Tuesday, December 6, 2011

Know the Specs: RAM

random access memory
Cool, so you need to buy 8 gigs of ram. well, obviously 8 gigs is better than 4 gigs, but what about if one type of ram has 10-10-10-30 timings and the other has 9-9-9-27? what do all the numbers mean? well... they are rarely explained and unless you try to hard to find them, no one seems to tell you.

The simple stuff:
Ram has two main characteristics, its size and its speed.
To find the size you should use for your ram, take the max size of ram supported by your motherboard and divide it by how many ram dimms you have. for example, if a motherboard that supports 16gb of ram and has 4 slots, you should purchase 4gb sticks for it. That way you get the maximum amount of memory without overpaying.

the second characteristic is the frequency. The faster the frequency, the faster the ram operates. however, faster clocked ram usually runs at slower timings (higher numbers)... however, by having higher frequency, it gives you the ability to lower the frequency and lower the timings.

  The Nitty Gritty:
ram CAS RAS tableThe first thing you need to know about ram, is that is is a 2d array of memory. imagine a sheet of graph paper, were you can shade in squares. this is how ram is arranged. memory is saved with an X and Y location physically (which we call rows and columns). however logical memory addresses are numbered sequentially.  If we want to access a memory address, our computer needs to tell the ram were that memory address is located. To do this, your computer sends a strobe over the rows to the columns where the data is needed.
the CPU activates the RAS (Row Access Strobe) line to specify the row where data is to be found. After a short time, the CPU sends the CAS (Column Access Strobe) to specify the column. After this, the data is sent to the output line, and then the data is sent on the next clock cycle.

The numbers in the ram timings dictate how long this takes. this only matters if you are trying to get maximum performance out of your computer, for the most part... these numbers are trivial to the 'every day computer user'
The RAM timings are ordered as follows: CAS latancy, CAS to RAS, Row precharge time, Row active time.
this is still obscure, so lets talk about what each of these timings mean.
  • CAS Latency (tCL) – This is the most important memory timing. As stated above, CAS stands for Column Address Strobe. If a row has already been selected, it tells us how many clock cycles we’ll have to wait for a result (after sending a column address to the RAM controller).
  • RAS to CAS Delay (tRCD) – Once we send the memory controller a row address, we’ll have to wait this many cycles before accessing one of the row’s columns. So, if a row hasn’t been selected, this means we’ll have to wait for the CAS latancy plus the RAS to CAS delay (tRCD + tCL) cycles to get our result from the RAM.
  • Row Precharge Time (tRP) – If we already have a row selected, we’ll have to wait this number of cycles before selecting a different row. This means it will take tRP + tRCD + tCL cycles to access the data in a different row.
  • Row Active Time (tRAS) – This is the minimum number of cycles that a row has to be active for to ensure we’ll have enough time to access the information that’s in it. This usually needs to be greater than or equal to the sum of the previous three latencies (tRAS = tCL + tRCD + tRP).
there are another few measurements that determine RAM performance, however the difference is so minimal that it is not listed on the ram package directly. by using an overclocking utility or system info program, you can get these measurements.

  • Command rate: this is either T2 or T1. A command rate of T1 had about a 2% to 3% performance increase over T2, but is less stable. if you have 4 sticks of ram, chances are you will HAVE to use T2.
  • Write to Write delay: the minimum delay between writes in clock cycles.
  • Read to Read delay: the minimum delay between reads in clock cycles.
  • Write to Read delay: the minimum time between a sequential write and read in clock cycles.
there are many other settings, but changing any almost guarantee your system crashing. if you want to mess around with these, download AMD overdrive [download]. most likely changing something will cause your computer to crash, but the changes are not permanent, and restarting your computer will fix any problems you might have caused.


heddin said...

This is apparently more complex than I imagined, I was content to think biggest number, best number and that's all there was to it. Thanks for opening my eyes :P

Sub-Radar-Mike said...

That was really informative! There is a lot more to RAM than I had thought. The whole 2d aspect was new to me.

Publius said...

awesome specs, so important

M Fawful said...

These simple explanations are great. I always find crazy technical explanations that don't help at all.

Vince said...

RAM is so important and the thing people seem to care about the least.

M Fawful said...

Btw, I found some cool evidence that IBM is working on 3d RAM, so you might have to update this post soon!

Brandon Hudson said...

Your post really cool and interesting. Thanks very much.

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