exxos's Atari, Amiga & retro forum

There was also this beast (from 1990) which made it possible to have up to 14 MiB of ST-RAM, by re-implementing parts of the MMU:

https://forum.atari-home.de/index.php/t ... #msg257897
https://forum.atari-home.de/index.php?a ... 9608;image

So, it's certainly possible.

mrbombermillzy wrote: 14 Oct 2024 12:29 Wouldnt the inclusion of the (in development) MMU at least entertain the possibility of the extra ram being ST-RAM?

Implementing more than 4MB ST-RAM on a FPGA based MMU is not a problem at all. At least the hardware side. But it requires either an enhanced GLUE version as well, or either connecting to some other motherboard signals besides the ones that are available at the MMU socket.

czietz wrote: 14 Oct 2024 17:21 There was also this beast (from 1990) which made it possible to have up to 14 MiB of ST-RAM, by re-implementing parts of the MMU:

A beast indeed :) But yes, implementing extended ST-RAM without replacing MMU entirely with a FPGA, is probably much more complicated. At least if you want the extra RAM to support DMA and video memory, which is the main purpose of extending ST-RAM, I guess. Quite an accomplishment for a 1990 development!

I may be misremembering and someone who's spent more time over on stardot than me may correct me, but I think there are mods for the early Acorn Archimedies machines to extend their RAM that work by adding a second MMU (or whatever the Acorn equivalent's called).

They have the same issue we do that the MMU is restricted to 4MB but the address space available is much bigger.

I don't know if a similar approach would be feasilbe in ST land where some different address lines are fed into a second chip, perhaps?

Maybe that's what that board Christian linked did? I see two sockets there.

BW

Heh, I wonder if this Bartosz Świderski is from Warsaw and is one who repaired my falcon ram and expanded/repaired my dad's tt back in the day (pre 2000). At some point he ditched Atari repairs if I correctly remember..

Badwolf wrote: 15 Oct 2024 16:48 I don't know if a similar approach would be feasilbe in ST land where some different address lines are fed into a second chip, perhaps?

From what I remember, which may well be wrong, the GLUE is responsible for DMA hand off to the MMU. So aside from the extra address decoding logic needed for the GLUE, you would need other signals intercepted as well. Mix in the video address decoding etc .. It quickly becomes a mess.

It's why I went down the road of FPGA as you can properly fix these issues in code. Plus long term, the sky is the limit in FPGA. Doing massive hacks to the original chipset plus headaches of debugging... It's simply a deadend after that anyway.

Either way, it seems replacing the MMU won't be enough for extra RAM. Assuming it's intention is STram.. unless you want the same limitations as alt-ram... In which case, why bother replacing the MMU..

exxos wrote: 16 Oct 2024 10:44 It's why I went down the road of FPGA as you can properly fix these issues in code. Plus long term, the sky is the limit in FPGA. Doing massive hacks to the original chipset plus headaches of debugging... It's simply a deadend after that anyway.

Yeah, I was just speculating as to the 90s solution. Not a lot of point these days, although the FPGA option has only just been solved, of course. :)

BW

Badwolf wrote: 15 Oct 2024 16:48 I may be misremembering and someone who's spent more time over on stardot than me may correct me, but I think there are mods for the early Acorn Archimedies machines to extend their RAM that work by adding a second MMU (or whatever the Acorn equivalent's called).

I think you are thinking of the Archimedes MEMC? By adding additional MEMCs you can expand from 4MB upto 16MB. But the ARM2/3 has a 26-bit address bus (not 16-bit like 68000) and IIUC these are actual memory controllers not traditional MMUs (but I imagine that the ST MMU is not really an MMU?) but they do have some unusual behaviour (They have a CAM)