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Oswald
Registered: Apr 2002
Posts: 5127 |
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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chatGPZ
Registered: Dec 2001
Posts: 11523 |
"The instability is only that the and'ing sometimes is skipped"
there is another problem when indexing crosses a page boundary, then the page the value is stored might be the same as the value itself. |
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Cruzer
Registered: Dec 2001
Posts: 1051 |
Groepaz: Interesting, guess I haven't tried crossing page boundaries. |
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Oswald
Registered: Apr 2002
Posts: 5127 |
"it could (and should and will) once the behaviour is exactly known :)"
why isnt it known? every single transistor has been reverse engineered. visual6502 anyone ? |
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Ninja
Registered: Jan 2002
Posts: 418 |
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. I intended to make that a bit more robust and write an VN article about it, but well, -EBUSY. I seem to recall that idea was later mentioned in this forum, too. |
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Ninja
Registered: Jan 2002
Posts: 418 |
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: 1661 |
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: 11523 |
"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: 1051 |
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: 1051 |
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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