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.