Computer memory units explained

Computer memory has grown exponentially in the past three decades, and each generation of memory brings with it new memory units with new terms to learn. Let's take a look at these units.

A memory unit is a computer component that stores data, instructions and information. It's also called primary memory. The size of a computer’s memory unit has an impact on its speed, power and capabilities.

Bits and bytes are the basic building blocks of memory.

"Bit" stands for binary digit. A bit is a one or a zero, on or off, which is how all computer information is stored. A byte is made up of eight bits. Eight bits, or a byte, was the original amount of information needed to encode a character of text. The number was later standardized as computer hardware evolved.

As computer memory is based on binary numbers, computer memory capacity is traditionally expressed in multiples of a power of two. In recent years, metric prefixes have dominated the way we refer to the very large numbers of bits and bytes we now use in permanent storage, such as SSDs. However, we still use binary prefixes to refer to memory densities.

In contrast to the International System of Units (SI) prefixes which are based on powers of 10, binary prefixes are based on powers of two. This means a kilobyte is 210 or 1,024 bytes.

In broad terms, memory can be categorized into primary and secondary memory. Primary memory encompasses various types, including: 

Cache Memory: This temporary storage, known as a cache, is more readily available to the processor. It’s often integrated directly into the CPU chip or placed on a separate chip with a bus connection to the CPU. CPU cache memory is extremely fast. 

RAM (Random Access Memory): This term reflects the ability for any storage location to be directly accessed by the processor. 

Dynamic RAM (DRAM): A type of semiconductor memory commonly utilized by a computer processor as main memory for storing data or program code necessary for its operation. DRAM is dynamic because it requires periodic refreshing to maintain stored data. 

Static RAM (SRAM): SRAM preserves data bits in its memory as long as power is supplied, unlike DRAM, which requires periodic refreshing. Typically, SRAM is used for the cache. 

Double Data Rate SDRAM (DDR SDRAM): This type of SDRAM doubles the amount of data transfer that can occur for any given clock speed. 

Double Data Rate 4 Synchronous Dynamic RAM (DDR4 RAM): The successor to DDR2 and DDR3, DDR4 RAM offers a high-bandwidth interface, lower voltage requirements, higher module density, and increased data rate transfer speeds, supporting DIMMs up to 64 GB. 

Double Data Rate 5 Synchronous Dynamic RAM (DDR5 RAM): The successor to DDR4, DDR5 RAM increases density and bandwidth, while lowering power consumption. It’s maximum DIMM capacity increases from 64 GB to 512 GB! 

Direct Rambus Dynamic RAM (DRDRAM): A memory subsystem capable of transferring up to 1.6 billion bytes per second, comprising RAM, a RAM controller, and a bus connecting RAM to the microprocessor and other devices. 
Read-Only Memory (ROM): A type of computer storage containing non-volatile, permanent data that can only be read, not written to, typically housing programming necessary for a computer to start up. 

Programmable ROM (PROM): ROM that can be modified once by a user, allowing customization of microcode programs using a PROM programmer. 

Erasable PROM (EPROM): A type of PROM that can be erased and reused, typically by exposing the memory chip to intense ultraviolet light. 

Electrically Erasable PROM (EEPROM): A user-modifiable ROM that can be erased and reprogrammed repeatedly using higher than normal electrical voltage, without requiring removal from the computer. 

Virtual Memory: A memory management technique enabling the use of secondary memory as an extension of primary memory. Virtual memory uses both hardware and software to compensate for physical memory shortages by temporarily transferring data between RAM and disk storage.

Depending on what you use your computer for, you generally want as much memory as your computer will hold. The make and model of your computer will determine what kind of memory is compatible with it, as well as the maximum amount it can use. Use the Crucial® System Scanner or Upgrade Selector tools to find compatible memory. To learn more about how much memory you should have, read here.

Storage drives — hard disk drives (HDDs) and solid state drives (SSDs) — use the same memory terms when describing the capacity of the drive. As video and photograph files get larger, increased drive capacities are necessary. Currently, solid state drives are sold in a number of different gigabyte and terabyte capacities. As with RAM, you can use the Crucial System Scanner or Upgrade Selector tools to find an SSD that's compatible with your system.