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phx · 29 May 2012, 11:21

It's at least 15 years ago when I last programmed the chipset, but this confuses me:

I want to display a soft-scrolling screen of 320x240 pixels. To use the scroll-delay in BPLCON1 I have to fetch an additional word each line. So the usual DDFSTRT of $38 becomes $30, with DDFSTOP still at $d8.

This means I have 42 bytes each line. For an interleaved 32 color bitmap I will point BPL1PT to the bitmap's start address, BPL2PT to bitmap+42, BPL3PT to bitmap+84 and so on.

Now the BPLxMOD. I thought that when the hardware has read its 21 words for a line it has to skip 4 * 21 words (for the other 4 interleaved bitplanes) to advance to the start address of the next line. So I set BPLxMOD to 4*42 = 168.

But that's wrong! It simply didn't work. With some experiments I got the expected result then setting the BPLxMOD to 166. Two bytes are missing. Why?

29 May 2012, 11:21	#1
phx Natteravn Join Date: Nov 2009 Location: Herford / Germany Posts: 2,496	BPLxMOD for interleaved bitmaps It's at least 15 years ago when I last programmed the chipset, but this confuses me: I want to display a soft-scrolling screen of 320x240 pixels. To use the scroll-delay in BPLCON1 I have to fetch an additional word each line. So the usual DDFSTRT of $38 becomes $30, with DDFSTOP still at $d8. This means I have 42 bytes each line. For an interleaved 32 color bitmap I will point BPL1PT to the bitmap's start address, BPL2PT to bitmap+42, BPL3PT to bitmap+84 and so on. Now the BPLxMOD. I thought that when the hardware has read its 21 words for a line it has to skip 4 * 21 words (for the other 4 interleaved bitplanes) to advance to the start address of the next line. So I set BPLxMOD to 4*42 = 168. But that's wrong! It simply didn't work. With some experiments I got the expected result then setting the BPLxMOD to 166. Two bytes are missing. Why?