The card's 32kB of VRAM is directly mapped, there's no need to select another page for writing, to add screen's extra pixel data.
Other 32kB CGA standards, like mode 40h or Tandy, also use direct memory mapping, without page flipping.
If it's there, but it isn't needed for general operation, what was the envisioned usage?
VileR wrote on 2024-08-29, 14:09:AFAIK it controls which page is being displayed, has nothing to do with writing.
According to https://www.seasip.info/VintagePC/plantronics.html , it has to do with writing. I assume, this is meant to be able to re-use standard CGA graphics routines by writing the low 2 color bits first, then swapping the planes in "user code" and re-call the library to write the other two color bits.
VileR wrote on 2024-08-29, 14:09:AFAIK it controls which page is being displayed, has nothing to do with writing.
I don't think so. It controls what page is mapped at B8000. The display in plantronics mode is always 2 bits from B8000 and 2 bits from BC000, both pages are displayed.
mkarcher wrote on 2024-08-29, 15:37:According to https://www.seasip.info/VintagePC/plantronics.html , it has to do with writing. I assume, this is meant to be able to re-use standard CGA graphics routines by writing the low 2 color bits first, then swapping the planes in "user code" and re-call the library to write the other two color bits.
That's a possibility, code that has fixed start address and works with 16k of data.
However Plantronics is from 1982 and in my archive I have 20 PC games from 1981 and about 200 from 1982, I don't know how many of them are text mode. I'm not sure we can talk about shared code, libraries and so on, that early in systems life. It's possible everyone was programming for themselves from same set of manuals.
I'm not even sure would CGA benefit from a page flip? Even if doable the card would take time to switch modes forth and back to allow writing to second memory bank. Maybe the second memory bank is accessible in normal CGA mode too and could be used as off screen vram. Certainly it doesn't appear so under 86box but that part might not be accurate emulation. I have real hardware I can check on.
