Regular readers may
recall that I wrote a star scrolling routine for the 8-bit models
that was published in issue 18. Intrigued by the possibilities of
doing the same on the ST I set about writing a similar program and
ST Star Scroller is the result.
The program itself is
really just a bit of fun but I hope that it will give programmers an
insight into using the VBI on the ST. ST Star Scroller scrolls 2
planes of stars in any resolution from right to left across the
screen in a VBI interrupt. When the program is run it attempts to
insert the routine into the VBL queue and, having done so, it will
return to the desktop with the stars scrolling. You can now run any
other program and, as long as it does not alter the VBL interrupt or
move screen memory somewhere else, those stars will keep on
scrolling! By running the program again you'll end up with another
set of stars and you can keep going until so much work is being done
in the VBL that the machine locks up!
The Assembler source code
is quite useful as its shows the correct way to set up VBL routines
(at least for use with the operating system). It also shows that the
GEMDOS routines for terminating a program without reclaiming the
memory occupied are of some use, even without multi-programming
capabilities.
Star Scroller source code |
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