1MB Chip Ram on Amiga 2000 Rev 4 (Germany 1986)

[LaFey]

I have a Rev 4 (Germany) Amiga 2000 "Model A (German design derrived from the A1000 w/ an expansion card sidecar unit)"

My memory map looks like this:



It has:

• 512kb Chip Ram on the Motherboard;
• 1Mb RAM Expansion a card on the MMU slot with another 512kb listed under Slow;

According to the Big Book:

"Memory in the lower bank of 16 chips is placed at Address: $C00000-C7FFFF, and is OS-detected. This is what would be the slow-fast 512K memory range in the 8370/8371 PLCC Agnus Rev 4.x B2000 motherboards, but is actual FastRAM (and not connected to the Agnus-side of the bus)."

• 2MB Fast comes from A2058 RAM expansion;
• Another 2MB Fast comes from A2091 SCSI controller;

I understand later revisions under 4.x can do a "simple" modification to achieve 1MB Chip RAM, which involve replacing the original Agnus with the fat 8372 variant, isolating pin, cutting traces and setting jumpers.

Is such a thing even possible (or desirable) in this computer?

 

[dava1973]

It would be desirable to have more chip RAM, as it would mean more memory for the custom chips, and thus greater storage capacity for graphics and sound (bigger screens, more colors, longer samples). However, I don't think this is possible—or at least not without major modifications to the system.

I have recently finished repairing a "Braunschweiger" Amiga 2000A, and as a result, I’ve studied the schematics extensively. The architecture of the 2000A is completely different from the later 2000B and 2000C models. The 2000A is still based on the Amiga 1000 design, whereas the later 2000 models are based on the newer Amiga 500 architecture.

There are some significant hurdles to overcome:

• the 2000A comes with the oldest Agnus chip (8367), which is in a DIP package. Newer Agnus chips use a PLCC package, so at a minimum, you would need an adapter from PLCC to DIP. But that alone won’t help, because...
• ...the 8367 Agnus chip in the 2000A cannot handle more than 512 KB of chip RAM and does not include the adress decoding circuits (see more to that below). So you would need one of the newer Agnus chips capable of handling 1 MB of RAM. However, the problems only begin there...
• ...as the 2000A handles address decoding via PALs and discrete logic chips. In later Amigas, address decoding has been integrated into the Agnus chip, with some functions moved to the new Gary chip (which does not exist in the 2000A). Therefore, you would need to reroute the entire address bus for Chip-RAM to pass through the new 1 MB Agnus, and somehow replicate the functions of Gary—all while ensuring that the DMA system, which is handled differently on the 2000A (via PALs, since there is no Buster chip yet), remains intact.

By the time you’ve completed all this, you will have essentially reinvented the Amiga 500/2000B architecture

So, unfortunately, we are stuck with 512 KB of chip RAM on the good old 2000A. That said, I still love this rare gem of an Amiga!

 

[LaFey]

But can't we perhaps dream of the day someone creates a Parceiro or a Rejuvenator for the Rev 4.?

I say this because these are A1000 specific cards that ought to be somewhat compatible with the 4 layer German motherboard. At least theoretically.

 

[dava1973]

Of course - dreamin' is important
I think, the concept of the Rejuvenator would fit our hypothetical scenario perfectly: It is an ECS chipset adapted to the Amiga 1000 board. So in theory, that concept would be modifiable for the Amiga 2000A. But it would have to be one of us, Aamiga 2000A owners, because outside of us, it's not really a huge market: Only very few Amiga 2000A exist, as much as I know, so there is little motivation to do something like this... Maybe once I am retired, I will try, but I would have to learn a HUGE many things to do something like this ...

I do not know the Parceiro well, but it goes on the cpu port, if I remember well, so it most probably doesn't include any connections to the custom chip side of things so it wouldn't help with chip ram, I would think...