Did I just get free RAM or is my Mac screwy?

February 21, 2005 2:28 PM

Macs and RAM. I just bought 1 Gig of RAM for my (single 1.6GHz processor) G5, and after installing the two DIMMs and rebooting, the system profiler is now reporting that I have TWO Gigs of RAM. What gives?

Did the salesperson inadvertantly sell me double the RAM? (I'm not entirely certain RAM even comes in 2-Gig DIMM pairs) Or is my machine just reporting it incorrectly? The DIMMs came in a pair, obviously, each stick in its own static-free pouch, each pouch with a label on it saying "1 GIG". The only other RAM that's in there is the 256 Megs that came pre-installed.

Either way my system is running a heckuva lot smoother, but I'd still like to know why I suddenly seem to have a whole extra Gig.

posted by Robot Johnny to computers & internet (20 comments total)

Sounds like you really did buy two 1GB sticks of ram, especially since you mention the static bags saying "1 GIG" on them. If you only paid for two 512MB sticks of ram, consider yourself very lucky!
posted by zsazsa at 2:31 PM on February 21, 2005

If the individual stick was labeled 1G, you were sold 2G of RAM, and it sounds like Applications/Utilities/System Profiler is confirming that for you. Modern G5s don't require (or see any performance improvement from having ) the sticks be paired - that was a relic of the motherboards of years past.

Don't believe the Macintosh computer can address more than 2 Gigs at the moment, which is why you're not seeing 2 1/4 G. Tiger, Mac OS 10.4, is supposed to be able to address some ridiculous amount like 8 exabytes.
posted by ikkyu2 at 2:34 PM on February 21, 2005

Yup, I concur. DIMMS don't come in pairs, you can install them individually. Hence, the bags, each labelled 1 GIG, are a gig each.

If you're curious, just pull out all your ram, and put in a single "1 Gig" stick. The system should then tell you if it's a GB or just 512 MB.
posted by defcom1 at 2:35 PM on February 21, 2005

ikkyu2 and defcom1, DIMMs are 64 bits wide. On PowerMac G5s it's required for ram to be installed in matched pairs due to their 128 bit memory bus. On the iMac G5, it's a little more lenient and if you have unmatched pairs or single DIMMs, it operates at 64 bits, which results in lower performance. If you can put a single DIMM into a PowerMac G5, it's may not be supported and will result in decreased performance anyway. If the original poster's G5 came with a single stick of 256MB, it'd probably actually increase performance to remove it from the machine so the matched pairs can be used to their fullest. Most PCs, G4s, and below have a 64 bit memory bus, so you don't need pairs. Some PCs let you use "dual channel" mode with mached pairs of DIMMs to fetch 128 bits at a time.

Back to the original question, I'm also thinking if the original poster asked for a "1 Gig DIMM pair," they probably understood it to be a "pair of 1 Gig DIMMs", and that's what they sold him.
posted by zsazsa at 2:47 PM on February 21, 2005

Zsazsa -- I only asked for "a Gig of RAM", I didn't ask for a pair of anything, and I paid for the price of 1 Gig total. The salesperson explained to me how RAM needed to be installed in pairs, so I assume now he just grabbed a pair of DIMMs, and thinking he needed to get a Gig, he mistakenly grabbed a pair that had "Gig" written on the labels.

So hot damn! Sounds like I scored me some free RAM.
posted by Robot Johnny at 2:55 PM on February 21, 2005

Sounds like you got a sweet deal, congratulations! Usually it works out the other way for some reason, you ask for 1GB and get 512MB dimms instead.

ikkyu2: OS X.3 can use >2GB of ram. The per-process address space on X.3 is 2GB IIRC, but that wouldn't affect the amount of total system ram showing up under System Profiler. Tiger provides the ability to address 16 exabytes, but you'll only be able to use 4TB of physical ram IIRC.

And the 256MB dimm isn't showing up because you do indeed want to install them in pairs. On preview, see what zsazsa said about the 128-bit memory bus.
posted by beaverd at 2:57 PM on February 21, 2005

FYI: the original 256 Megs IS a pair... I don't know why it's not showing up. When I originally installed the new RAM on top of the old, the machine would not boot, so I replaced the current DIMMs with the new ones, and all was fine.

I installed the older DIMMs then, on top of the new ones, and they don't show up in the system profiler.

(Not picky about the missing 256 now, though, knowing I do indeed have 2 Gigs)
posted by Robot Johnny at 3:04 PM on February 21, 2005

A thought occurs - older models of Macs, at least, had a stick of RAM directly built into the system that could not be removed, and one expansion slot. I have no idea if they still do that at Apple (never having owned one), but it was my first reaction when I heard this.

That said, it would seem more plausible in these 'modern' times for there just to have been a mistake in your favor. If so, congrats!
posted by Ryvar at 4:13 PM on February 21, 2005

ikkyu2 and defcom1, DIMMs are 64 bits wide. On PowerMac G5s it's required for ram to be installed in matched pairs due to their 128 bit memory bus.
-zsazsa

Didn't know that. Thanks
posted by defcom1 at 4:18 PM on February 21, 2005

If there is more RAM showing up in System Profiler than you think you have, you will want to ran memtest (from single-user mode) to make sure it's really all there and working. Actually that's not a bad idea regardless -- I recently got my first-ever bad RAM. Run it overnight. If it's one gig that is somehow showing up as two, everything may seem to work just fine until the computer actually tries to put something there, at which point the kernel will probably panic.
posted by kindall at 4:43 PM on February 21, 2005

zsazsa and ikkyu2, I believe that only the dual-processor machines (2.0 gHz and up) require the RAM to be in pairs (separate banks for each processor).

