RAM Nomenclature
MacDoc said:
SODimm - for powerbooks iBooks and G4 iMacs -
two basic flavours PC 133 ( PC 100 and PC 133 ) 32/64/128/256/512
and DDR (2100, 2700, 3200) 128/256/512/1 gig
Dimm - for desktops again two basic flavours as above ( there are earlier ones but not for this thread. Sizes also as above.
Bottom line with RAM - ask carefully it's complicated.
RAM deserves elaboration! This is going to contain a lot of technical jargon, so you may want to either get comfy or skip right past it. The use (or rather misuse) of RAM nomenclature is something that irks me, so I'll be going into some detail.
<strong>RAM</strong> == Random Access Memory. Memory, contrary to what some people have been told, is not storage. Memory == RAM. Storage == Hard Drive.
<strong>SDRAM</strong> == Synchronous Dynamic RAM. All RAM currently used in Macs falls under this general category, albeit divided into sub-categories. SDRAM is an improvement on DRAM (Dynamic RAM). While both store bits of information on individual capacitors, the Coles notes on the difference is that DRAM is asynchronous, responding to control inputs immediately, while SDRAM waits for the clock to pulse before responding. This allows for input to be pipelined, which allows for new input to be sent before the previous input is finished processing, which reduces overall latency.
Bottom line: SDRAM == Good.
<strong>SDR SDRAM</strong> == Single Data Rate SDRAM. This is what is found in G3 machines and a lot of G4 machines. The "SDR" part is a relatively new addition to the name, typically this is just called "SDRAM". It comes in a number of flavours, but basically all Macs that use SDR SDRAM use either PC100 or PC133, so that is all you really need to be concerned with.
PC100 == SDR SDRAM running with a clock speed of 100 MHz.
PC133 == SDR SDRAM running with a clock speed of 133 MHz. PC133 has the distinction of being the fastest SDR SDRAM approved by the JEDEC[1], allowing for a total bandwidth of almost 1100 MB/s.
<strong>DDR SDRAM</strong> == Double Data Rate SDRAM. Why double you say? Basically, the clock signal to SDRAM is like a wave, the clock alternating from one voltage to another, low to high. There is an rising part of the wave and a falling part of the wave. SDR SDRAM only acts on the rising part of the wave, whereas DDR SDRAM operates on both the rising and the falling part of the wave, essentially doubling the rate at which it can send/receive data, or, in more technical terms, effectively halving the required clock speed to archive the same data rate as SDR SDRAM.
DDR SDRAM is typically referred to simply as DDR RAM, and comes in a variety of speeds, each of which is referred to two ways.
DDR200 or PC1600 == DDR SDRAM running at 100 MHz (x2), delivering 1.6 GB/s (~1600 MB/s) bandwidth per channel. There are no Macs that used DDR200.
DDR266 or PC2100 == DDR SDRAM running at 133 MHz (x2), delivering 2.1 GB/s bandwidth per channel.
DDR333 or PC2700 == DDR SDRAM running at 166 MHz (x2), delivering 2.7 GB/s bandwidth per channel.
DDR400 or PC3200 == DDR SDRAM running at 200 MHz (x2), delivering 3.2 GB/s bandwidth per channel.
These are all the specs that are fully approved by the JEDEC, but also slated are DDR466/PC3700, DDR533/PC4200 and DDR600/PC4800. I don't know if these specs will receive full JEDEC approval and ECC support, but probably not as manufacturers say that it's hard to manufacture anything faster than PC3200 in significant quantities. Also, DDR2 is likely to start taking over later this year anyway.
There are a few other things to cover when talking about RAM, like the Dual Channel variant of DDR that Apple uses in the PowerMac G5. Basically, by using matched pairs of RAM, 2 separate pipelines are opened up from the CPU to each pair of RAM modules, meaning that each chip can be used essentially at the same time. The advantages of this are fairly plain when you consider that the system bus of the PowerMac G5 is often over 1 GHz and the RAM only operates at 400 MHz. The more RAM it can use at one time, the less overall latency, the faster your machine will run.
Now that I've covered what types of RAM there is, here is what forms it comes in.
<strong>SIMM</strong> == Single In-line Memory Module. Don't worry about this as the last Mac that actually used them came out before the G3 did.
<strong>DIMM</strong> == Dual In-line Memory Module. Predominantly used in desktop systems like the PowerMac G5, the iMac G5 or the PowerMac G4.
<strong>SO-DIMM</strong> == Small Outline DIMM. Used in basically all laptops, and small form factor computers. iBooks, PowerBooks, original iMac G3s, and iMac G4s[2] use SO-DIMMs.
All RAM used in todays Macs uses a DIMM or SO-DIMM. They vary slightly depending on whether it's SDR or DDR SDRAM.
PC100/133 SDRAM SO-DIMMs come in 72 or 144 pin configurations (usually 144 pin), and regular DIMMs have 168 pins. Full sized DIMMs have 2 notches.
DDR SDRAM comes in 200 pin SO-DIMMs and 184 pin DIMMs. Full sized DIMMs have one notch.
<strong><em>So</em></strong>, I <em>think</em> I covered everything. I'm tired though, so I might have missed a few pertinent things. If you have questions, feel free to ask.
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[1[JEDEC == Joint Electron Device Engineering Council. For more info, see
here.
[2] Technically speaking, only the user accessibly slot in the iMac G4 was an SO-DIMM slot. The internal RAM was a full sized DIMM slot.