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Old 07 August 2017, 09:21   #1
sandruzzo
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Display DMA

We know well how DMA slots with bitplanes are allocated. I was wondering if back in the day, having Display DMA usage as we know, only in the chip-ram were screen bitplanes read datas, and left other free chip mem, full of speed.
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Old 15 August 2017, 23:12   #2
Trachu
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Amiga OCS was created by design to operate at full spedd with 4 bitplanes lowres or 2 bitplanes high res, due to architecture of Motorola MC68000 CPU which can utilise memory access every second clock, leaving the rest for chipset.
If you use 5 bitplanes CPU speed drops by 25% in display time, while still being at 100% in offscreen area. Typically this means 10% overall slow down.
If you use 6 bitplanes CPU speed drops by 50% in display area which means about 25% overall.

Chip memory access is only one, you cant have one portion of it operating faster than the other, but the speed only depends on when in raster time you access this memory.

On AGA chipset the memory interface is much more complicated and has many strange behaviours. For example turing 4 bitplanes makes cpu operating faster than with 8 bitplanes, no matter is it hires or super hires.
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Old 16 August 2017, 06:40   #3
sandruzzo
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Aga was very bad designed
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