what's the deal with RAM?

Hmm, I think there is, at least with some machines, but it requires pretty specific knowledge about the machine itself - Example being, the tray loading (233-333MHz) iMacs will accept up to 512 MB of RAM (all of them), but only certain varieties. No more, due to the way the memory controller is (Grackle codename I think? Can't remember). Someone tried to take it to 1GB, but it was a fluke due to the RAM disk being on, or something to that nature.

So, I guess there are physical limits, but I have no idea how you tell. Apple developer notes? I have no idea.

Kyle-

Technology advances after the Mac is made and larger RAM chips are available that were not available at the time of manufacuring. The manufacurers specs are usually not updated which can be confusing.

As in the B&W - when it was designed there were probably only 128MB chips available to fit it. Since then 256MB chips have become available, but beware because not all 256MB RAM sticks will work. Usually you need to use double sided RAM (chips on both side of the stick) in stuff like that to get the maximum RAM amount.

The SE/30 can use 128MB if something is changed, maybe the ROM chip, but I can't remember. Just putting 16MB SIMMs will not get 128MB total by itself.

I find MacTracker (http://www.mactracker.ca/) to be awesome for finding out the actual memory limits. It lists both the Apple spec and the real spec.

-BW

+1 for RAM technology advancing after the Mac is made. If the advance goes in the direction the memory controller's designer anticipated, then it'll work. If it goes a different way, then no go.

Sometimes, there just aren't enough address lines on the motherboard.
For example, JT was working on a hack a while ago to add more RAM to the IIfx by adding another 2 address lines, effectively quadrupling the (physical) address space.

128MB SIMMs are known to work in the LC575s--has anybody ever tried SIMMs tested in a 575 in other 72pin SIMM Macs? I dunno. Somebody will have to try to know for sure.

Also, the SE/30 does take 16MB SIMMs without modification. I've got one with 16MB SIMMs. You're probably thinking either of the ROM hack necessary for 7.6 and 8.1 or the Mode32 software necessary to allow 32-bit addressing.

But it isn't always accurate.
For instance... the Quadra 605... claims its max ram is 36MB. But I now know for a fact that it will take a 64MB SIMM, making the max ram 68MB. Who knows? Maybe it will take a 128MB stick....

I just thought maybe there was information regarding why certain boards max out at certain MB and that this could be found somewhere.

I looked everywhere for the skinny on this ie what varieties of RAM will work. Ended up sourcing a 256 MB for a friend which only shows as 128MB. Lowendmac.com reckons it's pot luck, some machines will, some won't and that's the best they can come up with.

Can anyone actually specify what varieties work as 256s and what don't? And is swapping it to the bottom slot worth a try?

Yeah, we got to the bottom slot, it wasn't that hard.

Does your 256MB SO-DIMM have 16 tiny little chips on it, or 8? The RAM controller in the iMac is the same as the Beige and Blue & White G3, and has the same density limitations. If your SO-SIMM has 16 chips it *should* work. DIMMs with fewer chips usually have an incompatible internal layout.

Basically, imagine the machine as someone who's able to carry 256 pounds, but to do it *has* to be distributed as 128 pounds in each hand. Each SIMM can have one or two "banks". The bank size limit on the iMac's memory controller is 128MB. 256 MB DIMMs with fewer then 16 chips usually have it arranged as a single 256 MB bank. The reason being is it's usually cheapest for the manufacturer to put SIMMs together out of the highest density parts available, so most 256MB SIMMs you buy now can be thought of as being half of a double-banked 512MB. And the iMac can't lift 256MB with one hand.

I'd be curious if anyone's ever found an 8 chip module that works to full capacity. Or a 16 chip one that doesn't.

--Peace

Believe me I know all about the beige's banking problems. It surprises me that I couldn't find a single reference to the same issue in the iMac, and misleading information at the usually reliable lowendmac. Maybe my search fu was without wu-wei that day.

For whatever reason the Beige and Blue & White are more copiously and accurately documented. It seems that sooner or later this question would come up enough that someone would just add an addendum something to the effect of "Original iMac (233-33Mhz): 2 SO-DIMM RAM slots instead of 3 DIMM, bank limitations same as Powermac G3". Eh. Maybe no one really cares anymore.