What is RAM and why does it matter?

Buying Guides, How to Add Memory

Question: I am confused by the different types of random access memory (RAM) out on the market. I would like to know what separates each type from one another and if any computer can use any type of it? —N.R.

Answer: Welcome to this week’s can of worms. Luckily, technical expert Mathew Fiszer of Logicorp Service Group in Edmonton helped me sort it out. “There are basically two major types of memory: Dynamic RAM (DRAM) and Static RAM (SRAM),” he explained. “Dynamic RAM (or DRAM) is used for main system memory. SRAM is used for Level 2 Cache and comes in a few flavours.”

Before we get into the nitty-gritty, there are a few things you’ll need to know:

  • RAM speeds are measured in nanoseconds (ns).
  • One nanosecond is a measurement in time, the equivalent of one thousand millionth of a second.
  • You’ll also see reference to numbers as follows: x-y-y-y. The “x” refers to the number of clock cycles it takes to read or write the first piece of data of a group of four. Each “y” refers to the number of clock cycles each subsequent piece of data is dealt with in the group.
  • A clock cycle is the speed at which a microprocessor executes instructions. All computers have an internal clock that regulates the rate at which instructions are executed.
  • Clock speeds are measured in megahertz (MHz). One MHz is equal to one million cycles per second.

First let’s look at DRAM, which is probably what N.R. was after in the first place. Hold on to your propellers.

  • Fast Page Mode RAM or FPM RAM: This RAM has been around for years. It comes in speeds of 70ns and 60ns. The 70ns type is used in older systems, and 60ns is used in systems equipped with the Pentium 100, 133, 166, 200 MHz. Fastest access speeds in CPU cycles are 5-3-3-3 in burst reads.
  • EDO RAM (Extended Data Output RAM): EDO comes in 70ns, 60ns, and 50ns speeds, and should be used with most Pentium 100 to 200 MHz systems. It can pump data out in less clock cycles that FPM RAM. “If you’re buying EDO memory,” explains Fiszer, “aim for the 60ns or preferably the 50ns speeds. EDO cannot support bus speeds over 66 MHz.” (A bus speed is the rate at which data moves through the pathways in your system.)
  • BEDO RAM (Burst Extended Data Output RAM): This RAM is an improvement over EDO. The CPU is able to read data in a 5-1-1-1 burst. As with EDO, BEDO cannot handle bus speeds over 66 MHz. BEDO is only supported by a few chipsets: VIA 580VP, 590VP, 680VP. VIA 580VP and 590VP are used with Intel Pentiums and the Cyrix 6×86.The 680VP chipset is used in the Pentium Pro.
  • SDRAM (Synchronous Dynamic RAM): Its access speed is 5-1-1-1, the same as BEDO RAM, but it can handle bus speeds up to 100MHz. SDRAM is becoming very popular since it’s supported by the new Intel Triton VX chipset along with all new VIA chipsets.

  • SIMM and DIMM: Both are simply the packaging types that RAM comes in. SIMM stands for “Single In-line Memory Module” and DIMM stands for “Dual In-line Memory Module”. SIMMs have to be installed in pairs. For example, if you have eight RAM slots in your motherboard, you have to add them two at a time. You couldn’t install seven SIMMs and leave one slot empty. A DIMM can be installed one at a time.
  • [[ RAM modules ]] have contacts on them. They’re either gold or tin (a dull silver colour, actually). If you use one type, you shouldn’t mix them with the other type. The experts tell me that doing so can cause corrosion, possibly within a matter of months. Then there’s parity and non-parity RAM. You can tell if your RAM module has parity checking by counting the number of chips on it. If the count is odd, you have parity checking. By the way, you can’t mix parity RAM with non-parity RAM.

Earlier, I mentioned SRAM. It’s used primarily as a computer’s secondary (or L2) cache, which is a holding area for data outside the processor. It’s not integrated into the processor as primary cache is, so it’s expandable.

There are several types of SRAM:

  • Static RAM: This type of RAM is mostly used for Level 2 Cache.
  • Async SRAM (Asynchronous Static RAM): This type of SRAM has been used for years, ever since the first 386 with L2 cache came out. It comes in 20, 15 or 12ns speeds, but it is not fast enough to be accessed synchronously.
    “Simply speaking,” said Fiszer, “the amount of time the CPU has to wait for this RAM is only a little bit shorter than for DRAM.”
  • Sync SRAM (Synchronous Burst Static RAM): This type of SRAM delivers data in 2-1-1-1 burst cycles only up to the 66 MHz border. As soon as that border is crossed, it slows down to 3-2-2-2 bursts. Sync SRAM is not supported in the newer Pentium boards and is slowly becoming expensive as fewer companies manufacture it. It comes in speeds of 12 – 8.5ns.
  • PB SRAM (Pipelined Burst Static RAM): This is the most popular cache RAM of all. It is the fastest for bus speeds of 75 to 133MHz. It can deliver burst speeds of 3-1-1-1 regardless of processor clock speed.

Fiszer provided a lot of this information, but bits and pieces have also come from the web. I spent a lot of time on the Free On-line Dictionary of Computing web page. It’s a great jargon translator at http://wombat.doc.ic.ac.uk/foldoc/index.html. A comprehensive look at RAM can also be found Karbos Guide.

Finally, for people who want a simple primer, try Kingston Technology’s Ultimate Memory Guide at www.kingston.com/.

Also, be sure to see our How to Add RAM article.