Display DMA

sandruzzo · 07 August 2017, 09:21

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.

Trachu · 15 August 2017, 23:12

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.

sandruzzo · 16 August 2017, 06:40

Aga was very bad designed

Tigerskunk · 27 November 2017, 08:40

What do you do with the CPU anyway during raster time? Most of my game logic is usually done during offscreen...

07 August 2017, 09:21	#1
sandruzzo Registered User Join Date: Feb 2011 Location: Italy/Rome Posts: 2,302	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.

27 November 2017, 08:40	#4
Tigerskunk Inviyya Dude! Join Date: Sep 2016 Location: Amiga Island Posts: 2,775	What do you do with the CPU anyway during raster time? Most of my game logic is usually done during offscreen... Last edited by Tigerskunk; 27 November 2017 at 09:05.

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15 August 2017, 23:12	#2
Trachu Registered User Join Date: Dec 2015 Location: Poland Posts: 189	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.

16 August 2017, 06:40	#3
sandruzzo Registered User Join Date: Feb 2011 Location: Italy/Rome Posts: 2,302	Aga was very bad designed

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