Memory technologies

Here is a short list of common technologies used to for both volatile and non-volatile memory.

SRAM - Static Random Access Memory

The 'static' means it does not have to keep being refreshed by a clock signal in order to retain its data. It is faster than DRAM (see below) but more expensive than DRAM, so most computers will use DRAM as its main memory.

You may find SRAM in smaller high-speed memory areas such as inside hard disks, where better performance outweighs higher cost.

DRAM - Dynamic Random Access Memory

Cheaper than SRAM so it is very popular as main memory in personal computers. The 'Dynamic' means that it needs a clock to keep refreshing / retaining its data. The data leaks away because each bit is stored in a 'capacitor', which is a kind of tiny battery that can store charge. Each capacitor gradually loses charge and needs to be topped up by the refresh clock.

SDRAM - Synchronous Dynamic Random Access Memory

Just like the 'Dynamic' description above but now reading and writing data is locked in with the chip clock. This allows it to be faster than the older DRAM technology and has virtually replaced ordinary DRAM in personal computers.

DDR - Double Data Rate

This type of random access memory (RAM) can read and write data on both the rising and falling edge of the system clock. Effectively this doubles the speed of the memory. Older technology only worked on one of the clock edges.

PROM - Programmable Read Only Memory

This is a form of non-volatile memory. The PROM chip can be only be loaded once with data by inserting it in a special PROM programming machine.

For example, some memory chips of this type keep data permanently by blowing tiny fuses inside the chip. Once blown, they cannot be 'unblown'. PROMS are cheaper than EPROMS so they are good to use for data that is unlikely to change such as holding equipment configuration settings.

EPROM - Erasable Programmable Read Only Memory

These are also non-volatile memory, but they can be erased by shining ultra-violet light through a clear glass window on the chip for a few minutes. They can then be re-programmed by using electrical signals. More expensive that PROM, but very handy for prototype work as data may need to be changed often.

EEPROM

Electrically Erasable Programmable Memory. Unlike EPROM, this does not need ultra-violet light or a transparent window on the chip to erase data. It can do so using electronic signals.

For example, Flash memory found in USB memory sticks and solid state hard disks are a form of EEPROM.

2020-10