https://www.reddit.com/r/amiga/comments/1j10859/im_still_struggling_understanding_ram/
created by spacemidget75 on 01/03/2025 at 14:07 UTC
13 upvotes, 11 top-level comments (showing 11)
Could someone please dumb this down for me? So, a Amiga 500 OCS has a trapdoor expansion and a side/CPU expansion:
You can upgrade Chip RAM to 1MB via motherboard chip replacements or via the trapdoor expansion and jumpers??
How do I upgrade fast RAM? Is that only via the side/CPU (Zorro?) expansion, or *can it also be done via the trapdoor (****if you don't cofigure it as chip RAM****)*?
Where on earth does "slow" RAM sit in all the above? Is this only via the trapdoor IF you don't configure it for chip RAM, or is it the remaining RAM on the expansion above the 512Kb chip RAM updgrade?
I'm so confused :(
Comment by danby at 01/03/2025 at 15:15 UTC*
22 upvotes, 3 direct replies
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 VR-Geek at 01/03/2025 at 14:55 UTC
5 upvotes, 1 direct replies
The A500 has 3 main ways it can be upgraded.
1 the trap door slot that can be used to upgrade from 512k to 1MB of ram for all A500 models or from 1MB of ram to 2MB of ram for the A500+.
All A500 shipped with 512k chip ram that can be accessed my both the CPU and the chipset. With the trapdoor exanpansion slot being designed to allow you to double the ram by adding another 512k of ram. Which shows up as fast ram and which can only be accessed by the CPU but as the chipset cant access it directly it cant be used for graphics or sound directly just storing program data.
The sidecar expansion slot was designed to allow you to add an external harddisk, larger 8MB ram expansion and or a CPU upgrade.
Finally its also possible to fix internal expansions that sit between the sockets for the chips such as the CPU or other custom chips as they are socketed to upgrade things like adding modern HDMI out, or an internet RAM and Hd controller.
If you are just going to use your amiga to play games from a floppy drive or gotek drive 99% of the time all you need is the 512K trapdoor ram expansion to give you one 1MB of total system ram.
Now in resent years there have been a few games released that need 1MB of chip ram like the A500+ and A600 had as standard or to allow WHDload to let you play games from HD with workbench is using some of the ram.
Most A500 can be modded to turn the 512K in trapdoor slot to into another 512K of chip ram to give 1MB like the A500+ and A600 shipped with. But that request making some modifications to the motherboard and a newer revision of one of the custom chips. Most A500 with green power lights shipped with the right chip to do this but not all of them.
So before planning to do the 1MB chip ram upgrade you need to either use the Program SysInfo to check if you have a 1MB Angus chip, which can be downloaded from ambient if I remember correctly or open up the amiga and check the custom chip versions and your A500 motherboard revision.
Comment by starnamedstork at 01/03/2025 at 15:00 UTC
2 upvotes, 0 direct replies
A stock A500 (not plus) has 512kb chip. Chip ram is ram that is also accessible for the graphic and sound chips, meaning it is where you should store and graphics and sound that is in use, but it can also be used for everything else. Essentially it is somewhat similar to "shared" ram you see on some budget pcs with integrated graphics, where the graphics don't have it's own ram but it can use some of the normal ram.
A typical trapdoor expansion adds 512 "fast" ram. It can not be used directly for graphics or sound, but for anything else. This can also be used as chip ram, depending on your revision of the A500. If you have a Fat Agnus chip and you are handy with a soldering iron you also make this into chip ram.
A sidecar expansion can add fast ram, which is similar the the one in the trapdoor but faster, and can't be used as chip.
My 500 back in the day originally had Kickstart 1.3 and a 512 kb expansion. I later replaced the Kickstart with 2.0, converted the expansion to chip, and added a GVP sidecar expansion with 2mb fast ram and a 52 mb hard drive.
Comment by okapiFan85 at 01/03/2025 at 16:34 UTC*
2 upvotes, 0 direct replies
Thank you u/danby for the nice RAM summary. I just wanted to point out that you have a slight typo in the name of **David Dunklee**’s amazing **Parceiro** devices in case anyone wants to search the interwebs for them (of course the second link was just fine).
