In article <3hjkdb$20q@clyde.ics.uci.edu> lstrand@ics.uci.edu (Leif Strand)
writes:
>Is there a double-high res page 2?  That is, can you display a DHR
>screen that is composed of the HR page 2 RAM in the main and auxiliary
>banks (starting at $4000 if memory serves) instead of the HR page 1 RAM
>in the same (starting at $2000)?  Back when all I had was a //e (before
>I got my GS), I remember being very interested in this possibility, but
>unable to find any mention of it in the Apple docs.

Yes, there is a double hires page 2.  Using it is a bit of a pain, though,
since the double-resolution bank switching scheme doesn't work on it.  If
you want to write data into the auxiliary bank of DHR page 2, you have to
switch in the WHOLE auxiliary memory area ($200-$BFFF).

To display DHR page 2, flip the following switches:

     80-column bank switching OFF (STA $C000)
     80-column display ON (STA $C00D)
     graphics ON (LDA $C050)
     mixed mode OFF (LDA $C052)
     page 2 ON (LDA $C055)
     hires ON (LDA $C057)
     double-res graphics ON (LDA $C05E)

Be careful--the firmware does NOT like to find the machine in this state.
Many ROM routines (especially the 80-column firmware) turn on 80-column
bank switching whenever they discover that the 80-column display is
enabled.  This ruins everything, since DHR page 2 can only be displayed
when 80-column bank switching is off.

But since 80-column bank switching is off, there's no convenient way to
access the auxiliary page 2 buffer.  The only way to get at $4000-$5FFF in
the aux bank is with the RAMREAD ($C002,$C003) and RAMWRITE ($C004,$C005)
switches.  Since these switches map out the whole main memory area from
$200 to $BFFF, this places rather severe limitations on where your code can
reside.  The most convenient method I can think of is to put identical
copies of your drawing routines in identical locations in both the main and
aux bank.

By the way, DHR page 2 isn't the only "hidden" display mode.  By using
similar techniques, you can also display 80-column text page 2, and double
lores page 2.

              - Neil Parker
--
Neil Parker                 No cute ASCII art...no cute quote...no cute
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