I recently developed a popular board called RAMesses, which upgrades an Amiga 500 / 2000 / Commodore CDTV to 2MB of Chip RAM without any soldering / cutting of the motherboard. I figured I would post more about what it is and why I developed it.
Amiga computers have three main types of memory that are all handled differently. “Chip” memory is controlled by Agnus (Alice for AGA machines) and the co-processors inside the Amiga can access this using DMA. There is “Fast” RAM that is often called “Slow” RAM, this is RAM that only the CPU can use but is accessed via Agnus so is not as fast as it could be. When you put a RAM expansion in the trapdoor slot of an Amiga 500, this is usually what the upgrade provides. Then there is true “Fast” RAM which the CPU can directly accessed, most CPU accelerators include this.
While on the outside there isn’t much difference between one Amiga 500 and another, there were several motherboard versions with different capabilities. All Amiga 500s (excluding Plus) had a stock 512KB of Chip RAM the Agnus version in the motherboard could dictate how far it could be expanded. Most could only be expanded to 1MB of RAM.
Due to the address map and the addressing for the Agnus chip the maximum possible is 2MB RAM, this is only possible with the model “8375” Agnus (and only the 2MB versions of this).
Most games are happy with the stock 512KB, but there are many that either have enhancements when you have more or require 1MB of RAM. In addition there is technology called WHDLoad to effectively run a virtual Amiga from Workbench to run games from the hard drive, this can require more than 1MB of Chip RAM to operate correctly with some games.
Whilst in most cases you can upgrade a 512KB Agnus to a 1MB one just by pulling the old one and inserting a new one (and sometimes blanking off a pin) you still need to make some modifications to the motherboard so that any extra memory becomes Chip RAM.
In addition you cannot take a 2MB 8375 Agnus (typically used in an Amiga 500 Plus or Amiga 600) and plug is straight into an Amiga 500, due to the additional address lines the pinout is not compatible.
Matt Harlum (LIV2) created a board which converted an 8375 Agnus to work in a regular Amiga 500 and add 2MB of Chip RAM. It required you to desolder the Agnus socket (as an Agnus plug is either expensive or difficult to make), and use regular pins / socket for the board. Whilst it is nice and compact it is quite difficult to build. I used this as a basis to create a new version which is easier to build.
Whilst I liked this design there some things I still wasn’t happy about:
I therefore decided to design a board that solved these problems.
The RAMesses board uses a PLCC plug to plug into Amiga’s Agnus socket and an 8375 Agnus plugs into the underside of it. In addition there is an adaptor to go under the Gary chip for a few additional signals. It overrides any motherboard and expansion RAM so that it is easy to install no matter what your motherboard / configuration is. This means you do not need to make any modifications to the Amiga motherboard, and any modifications you have made in the past are bypassed. It is also smaller in every dimension than my previous board, to the extent that it even fits in a Commodore CDTV.
Above you can see the RAMesses prototype in purple (the project was called MightRAM back then), final version 1.1 and then 1.2. The only real differences between 1.1 and 1.2 are that 1.2 is easier for me to build and the “legacy mode” is configured slightly differently.
Speaking of “legacy mode”, you’ll note there is a jumper on the top of the RAMesses board. There are a few games that state they require 1MB but they actually mean 512KB Chip and 512KB Slow RAM. So this jumper reconfigures the board to provide Slow RAM. In 1.1 it switched from providing 2MB Chip to 512KB Chip and 512KB Slow, in 1.2 it switches to 1MB Chip and 512KB Slow.
RAMesses is now available on my new Retro Supplies store. I also have PCBs for my older design available. Both require a 2MB 8375 Agnus which are available on eBay and online stores but are difficult for me to obtain in bulk at a reasonable price.
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I wonder if a full FPGA replacement for Agnus could be done to remove most of its shortcomings.
I believe there is a project called ReAgnus which at the very least should be able to reimplement an 8375. If nothing comes of it I may well develop one myself in the next couple of years.
Which WHDLoad games requires 2Mb chip ram?
The answer to that depends on your hardware configuration. If you don't have Fast RAM available then pretty much every game that uses a ROM file. There are several games that take advantage of more Chip, off the top of my head Settlers is one.
Since it has to connect to Gary chip (like MegaChip) it is incompatible with certain accelerators like the Irix Labs Classic 520.
Do you have a link to more information on this? I don't have that particular accelerator but I can't see a reason why a Gary signal capture would make it incompatible (unless it has logic to override Gary?).
Edit: just checked their manual and it seems to suggest it is incompatible with Slow RAM upgrades with Gary adaptors, which is totally understandable. It means that RAMesses legacy mode won't work but the regular mode should. Otherwise the 520 would also be incompatible with the Amiga 500+.
This is the email reply i received today from Irix Labs:
Yes, confirmed. It will not work.
Get a rev. 6 or rev. 8 motherboard to use 1-2 MB Chip RAM.
Best regards,
Viktor K.
Those two statements are contradictory there is no way that the default mode of RAMesses can't work and the 520 can work on a rev 8. They are literally wired the same way.
If it doesn't work for some bizarre reason (I can't think of any for default mode, the logic on it isn't big enough for a full bus replacement like Vampire) I recommend getting a more Amiga compatible accelerator, all the ones I've tested do work.
this is nicebut still suffers from the problem of tapping the gary chip. upgrades like this will not work with the classic 520 accelerator and certain other accelerators.
You are the second person to comment that in the last 24 hours, I would be interested to know why people think that. It may well have compatibility problems with the legacy mode on RAMesses, as do other accelerators that use the ranger area, but normal operation should be fine. Otherwise the Classic 520 would be incompatible with an Amiga 500+.
Can I use any 2mb 8375 or do I need to use an NTSC one if in USA (use a SCART adapter to HDMI) …
Any 2MB will work. A PAL one will default to PAL, NTSC will default to NTSC. They are switchable in software. SCART will work fine, there is a very slight clock difference but that only seems to affect composite.
Any PAL or NTSC is fine, but can you clarify which 8375 has been used in the design. The 2MB 390544-xx series have a different pinout compared to the 2MB 318069-xx series. Thanks
The 2MB versions of those serials have the same pinout (except for maybe VBB which is catered for). The 1MB version has a different pinout as does the 2MB 8372B from the A3000.
Thanks for the reply. I am a little confused. This link mentions the pin incompatibility, which is why I asked. https://www.pmsoft.nl/amiga/amiga-parts-chips.html
I'm not sure, the content in there is a little contradictory. My guess is it is VBB that is the incompatibility. I have several test models in my workshop. Unfortunately I'm in Germany this week so can't check what they are until I get back.
In fact I just checked my photos from testing RAMesses boards. The first prototype was using a 390554-01, the first batch were sold with 318069-10 VBB chips (back when I could buy them in bulk). Now the VBB variant requires an capacitor to support it on a pin which should not be there for the non-VBB. It would likely cause compatibility issues on motherboards that support the 8375. This is why RAMesses has a configuration jumper for this on the bottom.
Is there hope for a second run of the board in the future?
There have been many runs of it, another batch should be ready in a week.
The 2MB chips 318069-18/318069-19 are also marked 8375, and are VBB versions of the 318069-03 (A3000), but are not pin compatible with the 2MB 390554-01/318069-10/318069-11 (A500+/A600).
https://www.amigawiki.org/lib/exe/fetch.php?media=de:parts:agnus-history.pdf
https://www.amigawiki.org/lib/exe/fetch.php?media=de:parts:agnus_reworks.pdf
https://www.amigawiki.org/lib/exe/fetch.php?media=de:parts:agnusliste_v09_120923.pdf
I've never seen one for sale, but that is true.