Memory, Technical

Q: What do the acronyms DRAM, EDO RAM and SDRAM mean, and how does each type of RAM differ?

A: DRAM stands for dynamic random access memory. As this type of memory requires a constant current to retain information, it needs to be refreshed hundreds of times per second. The memory uses the same circuit to store and retrieve data, so access times can be an issue. Memory is organized in pages, and when one page is accessed, it takes additional CPU cycles to switch to another page to access more memory.

EDO RAM stands for extended data out RAM. It's similar to DRAM, but EDO RAM operates between 10 and 15 percent faster because it starts accessing the next block of data while sending the previous block to the CPU. That makes it easier and quicker to synchronize data transfer than with regular RAM. EDO RAM is used in both SIMMs and DIMMs (see the next question), while regular DRAM is typically found only on PCs with SIMMs.

SDRAM stands for synchronized DRAM. It is significantly different from regular DRAM because it uses a clock cycle timing for data access and refresh. It operates at the same frequency as the system bus and synchronizes automatically with requests from the CPU. That makes it faster than DRAM and EDO RAM. SDRAM is typically found only in DIMMs.

Q: What is the difference between SIMMs and DIMMs? And what are RIMMs?

A: SIMM stands for single in-line memory module; DIMM stands for dual in-line memory module.

RAM chips are typically packaged in 8MB, 16MB, 32MB or 64MB modules that plug into a PC's motherboard. These modules are small, standard-size circuit boards that hold the actual RAM chips. Memory used to come in 30-pin SIMMs, but now you'll find these SIMMs only on older PCs. Pentium-based PCs have the newer 72-pin SIMMs-which hold more memory and can access it better-or the newest DIMMs. DIMMS can hold even more memory and typically have 84 pins active on both sides for 168 connections.

While unbuffered DIMMs are limited to 64MB, newly designed registered DIMMS can hold 128MB or 526MB. These registered DIMMs are found in servers and high-end workstations.

RIMMs, or Rambus memory modules, will be used with Intel's next-generation Rambus memory interface, which will support high-speed buses and provide much greater bandwidth than current memory (more on Rambus below).

Q: Does the speed of my RAM matter?

A: The newer the system, the more RAM speed matters. On older systems with SIMMs, speed matters less. A 60-nanosecond DRAM should work fine for all PCs, and some older systems can run on slower speeds of 70ns or 80ns.

SDRAM speed is measured in MHz because it is clocked, just like the system bus. Newer systems based on Intel's Deschutes Pentium II processors use a 100MHz system bus and require memory clocked at that speed. If your system uses EDO or SDRAM, make sure your RAM conforms exactly to the manufacturer's specifications. If you upgrade or replace RAM on a PC with DIMMs, you need to follow exact instructions in your system manual.

Q: Can I use my existing RAM in a new PC when I upgrade?

A: If your new PC has a system bus clocked at 66MHz or slower, and the PC uses a compatible memory module (SIMMs or DIMMs), then it is possible. Some systems are designed to take a mixture of SDRAM, EDO RAM and even DRAM, but many require a particular type of memory. You should check the precise specifications of your new machine.

If your new PC has a 100MHz system bus, you can't use the old RAM (unless your old system had a 100MHz system bus). Make sure the RAM in the new machine is designed to run at 100MHz, or else you'll see slower performance and even memory page faults that could crash the system.

Q: Is RAM for notebooks the same as RAM for desktop PCs?

A: Notebook memory chips are typically the same types of RAM as used in desktop PCs, but with different packaging. Many notebooks use smaller SODIMMs (small-outline DIMMs). These come in 72-pin and 144-pin modules. But many notebook manufacturers use proprietary memory modules, so if you want to expand RAM, you have to get memory designed specifically for that machine.

Q: Is the RAM on a graphics card the same as regular system RAM?

A: Graphics cards have special requirements because they must simultaneously move data rapidly into and out of graphics memory to the display. Therefore, most graphics memory is dual-ported, meaning it can send and receive data simultaneously. Graphics memory types include VRAM (video RAM), TPRAM (triple-port RAM), SGRAM (synchronous graphics RAM). Most current cards use SGRAM.

Q: What's cache memory? Can I upgrade it?

A: Cache memory is temporarily held data that's immediately ready to use, speeding up your system. The Intel Pentium and many other CPUs have this memory built right into the processor. That's level 1 cache, and you can't change it. Most CPUs now also have level 2 cache, used by the main system RAM. Cache memory is much faster than regular RAM.

Static RAM is a type of cache memory that usually requires no refreshing or synchronizing and returns information to the CPU virtually instantly. You can only upgrade cache memory if your system's cache memory socket is accessible and includes a larger secondary cache as an option. If your system has a Pentium II, you have to replace the entire processor to upgrade the cache because the system cache is inside the processor's housing.

Q: What about future developments?

A: As CPU speeds increase, memory must become faster to avoid bottlenecks. Two types of faster RAM are currently proposed. Intel is backing Rambus or RDRAM, a much more complex type of memory interface using a special 800MHz bus and a protocol- and packet-based system for transferring data. Because Intel plans to eventually double the bus speed to 1.6GHz, Rambus is also likely to be the fastest of the proposed suggestions.

A cheaper alternative is high performance SDRAM-DDR, or double-data rate SDRAM. SDRAM-DDR reads data at 200MHz, twice the 100MHz speed of current high-end PC buses. An advanced version of this, SLDRAM (SyncLink DRAM) will quadruple the data rate to 400MHz. The latter two alternatives are cheaper and easier to implement than Rambus. We may see Rambus in high-end PCs and SLDRAM in less-expensive systems.

None of the existing RAM solutions will transfer upward to these new systems. The RAM you're using today isn't likely to work in the PCs of tomorrow.