Smooth horizontal
Scrolling on the
STE
The Atari STE was the first 16/32-Bit computer of the
series to feature horizontal and vertical fine-scrolling.
While vertical fine scrolling merely requires the ability
to set the low-byte of the screen address accordingly, the
horizontal fine-scrolling requires the ability to have
lines larger than the current resolution and to make the
graphics chip to skip a certain amount of pixels at the
beginning of each line.
In other words, a few extensions with regard to the simple
ST were required. First, the video base address was given
a low-byte which the ST lacked, making up three registers
in total:
$FFFF8201 0 0 X X X X X X Address bits 16 to 21
$FFFF8203 X X X X X X X X Address bits 8 to 15
$FFFF820D X X X X X X X 0 Address bits 7 to 1
Please mind that the address bit 0 is always treated as "0"
so that the screen must start at an even address.
To also allow horizontal fine-scrolling, the STE has been
given a register nicknamed "Pixel-Skip", which is located
and defined as follows:
$FFFF8265 0 0 0 0 X X X X Number of pixels to skip
These registers allow the STE to scroll horizontally and
vertically with basically just setting the registers above.
However, when skipping a few pixels at the beginning of a
line, that would imply "missing" some pixels because the
Shifter always displays 320 pixels per line. Therefore,
the Shifter allows to ignore a given number of pixels
after having drawn 320 pixels per line. This means, there
are more pixels per line in memory than the shifter
actually displays. Then, of course, the Shifter needs to
be told how many pixels to skip after having drawn a line.
This is given in number of words (16 bits) in the following
register, called line offset register:
$FFFF820F X X X X X X X X Number of words to ignore
To also gain more flexibility in between the lines, the
video address counter, which was read-only on the ST, is
now a read/write register and located at
$FFFF8205 0 0 X X X X X X Counter bits 16 to 21
$FFFF8207 X X X X X X X X Counter bits 8 to 15
$FFFF8209 X X X X X X X 0 Counter bits 7 to 1
To have horizontal fine-scrolling on the STE, it now
seems that only the video base address registers, the
Pixel skip and the line offset during the VBL.
Unfortunately, the result will be a corrupted screen
every now and then. How come ? Atari's official state-
ment looked a bit like this:
"Setting the Video Base Address Pixel Offset during
the VBL from unequal to zero to zero might result
in corrupted display."
As a result - and so it is explained in the STE-FAQ as
well - this register should be treated in a HBL-Interrupt
rather than the VBL to avoid resetting it during the VBL.
However, for many programs, this means the introduction
of the HBL-interrupt as an additional interrupt source
and might mix up timing with music/digi-sounds etc.
Therefore, the HBL-Interrupt is not desired.
Often enough in magazines and on webpages a solution
without requiring the HBL-Interrupt is debated, but so
far, only fragments of a working routine have been
presented and the results were not very satisfactory.
Nevertheless, a simple solution is available even
during the VBL and even better, it works on the STE,
the Falcon VGA and RGB and even on the TT.
The procedure is quite simple. During the VBL, you
first write the screen base address (mind the order):
move.b adr_high,$FFFF8201.w ;High Byte
move.b adr_mid,$FFFF8203.w ;Mid Byte
move.b adr_low,$FFFF820D.W ;Low Byte
Now write the new pixel skip and line offset in no
special order
move.b line_ofs,$FFFF820F.w ;Offset between lines
move.b pxl_skip,$FFFF8265.w ;Pixel Skip (0-15)
This might actually lead to the Shifter interpreting
the following lines in the wrong way and therefore
doing corrupted screen display. Funny enough, the
Shifter will always perform correct display when
writing the address counters by hand:
move.b adr_high,$FFFF8205.w ;High Byte
move.b adr_mid,$FFFF8207.w ;Mid Byte
move.b adr_low,$FFFF8209.w ;Low Byte
Once these values have been loaded directly into the
screen address counters, the Shifter seems to correct
its faulty line interpretation and will correctly
display the next frame.
Tested on STE, MegaSTE, Falcon on both RGB and VGA and
on TT (Colour) and found working on all these machines.
Some of the previous articles state that this concept
seems to work only in low resolutions.
This cannot be affirmed so far, the concept seems to
work in any resolution at least on the MegaSTE, but
there are restrictions anyhow:
1.) The Falcon cannot scroll in monochrome resolution
2.) The TT can only scroll in 16- and 256-colours
The Paranoid / Paradox
Alive on a dead machine
... That's Paradox ...
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