Computer Memory: RAM and ROM

Computer memory is the internal storage areas in the computer. It comes in the form of chips or ICs (Integrated Circuits), verses the computer storage which exists on tapes or disks. Moreover, the term memory is usually used as shorthand for physical memory, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk.

There are two basic types of memory: ROM and RAM.

ROM - Read-only memory: On ROM, data is prerecorded for read only which can not be removed. ROM is nonvolatile and it retains its contents regardless the computer is on or off. Most personal computers contain a small amount of ROM that stores critical programs such as the program that boots the computer. In addition, ROMs are used in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs. There are a few variations of ROM such as programmable read-only memory(PROM), which is manufactured as blank chips on which data can be written with a special device called a PROM programmer.

RAM - Random access memory: whose contents can be accessed (read, write and remove) in any order. This is in contrast to sequential memory devices such as magnetic tapes, discs and drums, in which the mechanical movement of the storage medium forces the computer to access data in a fixed order. RAM is usually used for primary storage in computers to hold active information such as data and programs. Common forms of RAM are: SRAM (Static RAM) and DRAM (Dynamic RAM).

RAM ICs are often assembled into plug-in modules. Some standard module types are: Single in-line memory module (SIMM) and Dual in-line memory module (DIMM).

There are also some hybrad memory types which combine characters of both RAM and ROM, such as Flash memory, NVRAM and EEPROM.

Type Volatile Writeable Erase Size

Max Erase Cycles

Cost (per Byte) Speed

SRAM Yes Yes Byte Unlimited Expensive Fast

DRAM Yes Yes Byte Unlimited Moderate Moderate

Masked ROM No No n/a n/a Inexpensive Fast

PROM NoOnce, with

a device programmer

n/a n/a Moderate Fast

EPROM NoYes, with a

device programmer

Entire Chip

Limited (consult

datasheet)Moderate Fast

EEPROM No Yes ByteLimited (consult

datasheet)Expensive

Fast to read, slow

to erase/write

Flash No Yes SectorLimited (consult

datasheet)Moderate

Fast to read, slow

to erase/write

NVRAM No Yes Byte UnlimitedExpensive (SRAM + battery)

Fast

Computer Memory: RAM and ROM