Volatile RAM

The other major category of memory chips besides ROMs is RAMs. RAM chips store data bits just like ROM chips do, but with two major differences: RAM chips can be erased and reprogrammed almost instantaneously and the data stored in RAM chips is not permanent. RAM chips “forget” everything they're storing when you turn off their electricity. RAM chips are used in computers and other products that need lots of short-term memory storage.

RAM chips are inferior to ROM chips in almost every way, yet they are necessary because they work like electronic scratchpads. Even flash ROM chips take far too much time to erase and reuse to be practical for the main memory in a computer. RAM chips are more expensive and harder to use than ROM chips, but they're the most popular kind of memory chips made. The price of every new PC is determined, in part, by the spot price of RAM chips when the computer was built.

RAM chips come in two basic types: SRAM and DRAM. The initial S and D stand for static and dynamic RAM, respectively. Neither description is particularly useful to us here, and even the term RAM is confusing. Random-access is meant to indicate that a computer can access any bit stored in a RAM chip in random order. For example, it doesn't have to read out the bits in sequence, like watching a videotape from start to finish. RAM chips can provide any bit of data at any time, randomly.

Honestly, ROM chips can also read out data randomly, so RAM doesn't accurately describe the difference between them at all. Read-write memory (RWM) would be more descriptive, but harder to pronounce.

SRAM

SRAM chips are simpler to design, use less electricity in operation, cost less to manufacture, and are faster than DRAM. Overshadowing all these advantages, however, SRAMs have one major drawback: They can only store about one-quarter as many data bits as a DRAM chip. The capacity of both SRAMs and DRAMs increases year by year, but SRAMs are always a factor of four behind their DRAM cousins. Because of that, SRAMs are generally used only in products that run on small batteries (where SRAM's efficiency is valuable) or those that need to be very fast.

Unlike ROM chips, SRAM chips can store data as easily as they retrieve it. In fact, they can store data slightly faster than they retrieve it. This makes SRAM chips important for very fast computers. They are also used for the instruction cache and data cache inside microprocessor chips. An erasable ROM might be erased and reprogrammed with new data once per month, but SRAMs store new data thousands of times per second. SRAMs don't have to be erased all at once like EPROMs, either. They can change one bit of data while holding millions of other bits intact.

DRAM

DRAMs are the volume leader of the entire memory chip market. Computer makers and many consumer electronics products use DRAMs in enormous quantities because they have the greatest storage capacity for the money. SRAM chips might be faster and use less energy, but DRAMs are the cheapest way to get lots of erasable memory.

To the engineer, DRAMs have some drawbacks, but their economic advantages outweigh their technical shortcomings. For starters, DRAM chips use more electrical power to operate than SRAMs do. The difference can be dramatic (to an engineer, that is). Battery-powered products that run for hours with SRAM chips might run for only 20 minutes using DRAM chips. Laptop PCs, unfortunately, use DRAMs because of their lower cost per bit; SRAM-based PCs would have noticeably longer battery life but be far more expensive.

DRAMs also “forget” easily, a serious snag for a memory chip. In fact, the average DRAM can store data only for about one one-hundredth of a second, even if the power is on all the time. After that, it risks forgetting what bits it was storing. To prevent this, DRAM chips must have their memories refreshed about 100 times per second. This takes special circuits, and sometimes a separate chip, just to keep refreshing each DRAM's memory. All this requires even more electricity to operate.

DRAMs can't be used while they're refreshing, either. That means each DRAM chip is offline about 100 times per second, or about 5 percent of the time. These little timeouts aren't noticeable to the average person, but they complicate the design of every computer, which has to tolerate memory chips that sometimes work and sometimes just don't work.

DRAMs don't retain the contents of their memory when the power is turned off. This makes them volatile memory chips, like SRAMs. That's why most personal computers have a button or a menu option to shut down; it gives your PC a chance to copy the contents of its volatile memory chips onto the hard disk before the power goes away.