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Old 16 February 2011, 14:02   #1
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Garbage on screen during boot

I used to have an A500 (KS 1.3) and remember that sometimes when booting plain DOS disks (like Workbench), there would be a quick flash of garbage on the screen instead of plain white while the boot block (original unmodified) was loaded and did its thing. Once in a while the garbage would stay until the AmigaDOS window appeard, and the garbage would change with the floppy drive noise.

For some reason I suddenly remembered this "phenomenon" the other day, and am curious about why this happens. I've seen it on other Amigas, so it wasn't a problem with my particular computer. Is it a bug in the Kickstart that occasionally causes bitplane pointers to be set to random addresses, pallette issue, BPLCON, or what?
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Old 16 February 2011, 14:05   #2
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You can still use rippers after reset so I think RAM is not fully cleared.
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Old 16 February 2011, 14:20   #3
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It shouldn't matter if memory is cleared or not, as the hardware registers decide if anything is visible on screen. You're right about memory usually not being cleared on reboot, but I had an Action Replay III which does clear all memory IIRC. The pixel data of the garbage is likely system structures that exist in memory prior to booting, Kickstart code/data, or (if memory isn't cleared) whatever has been in memory earlier. What I wonder is WHY this pixel data is sometimes allowed to display briefly, sometimes longer, and often not at all?
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Old 16 February 2011, 14:54   #4
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I guess it's because of bitplane DMA that is enabled before bitplane pointers have been initialised.
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Old 16 February 2011, 15:52   #5
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Hmm, so DMA is always enabled slightly before pointers are initialised, and depending on the initial content of the pointer registers there may or may not be a flash of garbage. That makes sense for most of the cases, but what about the odd ones where the pointers aren't set at all, and the garbage stays on screen and animates for the few seconds it takes to open the AmigaDOS window? Race condition of some kind?
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Old 20 February 2011, 02:25   #6
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Maybe there is something dodgy with the boot block and it is a bit buggy. Are you certain that this is a clean C= boot block or has something else installed itself there (virus, maybe)?
Old 20 February 2011, 21:28   #7
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I can confirm I also saw this a lot, still happens from time to time on my CDTV nearby. And the bootblocks are usually the standard C= ones...
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