Re: Apple II VGA video card and the IIgs

[bieling@terra.es (Jorge Chamorro Bieling)]

Charlie <charlieDOTd@verEYEzon.net> wrote:

> "Jorge Chamorro Bieling" <bieling@terra.es> wrote in message 
> 1hbn9qu.174z4ykfxigtoN%bieling@terra.es">news:1hbn9qu.174z4ykfxigtoN%bieling@terra.es...
> 
> > Then you have to set some softwitch to enable bank $xx before writing to
> > the pixel map ?
> >
> > Does the IIgs use the same mechanism as the IIe to access the memory in
> > banks beyond the first 64k "bank 0"?
> 
> If you are running in 8-bit mode I believe the IIgs lets you access the aux
> memory bank in the same way as the IIe though I'm not 100% sure.  When you
> are in the 16-bit mode the 65816 processor has registers that set the bank
> for program execution and data (you can have the program running in one bank
> and the data in another).
> 
> > Or, given that it has a 24 bit address space there are not "64 banks" to
> > shuffle in and out but a contiguous memory map instead and no
> > softswitches to set ?
> 
> There are multiple 64K banks (depending on how much RAM you have) but the
> program bank register (and data bank register) acts like the most 
> significant byte of the 24 bit address when writing 16-bit code. It is
> trivial to have your program read/write data anywhere in memory.  A program
> can easily jump to code in another bank. I seem to remember that you can't
> have your program cross a bank boundary, though.
> In other words when programming it seems almost like contiguous memory.
> 
> > Another way of making the same question is: The IIgs specific video
> > modes are accesible while the IIgs is in Apple II compatible mode of
> > operation (if such a mode exists, I think it does) ?
> 
> Yes, you can use Super Hires modes (320x200 and 640x200) by writing to them
> as you would on a IIe and you can turn on the SHR graphics with a softswitch
> at $C029.  If bit 7=1 then SHR is on.  The pixel data is linear and 
> therefore much easier to handle than the older graphics modes.
> 
> > And if the memory map is not fragmented in banks, is there any chance
> > for a card sitting in a slot to guess somehow about the missing a17..a23
> > address lines ?
> 
> Well as mentioned above, the memory map is technically fragmented into 64K
> banks but I don't know enough about the slots to know whether there is any
> way for them to detect which bank is being used other than as mentioned by
> David Wilson the /M2SEL signal which indicates that bank $E0 or $E1 
> addresses are on the address bus.  Remember that banks $00 and $01 are
> "shadowed" onto $E0 and $E1 which run at 1MHz.

Charlie, 

I also found this interesting (640*400 !) stuff :
<http://web.pdx.edu/~heiss/technotes/misc/tn.misc.17.html>

I'm going to google search for "/M2SEL" "M2B0" "Apple II".
I'm starting to think that there may be a chance.

...
-- 
Jorge Chamorro Bieling