPX graphics screen

[paulhardy2 at btinternet.com]

On 06/21/2019 11:40 AM, simh-request at trailing-edge.com On Behalf Of Jon Elson wrote:
> The first picture looks like it is doing some kind of transliteration into Cyrillic, is it possible the font selection got switched?
> Jon

Not that simple. The fonts get corrupted one by one, and character substitutions aren't just Cyrillic - they are numbers and punctuation also.

I can provoke it fairly easily by starting three or four standard VMS applications - e.g. Paint, Fileview, and Calendar, maximising them to full-screen and using right click to lower them to see the underlying windows.

My suspicion is that it is memory trampling in the font caching implementation in the emulated VCB02 device in SimH. The VCB02 device uses the same memory planes for font caches as for screen bitmaps - the manual says:
1.6.1.1 Font Storage and Access Fonts are stored in an undisplayed portion of the bitmap. Ordinarily, a font is stored only in one plane and is transmitted from that plane to itself and
others when a character is written.  .... Fonts are normally stored in the off-screen portion of the VCB02 bitmap.

Calling Matt Burke - can you spot anything odd in your implementation of this aspect of the VCB02?

-- 
Paul  Hardy
web: www.paulhardy.net