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Forums > C64 Coding > fetch the 0-sprite. with open borders...
2010-05-17 18:59
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...

2010-05-17 19:42
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.
2010-05-17 20:10
Danzig

Registered: Jun 2002
Posts: 440
But it's fun still to see people trying... in somewhat like 2010!
2010-05-17 21:07
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!.
2010-05-17 23:50
Ninja

Registered: Jan 2002
Posts: 411
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.
2010-05-18 03:51
Oswald

Registered: Apr 2002
Posts: 5094
hmm does the cpu finish off all instructions or may it be stopped in the middle of one ?
2010-05-18 04:25
Ninja

Registered: Jan 2002
Posts: 411
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.
2010-05-18 04:28
Oswald

Registered: Apr 2002
Posts: 5094
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.
2010-05-18 05:06
Ninja

Registered: Jan 2002
Posts: 411
What do you expect to happen if you halt the CPU while it is driving R/W# low? :)
2010-05-18 05:17
chatGPZ

Registered: Dec 2001
Posts: 11386
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 :)
2010-05-18 06:09
Oswald

Registered: Apr 2002
Posts: 5094
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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