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Old 10 September 2018, 01:15   #1
Harry
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Join Date: May 2010
Location: Hamburg/Germany
Posts: 811
Some games are not random anymore

This happens especially for the WHDLoad versions. The disk versions are less affected, but the problem is noticeable too.

Two examples:
GemX starts mostly with the same mine.
Millenium 2.2 brings always the same asteroid for the Grazer.

I looked deeper in GemX and found the following:
The subroutine determining the next mine is at $5ade.
At $5aea is MOVE.W (A5, $6), D0 - A5 has $DFF000 in it, making it $DFF006.
Only the byte value is used, that is the horizontal beam counter.
This means the horizontal beam position is used as random value,
and this value is not as random as on a real Amiga.

I tested it also with WinUAE and versions 3.4 and 4.0.1 have the same problem. With 4.0.1, in most cases mine A10 is presented at start, and in a few cases A5-A9. WinUAE 2.4 is not affected.

So it looks as if the cycle-exact emulation is a bit too exact.
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