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Author Topic: Maximum RAM in an Amiga 2000??  (Read 9145 times)

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Offline Zac67

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Re: Maximum RAM in an Amiga 2000??
« on: June 10, 2010, 06:12:02 PM »
Quote from: runequester;563900
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.

 

Offline Zac67

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Re: Maximum RAM in an Amiga 2000??
« Reply #1 on: June 10, 2010, 09:14:02 PM »
Quote from: meega;563925
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).
 

Offline Zac67

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Re: Maximum RAM in an Amiga 2000??
« Reply #2 on: June 11, 2010, 06:56:32 PM »
Quote from: meega;563960
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. ;)
 

Offline Zac67

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Re: Maximum RAM in an Amiga 2000??
« Reply #3 on: June 11, 2010, 10:02:57 PM »
@rkauer
Add to that 32 bit Z3 RAM and you've got a list. And on the 1200 you may have 256 MB accelerator RAM.;)

@amigaksi
Depends on how you define things (I'm differentiating bus cycles and clock cycles) - but the 68000 can only utilize half the bus bandwidth, so if the chips use no more than the other half, there's no penalty.
« Last Edit: June 11, 2010, 10:05:46 PM by Zac67 »
 

Offline Zac67

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Re: Maximum RAM in an Amiga 2000??
« Reply #4 on: June 14, 2010, 12:56:47 PM »
Oops - must've missed the "500/2000" part, sorry! ;)

As a side note: accelerator RAM on 500/2000 is not limited to 128 MB (only by existing boards). Since the mobo provides no installation help, adding 32 bit RAM needs to be done 'manually' (boot ROM) regardless of size.
On the 3000/4000/1200 the system supports automatic installation of 128 MB (1200: 256 MB) with no additional logic on the CPU board.
« Last Edit: June 14, 2010, 01:05:35 PM by Zac67 »