Arcade / Console --> Amiga conversion discussions

[sandruzzo]

Originally Posted by Toni Wilen

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.

Even on ECS you can have "1mb" on chip mem?

[roondar]

ECS Amiga's (A500+/A600/A3000) support 2MB chip memory out of the box. Only need to add a trapdoor expansion on the A500+ or A600 to get it.

[sandruzzo]

Just a little bit off, but is it possible to overclock amiga chip set?

[robinsonb5]

Originally Posted by sandruzzo

Even on ECS you can have "1mb" on chip mem?

Yes - you don't even need the full ECS chipset for 1mb of chip RAM, just Agnus (8372 or higher) and a small hardware mod on A500 motherboards. You need the full ECS chipset for 2mb of chip RAM.

Originally Posted by Toni Wilen

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.

So am I understanding this correctly? You're saying that Agnus can actually access A500 trapdoor RAM even if the rest of the system doesn't recognise it as chip RAM? So what can be done with it? Can you blit to/from it? Store copperlists? Bitplanes? Audio?

[Toni Wilen]

Originally Posted by robinsonb5

So am I understanding this correctly? You're saying that Agnus can actually access A500 trapdoor RAM even if the rest of the system doesn't recognise it as chip RAM? So what can be done with it? Can you blit to/from it? Store copperlists? Bitplanes? Audio?

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)

[ReadOnlyCat]

Originally Posted by Toni Wilen

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.

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.

Originally Posted by Toni Wilen

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)

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.

[robinsonb5]

Originally Posted by ReadOnlyCat

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.

Actually quite a lot of A500s (including the one I had as a kid) shipped with the 8372A Agnus, and thus could access 1 meg of Chip RAM with a simple jumper mod on the motherboard - even though the rest of the chipset was OCS.

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.

No, the OS would need patching to translate addresses between the two ranges.

[dlfrsilver]

I have discovered that SF2 use 512 colors. basically, the game open 2 screens of 256 colors.

hint : the plane on guile's stage, indicate Capcom CR 512 for Capcom 512 colors. The megadrive version is CR 64 for 64 colors.