For the comparatively few **A1000** owners out there (relative to the A500), the **Parceiro II** sounds like a must-have upgrade (it’s on my to-do list). You can read about it at “The Best Amiga 1000 Upgrade in 30 Years: Parceiro II[1]” on AmigaLove[2].
1: https://www.amigalove.com/viewtopic.php?t=2394
[edited to add username u/danby because apparently I hit the wrong reply button, so my reply was to the original post and not u/danby ‘s reply… ]
Comment by 3G6A5W338E at 02/03/2025 at 10:01 UTC
2 upvotes, 0 direct replies
Chip RAM: Behind fat agnus, usable by both chipset and cpu.
Slow RAM: Behind fat agnus, but only usable by CPU. Still blocked for CPU when chipset is accessing Chip RAM.
Fast RAM: Exclusive to the CPU, never blocked by chipset.
Get this for 8M fastram + ide: https://amigastore.eu/en/929-ide68k-gottago-fastram-8mb.html
With these two expansions together, you're set.
Then you can use mkick (on aminet) to load newer kickstarts into fastram whenever you need access to modern AmigaOS, while having excellent compatibility with software from the era on power-on, using the kickstart in the rom socket.
Comment by CompuSAR at 01/03/2025 at 15:34 UTC
2 upvotes, 1 direct replies
I'm not sure people here said it (I have not seen it).
As people said, the Amiga had "chip RAM", which is RAM where graphics and audio can reside, and for which there is contention with the CPU, and fast RAM, which is RAM the CPU can access without contention.
What I haven't seen anyone mention is that the belly expansion for the A500 resides on the contended bus only. If your chipset supports 1MB of chip RAM, this means you can have that memory be chip RAM (in which case you need to cut the jumper, so it appears at the correct address.
If your chipset does not support 1MB chip RAM, the memory you place there will appear at high addresses, normally associated with fast RAM. It will not, however, have fast RAM characteristics. It is still on contended bus, and will still be slow. This is sometimes referred to as "slow fast RAM".
There used to be a program in the AmigaDos diskette that told the computer not to use that memory, as it would get in the way of some programs.
Comment by Mako_ at 01/03/2025 at 14:19 UTC
1 upvotes, 2 direct replies
I have no idea, and don't think I need to. When I got my 500 there was a bunch of software that wouldn't run. I bought a 512k expansion (trapdoor) from Amigakit and now everything runs nicely. The point is it's not that complicated.
Comment by Status-Ad-5543 at 01/03/2025 at 14:19 UTC
1 upvotes, 0 direct replies
Chip ram is for the graphics sound chip hence chip ram Fast ram for cpu
Comment by laAndecIunson at 01/03/2025 at 14:29 UTC
1 upvotes, 0 direct replies
Yeah.. if I remember correctly.. the Amiga 500 I met back in the day all had a trapdoor "extra memory" as it was called here in Sweden.. most of them with a clockchip that needed a battery.
This was needed for many games and increased your chip ram.
I've since learned that a really old Amiga 500 couldn't really use the additional ram as real chip ram and would turn up in your os as fast ram but really not be fast ram.. and this is called slow ram.. funny that..
Fast ram.. the kind of memory the CPU didn't have to share with the rest of the computer, was expanded via the Zorro bus on the side.. my mate had a 52mb disk drive with 3 meg's of ram. Dunno what it was useful for though cause it didn't help when you were gaming . Could actually break some games iirc.
The fast ram, if it only were a memory upgrade could support 8 of those.. If you got a coprocessor or turbocard.. you could add up to 128mb.
I don't know how to identify your Amiga so I just give you the wish of best luck..
What do you want to do with your old Amiga?
Comment by Swimming_Ad4577 at 01/03/2025 at 14:41 UTC
1 upvotes, 0 direct replies
The trapdoor expansion on an A500 gives you "slow" ram. It's like fast ram, but, eh, slow.
Any OCS game requiring 1meg, this is what they mean.
Actual fast ram with an accelerator on the side bus would provide the same functionality for games, but be fast. Well, faster.
Not sure about upgrading the chip ram on a base A500, would need replacing other ICs at least.
I think the A500plus came with 1meg but it was all chip ram.
Comment by Beneficial-Area2386 at 03/03/2025 at 03:13 UTC
1 upvotes, 0 direct replies
I got the trapdoor expansion specifically to be able to play Dungeon Master. Woohoo! 1024K memory!