When is the modulo added to the bitplane pointers?

DanScott · 06 March 2016, 13:18

Is it as soon as DDFSTOP is reached, or is it at DDFSTART position? or some unknown position between the two?

Toni Wilen · 06 March 2016, 14:05

Modulo is added when plane's last DMA fetch is done, after DDFSTOP, plane by plane.

Remember that DDFSTOP (quite unintuitively) means that following complete "bitplane block" (all planes fetched) will be the last one, DDFSTOP isn't actual stopping position.

DanScott · 06 March 2016, 14:19

Thanks Toni, Good to know.. Not something I ever really thought about before..

just that I was thinking of using modulo rather than bitplane pointers on a particular scanline, to do the split, for vertical scrolling.

EDIT: Seems I can't

Would be a modulo of 40,000 and something... or.. negative number in word size

Photon · 07 March 2016, 18:24

HPOS for modulo read can be checked quickly with a test-Copper, grr!

To bring down the modulo to +-$7ffe you can de-interleave completely or partially. Which may affect the bob routine speed, of course.

06 March 2016, 13:18	#1
DanScott Lemon. / Core Design Join Date: Mar 2016 Location: Tier 5 Posts: 1,211	When is the modulo added to the bitplane pointers? Is it as soon as DDFSTOP is reached, or is it at DDFSTART position? or some unknown position between the two?

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06 March 2016, 14:05	#2
Toni Wilen WinUAE developer Join Date: Aug 2001 Location: Hämeenlinna/Finland Age: 49 Posts: 26,502	Modulo is added when plane's last DMA fetch is done, after DDFSTOP, plane by plane. Remember that DDFSTOP (quite unintuitively) means that following complete "bitplane block" (all planes fetched) will be the last one, DDFSTOP isn't actual stopping position.

06 March 2016, 14:19	#3
DanScott Lemon. / Core Design Join Date: Mar 2016 Location: Tier 5 Posts: 1,211	Thanks Toni, Good to know.. Not something I ever really thought about before.. just that I was thinking of using modulo rather than bitplane pointers on a particular scanline, to do the split, for vertical scrolling. EDIT: Seems I can't Would be a modulo of 40,000 and something... or.. negative number in word size

07 March 2016, 18:24	#4
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,602	HPOS for modulo read can be checked quickly with a test-Copper, grr! To bring down the modulo to +-$7ffe you can de-interleave completely or partially. Which may affect the bob routine speed, of course.

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