The 1.6 referenced in the original post does not require pairing.
posted by omnidrew at 4:43 PM on February 21, 2005

I'm also a little unsure about your claims zsazsa. I'm running a Powerbook G4 with one stick of 512, and another stick of 256, and I have no performance problems at all. This is a vast improvement over the single 256 stick. I'm fairly confident that most all modern systems do not require paired memory, especially the newest macs, but please correct me if I'm wrong.
posted by odinsdream at 5:54 PM on February 21, 2005

According to suchly, the PowerBook G4 has a 64-bit memory bus. The new G5 has a 128-bit one, and (accd. to Apple) memory must be installed on Power Mac G5s in pairs. The iMac does not, but this would entail a speed penalty (the CPU still needs to retrieve 128-bits at a time, but the RAM controller just hits RAM twice to satisfy the CPU's demands - afaik).
posted by mrg at 6:19 PM on February 21, 2005

omnidrew, Apple still says that the single-processor PowerMac G5s need pairs of DIMMs. Also, SMP machines such as the dual-processor G5s share a common memory bus. If each processor had a separate bank, you'd need twice as much RAM as everything would have to be duplicated between the two. The reason for the DIMM pairing on all of these machines is so the G5 can get 128 bits in a single fetch. When a new memory standard comes out that's 128 bits wide or more, then we won't need to pair up any more.

odinsdream, unfortunately we're back to pairing memory. The DIMM format was made to satisfy processors like the earlier Pentiums and PowerPCs, which had 64-bit memory buses. Now that CPUs like the PPC G5 and Athlon64 are demanding wider 128-bit memory fetches for full performance, we're back to pairing memory modules, as we once were in the mid 90s when we had to pair up 32-bit SIMMs to satisfy our 64-bit Pentiums and PowerPCs.
posted by zsazsa at 6:52 PM on February 21, 2005

being a newly employed guy at a VERY small mac only shop (and realizing how small profit margins are for computer related goods (for the most part), i have to ask the ethics question. will you return the GB you didnt pay for?
posted by ShawnString at 7:15 PM on February 21, 2005

Fair question, ShawnString. I've pretty much made up my mind to keep it, yes. I feel slightly guilty, and perhaps karma will come back to bite me in the ass later, but maaaan does Photoshop fly.
posted by Robot Johnny at 8:18 PM on February 21, 2005

when we had to pair up 32-bit SIMMs to satisfy our 64-bit Pentiums and PowerPCs.

AFAIK, the only 64bit Pentium is the newish Intel EM64T stuff, which just clones (to a point) the AMD64 instruction set.

Everything previous from Intel was 32bit. pre-G5 PowerPC (again, AFAIK) was 64bit internal, 32-bit external.
posted by mrbill at 9:21 PM on February 21, 2005

Yes, such Pentiums and pre-G5 PPCs were 32 bits in that they could only address 32 bits of memory (the 4GB limit), but indeed did have a 64-bit memory bus so they could grab 64 bits of data per cycle when fetching. The new AMD64/EM64T stuff can now address 64 bits of memory, and have up to 128-bit memory busses.
Okay, I think we're off-topic enough here. I've had enough bits for today.
posted by zsazsa at 9:31 PM on February 21, 2005

Not only must you install 64-bit DIMMs in matched pairs, but you must install them in exactly the proper slots. You can't just put two DIMMs in any two available slots; you have to put one in one bank of DIMM slots and the other in the symmetrically opposite slot in the other bank. That is, put both in the innermost slots, or the second innermost, or the outermost, or whatever.

If you do not do this, it would not suprise me a bit if the memory controller thought you had anyway. If you put two 512MB DIMMs in bank "A" and none in bank "B", the system might assume that you have other 512MB DIMMs in bank "B" anyway, and not fail until it tries to access thoes addresses, as kindall said. This is just one of those configuration things - if your 120GB hard drive says it's a 160GB volume, the OS believes it until it tries to use storage that isn't really there. Then the errors and the weeping start.

This is easy enough to check: launch Apple System Profiler and look at the "memory" pane under "hardware." There should be no unmatched pairs, and the number of non-empty slots should exactly match the number of installed DIMMs. Ignore the total - look at the individual memory slot figures. If you see two 1GB DIMMs and two 128MB DIMMs, your system is fine and you scored free RAM. If you see four 512MB DIMMs and two 128MB DIMMs, you've installed them incorrectly and the system doesn't know it.

I'm not laying odds on it going either way, but that's how to find out.
posted by mdeatherage at 11:13 PM on February 21, 2005

I know I'm late to the party, but there is one more way to check that might be faster. Just google the part number. I do that with unknown RAM frequently.
posted by bh at 10:02 AM on February 22, 2005