How RG Rampage works

RG Rampage is a platform-based collect ’em up which was put together over a couple of weeks for issue 100 of Retro Gamer magazine, where it appeared as a type-in listing. And, at least according to the changelog kept in the game’s original source file, the final build was on the 11th of January 2012 which was a day over six years ago and that doesn’t make me feel old and decrepit in the slightest! So let’s take a look at what’s going on “under the hood” – a more human-readable version of the source code and some work files have been pushed up to GitHub – in order keep the game under 3.9K uncompressed.

Most of the savings come from the graphics which are optimised in a couple of ways to keep the overall size down; the character set is just cloned from the C64’s ROM whilst skipping the third byte of each character to make it look different, whilst the platform tiles – sixteen characters in total with one more for the titles scroller – are copied into that same 2K block of character RAM. Similarly, any object which can point in either direction like the player or some of the drones is actually stored just the once in the sprite data where it’s looking to the right, with the flipped version being generated on starting up.

Another place where things are reasonably compact is the level data; each platform uses five bytes for X and Y start position, character pair, colour and width and an entire screen only needs about a hundred bytes in total when adding the player start position, enemy patrol data – basically just another five bytes per nasty saying which object to use and giving two sets of X and Y positions to track back and forth between – and a stream of coordinates for the pick-up items. The level layouts were rather arduously worked out on printed sheets because getting a bespoke editor done would’ve taken too long.

I couldn’t include music because it would’ve added too much to the program’s length, so my (t)rusty “Roundasound” engine was dragged out of storage and given a bit of a dusting down; this is pretty much the same routine which was used for Quota by Chris Young and, with some modifications from Sean Connolly, in Flair Software’s Turn ‘N’ Burn so if anyone fancies a laugh they can have a prod through the source code for that. The routine is almost painfully primitive looking back and there’s a far better version now which is the base of my Atari 8-bit sound routine too, but it was compact and did the job.

The final “trick” was creating a BASIC listing for publication which, to keep the chance of errors sneaking in to a minimum, was handled by a truly hacky piece of BlitzMax code that yanked the assembled binary file in, generated a simple checksum by adding all the bytes together and then wrote out a text file containing the game as DATA statements and a small BASIC program with a FOR/NEXT loop which the code out to memory. That text file could then be tested by copy/pasting the contents into a freshly-loaded emulator and, when everything was deemed complete, sent off for publishing.

When Darran asked me to produce a type-in for issue 100 it seemed like a fun thing to try programming but I honestly didn’t expect anybody to actually type it in! There were a few brave souls who surprised me though and, judging by the feedback, they seemed to enjoy the experience of playing it as well. Going back to RG Rampage this afternoon for the first time in a couple of years to grab screenshots, I can’t help feeling with the advantage of hindsight that the balancing was a little off – almost certainly due to the dash for the finishing line – but it still feels fun to play.

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