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Testa
Account closed
Registered: Oct 2004
Posts: 197 |
fetch the 0-sprite. with open borders...
hi,
ik have a little problem.. i want to open the sideborders with the 4 lowest sprites... i have two questions about it..
when i do the D016 write i use a dec d016 or a lsr or ror as opcode... why does a sty, sta, stx not work....
has it something to do with cpu takeover cycles at that point...
second question... what to do on a badline....
there are not enough free cycles. with 4 sprites for a inc, ror or lsr d016 instead of a sta, sty or stx...
i realize it is common knowledge.. but sorry i dont. know it...
bye...
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Radiant
Registered: Sep 2004
Posts: 639 |
I don't really understand your first question. As for your second question, opening the sideborder with sprite 0 enabled on a badline is impossible because BA will be low at cycle 56. |
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Danzig
Registered: Jun 2002
Posts: 429 |
But it's fun still to see people trying... in somewhat like 2010! |
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Ventura
Account closed
Registered: Jan 2006
Posts: 67 |
Quote: But it's fun still to see people trying... in somewhat like 2010!
Well, can't argue with that, but hey psst: don't tell how any of this works!. |
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Ninja
Registered: Jan 2002
Posts: 406 |
See this graph: In cycles marked with "X" (during takeover), writing is possible, reading not. RMW opcodes are special because their last *two* cycles are write-cycles. This way, you can still access the cycle yo need for opening the border. STA has just the final cycle as write cycle, that is not enough. |
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Oswald
Registered: Apr 2002
Posts: 5022 |
hmm does the cpu finish off all instructions or may it be stopped in the middle of one ? |
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Ninja
Registered: Jan 2002
Posts: 406 |
If that cycle at the first "X" is a read-cycle (and most are read cycles), the current opcode will be halted at this point. |
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Oswald
Registered: Apr 2002
Posts: 5022 |
so why is this whole BA mechanism at place at all then? I thought its purpose is to let instructions finish before letting the bus go. Atari 8 bits simply stop the cpu when needed, there's none of this messy ready cycle business. |
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Ninja
Registered: Jan 2002
Posts: 406 |
What do you expect to happen if you halt the CPU while it is driving R/W# low? :) |
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chatGPZ
Registered: Dec 2001
Posts: 11130 |
Quote:Atari 8 bits simply stop the cpu when needed, there's none of this messy ready cycle business.
the _c64_ does also only stop the cpu (exactly!) when needed - using that "messy" ready cycle business :) the atari stops the cpu _more_ than needed, because it does not have such mechanism :) |
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Oswald
Registered: Apr 2002
Posts: 5022 |
Quote: What do you expect to happen if you halt the CPU while it is driving R/W# low? :)
what does the "lets wait for a read cycle" rule has to do with that ? a read cycle doesnt drives the R/W(rite)# low ? |
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