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Cruzer
Registered: Dec 2001
Posts: 1048 |
Unstable Illegal Opcodes
I have always avoided the opcodes that are marked as unstable on Graham's opcode list:
http://www.oxyron.de/html/opcodes02.html
But maybe some of them could be used in some cases after all? Anyone who can shed some light on when excactly they are unstable?
According to Graham's list there are two kinds of unstableness, the first being "unstable in certain matters", which applies to these opcodes:
ahx
shy
shx
tas
The other one doesn't leave much hope: "Highly unstable (results are not predictable on some machines)", but only these two are that bad:
xaa
lax #imm
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Oswald
Registered: Apr 2002
Posts: 5017 |
usually there's a way to do what you want with illegals, and a way with legals, and 95% of the time the cyclecount are the same. You cant really generalize where and what is useful, using illegals needs quite specific code. But some will prove me wrong lets see :) |
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Ninja
Registered: Jan 2002
Posts: 404 |
Those opcodes with "AND PCH+1 drops sometimes off" are not that mystical. It drops off, when VIC takes over the bus. Try disabling badlines and it will never drop off (at least on the various machines I checked). Activate a sprite and it will drop off again. Maybe you can time your code to that :)
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algorithm
Registered: May 2002
Posts: 702 |
Any examples where Illegal opcodes have been used in VIC trickery? One example I know of is darwin/dreams where 6 sprites over full screen FLI were achieved.
Any more? |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
@algorithm: When Crossbow did 5 sprites over FLI :)
@ninjadrm: What does "drop off" mean in this case? That it doesn't perform the and, or that it adds 1 to the and value?
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Ninja
Registered: Jan 2002
Posts: 404 |
@algorithm: Surely some more VIC-tricks have used basic stuff like LAX or SAX.
@cruzer: The AND drops off, so in case for SHX and SHY you have the pretty nice STX abs,y and STY abs,x opcodes. If you time it to VIC access that is :) Geez, I really could finish the article about those one day...
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HCL
Registered: Feb 2003
Posts: 716 |
Often you need to use illegals if you want to put magic colors on the leftmost 3 chars of a normal FLI-picture. That's more a conisidence though.. |
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Oswald
Registered: Apr 2002
Posts: 5017 |
ninja, now thats interesting !! could you shortly describe how and what happens ? |
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Frantic
Registered: Mar 2003
Posts: 1627 |
@Ninja: So.. Does that mean SHX/SHY works in the border or do you have to disable VIC? (I could try it, but only on one machine..) |
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Oswald
Registered: Apr 2002
Posts: 5017 |
this means imho that if the vic takes over the bus during shx/shy etc, then the and effect is dropped, or something like that :) |
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JackAsser
Registered: Jun 2002
Posts: 1987 |
What bugs me with the VIC-take over explanation is the fact that the VIC has taken the bus... i.e the CPU doesn't do anything at all. |
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