Thursday, 14 April 2016

On optimisations

Over on the AtariAge forums, user mariuszw has been porting the ZX Spectrum version of Pentagram (the 3rd filmation title) to the 6502-based Atari 8-bit computers. He started with an automated translation of Z80 to 6502 which he got running but, not surprisingly, was too slow.

He has spent some time both profiling and analysing the code, and aside from actual pixel rendering has identified areas in the Z-order rendering logic that can be significantly optimised. I was aware that it took a rather brute force approach and that it spent quite a while in there, but was surprised to learn that the code can spend up to 40% of CPU time in that routine!

And to give some idea of the effect of these optimisations, one user observes that the code now runs as least as fast as the Spectrum version, if not slightly faster. And keep in mind that significant portions of the code are still implemented from the auto-translation process. Impressive!

The dilemma I face now is how far do I go with implementing any optimisations on the Coco3? The original intention was to port the game faithfully, warts-and-all. I have no issue rewriting the platform-specific portions of the game - such as rendering routines - but re-implementing the sorting logic encroaches on modifying the way the game works. You could argue that if it doesn't change the outcome (ie. the same objects are rendered each frame) then it's still the same, but the purist in me is struggling with that argument regardless.

Either way, I'll study the code for interest's sake, if nothing else, then make a judgement call. I guess more people are likely to play it if it runs smoothly, and won't really care what's going on underneath the hood.

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