Maximum RAM in an Amiga 2000??

[MelbourneBen]

Hi,

I've read that the A2000 has a maximum capacity of 8MB of RAM without using a 3rd party accelerator card. Does this include chip ram?? meaning I could only have 6MB fast and 2MB chip.... or could I have 8MB of fast ram and 2 MB Chip ram...totalling 10MB?

Cheers

Ben

[psxphill]

Hi,
 
I've read that the A2000 has a maximum capacity of 8MB of RAM without using a 3rd party accelerator card. Does this include chip ram?? meaning I could only have 6MB fast and 2MB chip.... or could I have 8MB of fast ram and 2 MB Chip ram...totalling 10MB?
 
Cheers
 
Ben

68000 based amigas can have:
 
2 meg chip (0x000000-0x1fffff)
8 meg fast (0x200000-0x9fffff)
1.5 meg slow (0xc00000-0xd7ffff)
 
Adding 1.5 meg of slow ram to an a2000 may be a pain.

[meega]

That 8MB is Fast/Other RAM, in addition to whatever Chip/Graphics RAM your system supports. If you have full ECS then yes you can have 10MB overall with a base 68000 CPU.

[psxphill]

That 8MB is Fast/Other RAM, in addition to whatever Chip/Graphics RAM your system supports. If you have full ECS then yes you can have 10MB overall with a base 68000 CPU.

You'll need a megachip to get 2meg of chip ram. Just having an ECS agnus will give you 1meg.

[Xanxi]

That 8MB is Fast/Other RAM, in addition to whatever Chip/Graphics RAM your system supports. If you have full ECS then yes you can have 10MB overall with a base 68000 CPU.

What is exactly slow ram? I have only heard of this in WinUAE and never before in a real amiga. How can it be added to a real amiga setup?

[spaceman88]

What is exactly slow ram? I have only heard of this in WinUAE and never before in a real amiga. How can it be added to a real amiga setup?

If you add a trap door RAM card to your A500, that's slow-fast RAM (as long as you don't do the mod to make it CHIP RAM). If you add RAM to the side bus on the A500 that's "true" fast RAM. Something to do with no bus contention or something else I barely remember :-).

[gertsy]

You can put as much memory in as you want.. But you can only use 8MB of Fast and 2MB of Chip (With an Obese Agnus). Un-spunked of course.

[runequester]

What is exactly slow ram? I have only heard of this in WinUAE and never before in a real amiga. How can it be added to a real amiga setup?

Slow RAM is pretty much the same as chip RAM as far as I know.

[Zac67]

Slow RAM is pretty much the same as chip RAM as far as I know.

... just without the benefit of being chip RAM...

Maybe a bit of basic knowledge helps here: the Amiga system architecture is divided into two parts: the CPU part and the chipset part. Both can work independently from each other. On a single bus, only one device at a time can be active.

So the CPU can read/run code/data from the ROMs or  - if present - true fast RAM while simultaneously Agnus is e.g. shovelling bitplane data into Denise or the Blitter's shifting bits around. When the CPU needs to access the chip side, the busses need to be connected (asynchronous CPU boards require synchronization here) and the CPU must wait for a free cycle to use, effectively slowing it down.

On the CPU side are: CPU, ROMs, CIAs, fast RAM and on the chip side it's the famous DMA architecture sporting graphics, sound, floppy - all within chip RAM and controlled by Agnus.

Now, slow fast RAM is memory that exists on the chipset side, but due to architecture limitations, it's not usable as chip RAM. But since the CPU needs to connect to the chipset side it's just as slow as chip RAM.

[meega]

Also:

The 68000 and the custom chips alternate in their accesses, even cycles being for one and odd cycles being for the other. This means the CPU can never take more than 50% of the available chip/slow-ram bus cycles.

Further, the 68k has the lowest priority in the system and will grant all bus access requests made by anything else whatever. The custom chips, on the other hand, can lock the CPU out entirely if they are busy, the CPU is forced to wait until it gets let back in (only on its appropriate half of the cycles, possibly wasting more time until that condition is met).

True "fast" ram is not directly accessible by the custom chips, the CPU has unfettered access and achieves full-speed. You can see why fast ram is nice to have.

[amigaksi]

... just without the benefit of being chip RAM...

Maybe a bit of basic knowledge helps here: the Amiga system architecture is divided into two parts: the CPU part and the chipset part. Both can work independently from each other. On a single bus, only one device at a time can be active.

So the CPU can read/run code/data from the ROMs or  - if present - true fast RAM while simultaneously Agnus is e.g. shovelling bitplane data into Denise or the Blitter's shifting bits around. When the CPU needs to access the chip side, the busses need to be connected (asynchronous CPU boards require synchronization here) and the CPU must wait for a free cycle to use, effectively slowing it down.

On the CPU side are: CPU, ROMs, CIAs, fast RAM and on the chip side it's the famous DMA architecture sporting graphics, sound, floppy - all within chip RAM and controlled by Agnus.

If you don't have an accelerator (default condition for A500/A2000), then fast ram is still being accessed at same speed as chip RAM so I guess it's just "fast" because there's no DMA cycles getting in the way.

Now, slow fast RAM is memory that exists on the chipset side, but due to architecture limitations, it's not usable as chip RAM. But since the CPU needs to connect to the chipset side it's just as slow as chip RAM.

And that's the dark side of the force.

[Zac67]

The 68000 and the custom chips alternate in their accesses, even cycles being for one and odd cycles being for the other. This means the CPU can never take more than 50% of the available chip/slow-ram bus cycles.

Indeed - but the 68000 isn't able to utilize more than half the cycles anyway, so fast RAM isn't faster than (nearly) idle chip RAM (different with faster CPU/RAM, of course).

[meega]

Indeed - but the 68000 isn't able to utilize more than half the cycles anyway, so fast RAM isn't faster than (nearly) idle chip RAM (different with faster CPU/RAM, of course).

What happens if the CPU takes an odd number of cycles to complete an operation?

[MelbourneBen]

Thanks, I thought that was the case....as a side note does anyone know if any Amiga retailers sell ECS denise's? I've been looking on ebay for a while but only ever see the OCS denise advertised for sale.

[Zac67]

What happens if the CPU takes an odd number of cycles to complete an operation?

Nothing special, since a single CPU cycle consists of two bus cycles.