This article broadly introduces Fine Scrolling
but some prior knowledge of Display Lists and screen memory is
needed. It is recommended that the listing is studied alongside
the article.
Scrolling allows you to use the screen as a window
to move across a picture much larger than the television could
normally accommodate. To do this on other computers, you would
have to move thousands of bytes but on the Atari only a couple
of addresses need to be changed to manipulate the screen area in
RAM. This results in quick motion and is easily programmed but
there is a problem in that characters or graphics blocks jump across
the screen to give very coarse motion.
Atari, being the machine it is, provides an answer
to this problem by having two registers which enable fine scrolling.
HSCROL at 54276 ($D404) allows horizontal scrolling and VSCROL
at 54277 ($D405) allows vertical scrolling. These registers can
only contain numbers in the range 0 to 15 and therefore can only
scroll 16 colour clocks horizontally or 16 scan lines vertically.
To get continuous fine scrolling of a whole picture, you must first
execute fine scrolling until it reaches its limit, then reset the
register to zero and execute a coarse scroll.
In order to write a scrolling program, the display
list must be altered. Firstly we must find the start of the display
list.
10 DL=PEEK(560)+256*PEEK(561)
The next step is to enable the horizontal or vertical
scroll and also let the display list know where to find the area
of display memory in RAM which holds the picture to be scrolled.
Note that for horizontal scrolling RAM must be allocated so that
each line is wider than the TV screen. We must change the third
byte of the display list and every 3 after it by adding to the
ANTIC mode number, 16 to enable horizontal scrolling, 32 to enable
vertical scrolling plus 64 to indicate a Load Memory Scan instruction.
For example, to enable both horizontal and vertical scrolling in
Graphics 2.
20 FOR I=DL+3 TO DL+37 STEP 3:
POKE I,119: NEXT I
Then by POKEing DL+4 and DL+5 and every 3 after them
with the low and high address of the display memory of each line,
the display is ready for scrolling.
30 LO=0: HI=PEEK(106)-20
40 FOR I=DL+4 TO DL+37 STEP 3
50 POKE I,LO: POKE I+1,HI
60 HI=HI+1: NEXT I
To draw a picture in the display memory, you must
POKE values directly into the appropriate area as in the accompanying
program.
To actually implement the scroll is quite easy. For
coarse horizontal scrolling, increment (or decrement) the low address
byte (i.e. DL+4,DL+7 etc.) of every mode line. For coarse vertical
scrolling, change it by the length in bytes of the whole line.
In order to achieve fine scrolling, change the relevant scroll
register between changing the address byte.
The accompanying program demonstrates fine horizontal
scrolling. It scrolls a terrain across the screen in a continuous
loop. The terrain is about 12 screens wide and takes approximately
37.4 seconds for one cycle. It is drawn with a custom Graphics
2 character set as bit mapped graphics take up too much memory.
A small spaceship flies above the landscape to show how player-missile
graphics are not affected by the scrolling. To ensure smooth flicker-free
movement, the scrolling is done in the Vertical Blank Interrupt
as it is not possible to achieve such good results in Basic.
After the program is typed in, save it before running
it in case you have made a mistake. Upon execution, the new display
list will be set up, the PM image will be drawn and lastly the
terrain will be drawn. The latter takes quite a while as there
is so much of it, but when finished it will all scroll smoothly
across your screen.
This article and program first appeared in Inside
Info, the journal of Atari Computer Enthusiasts (N.S.W). Used
with permission.
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