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Old 29 June 2015, 15:32   #1
titan sucks!
Join Date: Dec 2012
Location: munich/germany
Posts: 52
Copper SKIP after SKIP


I'm playing around with Copper SKIPs and JUMPs and I've noticed that a SKIP directly after a successful SKIP is executed though. Copper code is something like:

SKIP - this one is successful, so it should skip over the next SKIP to JUMP, but he continue just right ....
SKIP - here... :-P
copper.cpp, line 6318:
if (cop_state.i1 & 1) { // WAIT or SKIP
	cop_state.ignore_next = 0;
	if (cop_state.i2 & 1)
		cop_state.state = COP_skip_in2;
		cop_state.state = COP_wait_in2;
	if (debug_dma)
		record_dma (0x8c, cop_state.i2, cop_state.ip - 2, old_hpos, vpos, DMARECORD_COPPER);
	if (memwatch_enabled)
		debug_wgetpeekdma_chipram(cop_state.ip - 2, cop_state.i2, MW_MASK_COPPER, 0x8c);
Here ignore_next is reset to 0 instead of emitting a 0x1FE/NO-OP like in the MOVE command.

Is that intended and can I find it in the hardware docs?

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Old 29 June 2015, 15:42   #2
Toni Wilen
WinUAE developer
Join Date: Aug 2001
Location: Hämeenlinna/Finland
Age: 47
Posts: 25,364
It is correct and matches real hardware.

SKIP does not actually skip anything.

It only sets internal variable that causes following MOVE (only if following instruction is MOVE) to do the move to dummy address. MOVE cycle sequence is still the same, "skipped" or not.

Nothing happens if following instruction is another SKIP or WAIT.
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Old 29 June 2015, 15:54   #3
titan sucks!
Join Date: Dec 2012
Location: munich/germany
Posts: 52
Okay, I see. So no way to do negative SKIPs =/

neoman is offline  
Old 29 June 2015, 16:46   #4
titan sucks!
Join Date: Dec 2012
Location: munich/germany
Posts: 52
Okay, I solved my problems with a sequence like

That's what I meant by "negative" SKIP. Basically I needed if then else and this is working fine now =) Thanks again.
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