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Oswald
Registered: Apr 2002
Posts: 5094 |
SHX/SHY
I cant seem to get them working. I'm not sure wether the value for the AND comes from the PC counter or from the destination address, anyway neither running code at $ff00 or storing to $ff00 does work. maybe vice does not support this illegal?
edit: vice 2.2 unstable, x64 on win7. |
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Ninja
Registered: Jan 2002
Posts: 411 |
Oh, and visual6502 won't help you since you need interaction with VIC. Note, that XL and AppleII users never noticed dropping off the AND (as far as I found out). |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Whoever feels like it, please go ahead and write a few lines about SHX/SHY and put it in the following section of Codebase64:
http://codebase64.org/doku.php?id=base:6502_6510_coding#illegal..
:) |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
"It is known. I somewhere have a test-program which reliably does SHX/Y with no AND happening because it waits for the VIC to take over the bus."
thing is that fiddling with BA does _not_ explain that behaviour in any way. so while it is known that the behaviour is somehow connected to DMA, it is not known what exactly happens =) |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Ahh, the inexplicable mysteries of the C64, isn't that what makes it magic. |
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Zyron
Registered: Jan 2002
Posts: 2381 |
That's where its soul is. |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Quoting FranticWhoever feels like it, please go ahead and write a few lines about SHX/SHY and put it in the following section of Codebase64 I felt like it the other day. :)
Btw, I also tried what happens with page crossing, and it turned out much weirder than expected, since it stores the value to a completely different page. Double confirmed on VICE and real hardware. Gotta do some more testing before I can figure out the logic behind which page it ends up on, since I can't find anything online. But since VICE emulates it, I guess the knowledge has to be out there somewhere. :) |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Quote: Quoting FranticWhoever feels like it, please go ahead and write a few lines about SHX/SHY and put it in the following section of Codebase64 I felt like it the other day. :)
Btw, I also tried what happens with page crossing, and it turned out much weirder than expected, since it stores the value to a completely different page. Double confirmed on VICE and real hardware. Gotta do some more testing before I can figure out the logic behind which page it ends up on, since I can't find anything online. But since VICE emulates it, I guess the knowledge has to be out there somewhere. :)
Nice :) |
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Krill
Registered: Apr 2002
Posts: 2980 |
Quoting CruzerDouble confirmed on VICE and real hardware. Gotta do some more testing before I can figure out the logic behind which page it ends up on, since I can't find anything online. But since VICE emulates it, I guess the knowledge has to be out there somewhere. :) So, why don't you simply take a look at the VICE source code? Avoiding confirmation bias and expecting to come up with different results? :) |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
or why not just read No More Secrets v0.91 ? :) |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Because experimenting is fun. :) And I guess I could have just read Groepaz' earlier comment saying that the page becomes the value stored. I.e. for SHX: Page = (H + 1) & X and for SHY: Page = (H + 1) & Y. Codebase article updated. |
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