How do I interpret the specification of memory (RAM)?

08
2014-07

KronoS

When looking at memory there are a few specifications that I don't understand and was hoping to have clarification on. What do these terms mean, and how do they affect the systems performance? Feel free to give technical data and answers to these, but not specific to the specs that I list as example below.

Speed: DDR3 1600, DDR2 800
Timing: 9-9-9-24 (what do each of the numbers mean?)
Voltage: 1.5V (I know what voltage is, but how does it affect my system?)
Multi-Channel Kit: Dual, Quad

Answers

techie007

Speed

The numbers are in MHz, and represent the frequency of the clock signal at which the RAM operates (x2 for DDR RAM, so DDR2-800 is running at 400MHz). The DDR means "Double Data Rate" which means it transfers data on both the rising AND falling edges of the signal (instead of just signal on vs. off). So, for example, DDR gives you the effect of 800MHz while actually still only being at 400MHz. DDR2 and DDR3 are superseding versions of the DDR spec. (ie: DDR3 is "double data rate type three").

Timing

Memory timings (or RAM timings) refer collectively to a set of four numerical parameters called CL, tRCD, tRP, and tRAS, commonly represented as a series of four numbers separated with dashes, in that respective order (e.g. 5-5-5-15). However, it is not unusual for tRAS to be omitted, or for a fifth value, the Command rate, to be added on (from Wikipedia).

CL (CAS Latency)

The CAS latency is the delay, in clock cycles, between sending a READ command and the moment the first piece of data is available on the outputs.

LRCD

Row Address to Column Address Delay - tRCD is the number of clock cycles taken between the issuing of the active command and the read/write command. In this time the internal row signal settles enough for the charge sensor to amplify it.

tRP

Row Pre-charge Time - tRP is the number of clock cycles taken between the issuing of the pre-charge command and the active command. In this time the sense amps charge and the bank is activated.

tRAS

Row Active Time - tRAS is the number of clock cycles taken between a bank active command and issuing the pre-charge command.

See here for more info on these and other RAM timing elements.

Voltage

The listed voltage is the minimum/recommended voltage required to power the RAM module. Not enough and it can't power the module, too much and you can damage the various chips on the module.

Multi-Channel Kits

These 'kits' are simply multiple single, similar (identical as possible) RAM modules packaged together. The intention (these days) is for them to be used in motherboards that have dual and triple (etc.) RAM channel capabilities. IE: since you need 2 sticks to do dual channel, and that became standard/regular for new systems a while back (before triple channel, quad, etc.), the memory manufactures started marketing their existing 'kits' as 'multi-channel kits'.

Previously the kits were sold mainly to give a bit of a break on price when buying multiple modules (ie: Two 1GB modules in a '2GB kit' is cheaper than buying two individual 1GB modules of the same model).

slhck

Speed:

The first part is the type of memory. DDR2 is Double Data Rate 2. The second is the speed in MHz that the memory operates at, in general the faster the better (to a point)

Timing:

The numbers are the number of wait cycles that must happen between different memory operations. Lower is better (more in depth).

Voltage:

The voltage that the memory operates at. In most cases this is just for reference but some systems require certain voltage memory. For example the new Intel core I chips need lower voltage (1.5v iirc) than the older Core 2 chips.

Multi channel:

Memory is sold either by the individual module (stick) or in kits for motherboards with multiple channels of memory. Most current boards have dual with the Intel socket 1336 have triple channel. All the packing does is make sure you are getting two of the exact same memory modules (same speed, timings, and size) which is required for multi channel RAM.

Related Answers

Seasoned Advice (cooking)

I have a couple of laptops with SSDs - a recent MacBook Air and a 64GB Kingston V100 placed into a Lenovo Thinkpad T60p. Both are fast - the Thinkpad is much faster than previously, with boot times below 30 seconds, and much improved battery life.

Your points:

Controller (this can affect performance and endurance more than all other factors combined)

I don't fully agree - if both the drive and the hard drive controller follows SATA 1.5, SATA 3.0 (aka SATA II) or SATA 6.0 (aka SATA III) standard, they'll probably work together: the performance limits in the standard are UPPER LIMITS: many things can make your performance worse. I have not yet found a source of reliable test results allowing SSD performance comparison across brands, devices and OSes.

Bus Technology

Some SATA II devices claim SATA III compatibility but really just take advantage of SATA III's downward compatibility with SATA II. It's easy to see that only few of the spinning SATA III drives actually push more than 3Gb/sec of data. SSDs have an easier time of it, but I've not seen benchmarks showing any pair of SATA III SSDs + SATA III controllers reliably pushing anywhere near 6 Gb/sec.

Form Factor (Physical Size)

Some devices expect a 9.5mm 2.5in drive; the 7mm 2.5in drives don't fit as well. The Kingston drive fit perfectly into the T60p's 9.5mm slot.

Capacity

This seems linearly correlated to price, as you'd expect. Increasingly, SSDs are over-provisioned, where a 64GB drive actually has 72GB addressable. The excess allows for longer drive life in the face of SSD "bits" write cycles being limited to about 100,000. To compensate, SSD controllers move rather than overwrite changing file sectors, in order to balance the limited lifespan. Some OCZ devices destined for servers are reputedly 25% over-provisioned.

NAND or NOR technology

NAND technology appears to have won: NOR technology either isn't as cost-effective, or it is hidden under more meaningful wrappers, such as wear-levelling, mean-time-between-failure (MTBF), and ever higher rates of over-provisioning

Power Consumption during Read, during Write, when Idle

It is difficult to measure this: as with many parameters for electronic devices, you're trusting the reliability of manufacturer vs. the difficulty of measuring these by reviewers or consumers. Having said that, common SSDs are said to consume ~2W during operation, and 0.5W while idle.

Read/Write Burst and Sustained Throughput

Complex: Some SATA II SSDs can't saturate SATA I's 1.5Mb/sec pipe: others can. And you're taking manufacturer's word for it, despite the fact it's possible to measure, with low precision, at least.

But other issues can obscure the results. For example: TRIM, which must be supported by the OS, the controller and the drive. TRIM is a technique to prevent SSDs from experiencing progressively slower write performance over time. Because SSD sectors are written all at once, if a previously used, partially full sector is to be written to, the drive must read the existing sector, add it to the new data in cache, then write the whole sector, then update the file table, which may require the same read-modify-write cycle. This can take some time.

Recently some reviewers tested whether Macs running Snow Leopard really needed TRIM, which isn't supported by Snow Leopard: the reviewers built some seemingly solid tests, and reported some interesting but reasonable results. But other commenters pointed out the researchers assumed Apple's secure formatting tool worked as promised: overwriting every bit of every sector with zeros, even for un-used file space. It turns out that Apple's tool might not actually do this, and if the zero'd status of the tested SSD isn't reliable, maybe the performance tests of new vs. "dirty" disks wasn't reliable, either.

Home