MAT6013@DMZRZU71.BITNET (Matthias Kapffer) (12/15/87)
I've waited some time to post this but now I feel I have to rectify the
following issues:
1) Re: GAFRO
(posted by: Sean Kamath)
The alternate display mode does *not* mean
a choice between text and graphics !
The Apple IIgs video buffers are all located in the 128K bytes of RAM in
banks $E0 & $E1, which is accessible at only 1 MHz because of the sharing with
the Video Graphics Controller (VGC). For compatiblity with older Apple II's
they are "shadowed" to banks 0 & 1, the fast RAM, in the standard address
ranges. Shadowing means that data written to the fast RAM will be copied to
the video RAM by the hardware transparent to software. But for reasons far
beyond my comprehension Apple engineers "forgot" to implement shadowing of the
memory at $800-$BFF which is a painful incompatility with a lot of existing
software using page 2 for text (40 & 80 col) or block graphics (normal &
double).
(Note that _all_ modes are available with buffers in the video RAM - just the
shadowing is incomplete !)
To give you some help with this problem a software solution was created (the
(in)famous "Alternate display mode") which copies text page 2 into the
appropriate video buffer during every tenth vertical blanking IRQ - stealing
you the CPU time to move 6K/s into the slow RAM ! (So you could also calculate
SIN(X) 1000 times - slowdown will be the same as with output.)
Besides delaying the machine some older software will crash with running
interrupts while other just locks out the IRQ disabling the software
shadowing.
BTW, the german Apple IIgs Owners Manual makes you think the alternate
display mode is for selecting the MouseText character set - and this
misunderstanding is so widespread that even the german ProDOS 16 system disk,
which contains translated texts for the ones in the ROM to be installed at
boot time, substitutes "Turn Alternate Display Mode On" with "Turn MouseText
mode on" (of course in German) !
2) Re: HELP! GS SHR Display strange
(posted by: Larry W. Virden, TMPlee@DOCKMASTER.arpa)
While this is *not* a solution of the SHR problem I will explain you some of
the effects of the b/w versus color monitor selection in the control panel:
Hires and double hires both have a tradeoff between color and resolution: You
can look at every bit as a pixel or you can group bits together to yield a
hard-coded color value (i.e. a colored but wider dot) somewhat depending on
the type of your monitor.
The Apple IIgs allows to switch the interpretation of the VGC to make both
approaches useable on monochrome and RGB monitors. MouseDesk uses b/w double
hires and therefore automatically selects this mode, but not permanent (i.e.
in the clock's RAM) so that entering the control panel from the desktop
switches back to whatever mode happens to be stored as your personal
preference. This illustrates why the color mode causes that "smearing" when
you go out again.
Other software (like paint programs and games) may of course require the
color monitor selection, i.e. the choice doesn't depend on the connected
display device but type of graphics an application generates. (Even both can
make sense for one application. Just have a try.)
Sadly, in this form this is only true for double hires. Somehow the
suppression of color is tied to annunciator 3:
When on (reference $C05F), which is the default (after RESET etc.), hires or
lores graphics are not affected by the current type of monitor.
When off ($C05E; one of the states necessary for double hires and lores) the
color bit (bit 7 in every displayed byte) is ignored disabling the half point
shift (it was formerly an official feature and is often used !) regardless of
the monitor selection. In color mode this will force blue and red to appear as
violett and green while finally in b/w mode we get the monochrome 280 dots/row
resolution but no pseudo 560 positions per scan line (as stated above).
The influation of hires seems to be a side effect - not a design goal: The
dependence of annunciator 3 makes it impossible to use b/w hires with 80 col
mode and other software which resets the machine somewhere at the beginning.
The situation is even worse with lores: b/w monitor with ann 3 off doesn't
give you block graphics with 16 shades of gray but mapes the 16 colors to the
4 of the restricted hires color mode (see above) making nearly every image
rather cryptic.
Super Hires and text mode are of course not affected - there is no tradeoff
between color and resolution.
Matthias Kapffer
<MAT6013@DMZRZU71.BITNET>