ReadOnlyCat |
23 July 2018 10:36 |
Quote:
Originally Posted by Toni Wilen
(Post 1253382)
Sorry for not being last off topic post because conclusion was incorrect and needs correction :)
Only requirement is ECS Agnus because ECS Agnus always see trap door in usual upper half of 1M chip RAM address space ($80000-$fffff).
Only hardware difference between 0.5M+0.5M compared to "real" 1M chip config is address space where CPU sees the memory: $c00000 or $80000. Agnus' view of address space does not change. (There is even one demo that uses this trick)
This is yet another less known hardware feature.
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Thanks for the correction!
After looking at the schematics again, indeed, even on OCS machines Agnus has access to the expansion data bus (on the rev6 A500 specifically, no idea on the A1000).
However, to be more precise. As far as stock machines are concerned, ECS is definitely needed for the chipset to be able to use 1MB of chip RAM.
Sure, one can simply upgrade Agnus but not everyone modifies their machines and knowing what to expect from the stock versions is important.
This is an interesting point since despite being physically connected to the slow RAM bus, there was no way for Agnus to access the corresponding memory before ECS Agnus was built. This leads me to suspect that the hardware engineers had already planned to expand Agnus to support >512Kb of RAM (or had already done so at the prototype stage) when they designed the A500.
Quote:
Originally Posted by Toni Wilen
(Post 1253415)
Yes to all. From Agnus point of view it is normal upper half of 1M chip RAM. Use it like it is normal 1M Chip RAM when accessing Agnus DMA registers, add ($c00000-$80000) offset to addresses when CPU accessing same RAM.
http://eab.abime.net/showthread.php?t=36905 has some more details.
(Note that in WinUAE this is only emulated in compatible modes because this kind of address mirroring breaks JIT)
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Is the OS capable of using that RAM as chip RAM? I twiddled and experimented extensively with slow/chip/fast RAM back in OCS and ECS days and I do not remember being able to use my OCS A500 512Kb expansion as chip RAM back then.
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