Having spent four weeks looking at “bog standard” demos created by C64 users during the 1980s, it’s time for more of what what the author calls “showing off” on your correspondent’s part with the latest C64CD release, Standard Clone. Although we looked at a lot of “bog standard” over the last four weeks, four in particular were the inspiration for this release; Real Sky-Runner had a nice sprite logo in the upper border which was expanded on, the bitmapped picture with high resolution sprites overlaid for detail was based loosely on Future Shock, the greeting sprites are similar to the Airwolf Demo and the “tilted” lower border scrolling message is like Xess 1 – Rendezvous with just a dash of side border from Metal Bar 2.
We’ve previously covered the basics of how the upper and lower borders can be opened and the side borders rely on a similar technique, although the timing required to do so is significantly more precise; the loop opening the side borders is called sb_loop and there are two commands by that label which change the horizontal smooth scroll register at just the right moment to make the C64 “forget” to enable the borders. Changing those two references to $d016 to $d021 and assembling the code gives an idea of where these changes need to take place and will also show that the routine isn’t perfectly timed – it was written this way because most lower and side border scrolls from the 1980s do the same thing.
As is always the case, the code for Standard Clone was written from scratch – some of the routines have been reused from Clone Invasion but that kind of recycling is perfectly normal in programming circles – with both the source code and many of the work files available at Github for those wondering where the colour in the upper border came from – have a look for ub_loop which splits the sprite multicolours for one hint or the sprite definitions themselves for another – or how the sprites are being manipulated in the lower border so that just seven can cover a space which usually requires eight.
Finally dear reader, we’ll just pause to remember that the demos that inspired this demonstration are what C64 owners were doing and indeed wanted to be doing with their computers in the 1980s; the author would like people to believe that his claims of excessive difficulty are real, but the sheer numbers of user-generated programs like the demos being covered in the Code Notes series very clearly demonstrate that he is rather staggeringly wrong.
 The author has, as is usually the case, missed the point when claiming that your correspondent is “showing off” – the irony of being accused thus whilst writing about programming is staggering in its own right – because replicating code that’s nearly thirty years old really isn’t challenging!