Re: [stella] Asymmetrical Reflected Playfield

Subject: Re: [stella] Asymmetrical Reflected Playfield
From: "Thomas Jentzsch" <tjentzsch@xxxxxx>
Date: Fri, 21 Sep 2001 09:44:41 +0200
Glenn wrote:
> If someone can explain what's happening at the tail end of the scanline for 
> why I don't seem to have as many cycles left as it seems, I'd appreciate it.

I'll try :)

1. You have an error in your cycle counting, the second write to PF2 needs 3 cylces, so this operation ends at cycle 48 (not 47).
2. You have to count the additional code arround your PF-writes too. The maximum cylce count for this is 27 (19 before, 8 after the PF-writes).

So, you need a total of 48+27 = 75 cylces. That's why any additional nop will break that kernel.

But there might be (as always :) some optimizations possible to reduce those 27 additional cylcles. 
- You don't have to decrease scanLine every loop, it would be enough to decrease only when X reaches 0. This requires just a different scanLine starting value. 
- You could use Y to save more cycles. 
- It might be also possible to avoid WSYNC by making the cycle count constant. But that will make the kernel very fragile, so I would do that near the end of the optimizing process.
- Put the 3 nops directly behind WSYNC and join those free cycles with the other free cycles before WSYN to gain more flexiblity here.
- ...

Have fun!
Thomas Jentzsch         | *** Every bit is sacred ! ***
tjentzsch at web dot de |

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