It’s easy to confuse the different types of computer memory, but they aren’t completely interchangeable. Here’s a look at the various different types of computer memory chips.
Dynamic RAM comes in several varieties, and we’ve been using one variety or another as main computer memory since the 1980s. Today the common types include DDR3 and DDR4. For our purposes, the difference between the memory of today and the memory of yesterday is speed. Today’s memory is faster and uses less power, but it still works the same way it did then. When you shut off the power, you lose the contents of the memory.
DRAM chips usually come on plug-in modules called DIMMs containing multiple chips, making up a quantity of several gigabytes of RAM. In phones and routers, you may see a small number of DRAM chips soldered to a circuit board connected to the CPU.
Flash memory is somewhat newer than DRAM. Its main advantage is persistence. When you turn off the power, the memory keeps its contents. The downside with flash is speed. DRAM is much faster. We use flash memory in places where speed isn’t critical, such as system ROM, where the ability to reprogram it adds convenience. Flash memory means we don’t have to swap ROM chips anymore. When we use flash memory as ROM, we call it firmware, since it is changeable, but you have to take special measures to change it.
Flash memory also powers SSDs, USB thumb drives, and digital camera memory such as micro SD cards. While flash memory is slower than DRAM, it’s many times faster than hard drives.
When I was a kid, they taught me there were two types of memory: RAM and ROM. You can write to RAM, but not ROM. Today ROM chips are less common. It’s more convenient to put a flash memory chip on a board and use it to store what we used to store in ROM. That way, in the event of needing to make a change, you can reprogram the flash memory, rather than having to swap a ROM chip. It lets you get product to market much faster.
EPROM is a special kind of ROM chip that you can erase and reprogram with ultraviolet light. This is much more convenient than hardwired ROM chips, but not as convenient as flash memory, which requires no special equipment to reprogram.
Static RAM is faster than DRAM, but it’s also more expensive. Modern computers use a small quantity of static RAM to cache a large amount of DRAM for extra speed.
Within a few years we can expect to see a memory technology that combines the persistence of flash with the speed of DRAM. As the cost comes down, we can expect this to change the way computers are designed. But that future isn’t here just yet.