21 upvotes, 3 direct replies (showing 3)
View submission: I'm still struggling understanding RAM types/upgrades for OCS 500.
Where on earth does "slow" RAM sit in all the above?
The thing to understand is that the Amiga has two RAM buses. A contended Chipset bus and an uncontended CPU bus
The contended Chipset bus is conventionally called the Chip RAM. The uncontended CPU bus is conventionally called the Fast RAM. On the contended bus the CPU and Chipset must take turns to read/write to the RAM. This makes access slow from the POV of the CPU. On the uncontended bus the CPU can read/write on any CPU cycle, so the CPU has no wait states on this bus and access is fast for the CPU
However, the Chip RAM bus is divided in to two RAM address pools. One pool the graphics and audio chipset and the CPU can access. The other pool of addresses can only be accessed by the CPU. This led to a new convention of referring to the contended Chip RAM addresses *that only the CPU can access* as "slow RAM",
Today, it would probably actually make more sense to talk about Fast and Slow RAM and to regard Chip RAM as a special subset of the Slow RAM. But that isn't how these terms were used and emerged historically.
Coming back to the A500:
The DMA controller for the A500 only allows the Chipset access to the first 512kb of Chip RAM addresses. This is a fixed limit. Trapdoor upgrades add 512kb of RAM to the contended bus, but in the region that only the CPU can access (i.e the Slow RAM). It is possible to add more chip RAM and more Slow RAM to an A500 but you need specific hardware and adapter mods, like the Gary chip mods and Angus chip upgrades or you need specific revisions on the A500 motherboard.
The side slot on the A500 presents a zorro II(ish) bus to the CPU and this provides the CPU with fast RAM. addresses. Fast RAM is not available via the trapdoor.
So... upgrades:
Most sidecar expansions for the A500 will give you fastRAM slots. Mostly you'll need to find those second hand but the ACA500plus and pacerio500 are modern sidecar upgrades that also provide all sorts of extras.
https://wiki.icomp.de/wiki/ACA500plus
https://www.amigalove.com/viewtopic.php?t=2642
Alternatively if you want RAM and a HDD interface then the IDE68K + gottagofasta, is a very good and affordable option (though you will need their modded kickstart to be able to boot from the HDD)
https://amigastore.eu/en/929-ide68k-gottago-fastram-8mb.html
For upgrading the chipRAM you will need a specific chipram upgrade and you need to know which motherboard revision you have as there maybe jumpers you need to change/cut. The latest A500s actually had the A500+ motherboards in them and you can just solder on the RAM chips to the board. For the earlier A500s you need some kind of hardware adapter/mod. Here's one, but you can shop around for others
https://www.retropassion.co.uk/product/ace2b-amiga-2mb-chipram/
Comment by jrherita at 01/03/2025 at 15:43 UTC
2 upvotes, 1 direct replies
OP - definitely look at the ACA500+; it makes upgrading from 512KB to 1MB chipram on an older A500 pretty easy while also giving you fastRAM, a faster CPU, and a HDD interface (several upgrades in one). Kickstart/Workbench is even built into the device. For 1MB chipram you just need to buy an ECS Agnus and a 512KB trapdoor expansion. (The 512KB trapdoor expansion is common). No soldering/wire cutting:
1M Chip RAM expansion: This option allows to double the memory that is available for customchip access. This requires a physical trapdoor memory expansion of exactly 512k as well as an ECS Agnus, so it is not available on early OCS Amiga 500s.
Comment by spacemidget75 at 01/03/2025 at 17:58 UTC
2 upvotes, 1 direct replies
Thank you!
Comment by Daedalus2097 at 01/03/2025 at 21:58 UTC
1 upvotes, 1 direct replies
Good explanation! Just to add, while slow RAM is on the same bus as chip RAM, it's actually considered fast RAM by the OS and software because it can only be accessed by the CPU, despite being as slow as chip RAM.