Release notes – Stercore XD (C64)

The RGCD 16K cartridge competition is running during 2019, so your correspondent decided to have some fun and use it as an excuse to complete and release Stercore XD, a reworked version of the ported Spectrum shooter of a similar name which eschews software-based sprites in favour of larger, more colourful hardware ones and replaces the chunky scrolling backgrounds with something a lot more detailed. The competition rules state that entries must fit onto a 16K cartridge ROM and Stercore XD manages that even if it only just slides under the bar, with less than a hundred bytes of free space.

That doesn’t mean there aren’t some improvements to the original Stercore however, code for handling enemy movement patterns has been grafted in from Super Hyperzap to make the gameplay more interesting and the background scrolling speed slowed down to five pixels per frame rather than the original’s eight in an attempt to add further longevity to the game. The scrolling background’s map is 9,360 bytes wide by 20 high which means that there’s somewhere in the region of 182K of data passing by during the course of the game which, due to the slower movement speed on this version, will take just under five minutes to traverse.

Stercore XD (C64)As with the original Spectrum game there are two levels of background graphics, a darker layer which is always behind the hardware sprites and the mostly lighter one which passes over them. The technique is the same one used for games like Gryphon or Action Force and relies on one of the two character multicolours always staying behind the sprites regardless of what the hardware priority register is set to; this and the background colour are used for one layer whilst the other multicolour and various character colours provide the other. It does require a little more care than usual when designing the graphics but the effect works reasonably well.

Stercore XD (C64)

Since the conversation here of late has been about hardware-based sprite to sprite collisions on the C64 it’s interesting to note in passing dear reader that this particular game exclusively uses software-based checks; that was a deliberate design choice for a number of reasons – this is something that your correspondent already has a longer discussion post in the works to cover more thoroughly – but it also harks back to the original Stercore and its subsequent C64 port, both of which rely on software sprites and, by association, collisions.

Stercore XD (C64)Along with the visual reworking, your correspondent also took some time to extend the original title music and tweak some of the sounds a little and most of the work files for the game are available over at Github including the Char Pad project containing the scrolling background for those wishing to understand more about how this works. It’s worth noting however that the source code is a little harder to read than usual, partly because chunks of the actual program are generated on starting up in order to meet the 16K limitation of the competition.

The source code even includes a macro since the author has previously talked about them. In this particular case the macro is called “print”, appears at the beginning of the source and is responsible for pushing text to the screen memory; the assembled code is slightly less efficient this way since your correspondent would normally optimise things by having one loop outputting multiple lines of text, but it does serve as a practical demonstration of how macros work since the author’s description was rather lacking.

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