About ROM

Tharindu Weerasinghe, Computer Engineer

ROM is an acronym for Read-Only Memory. It refers to computer memory chips containing permanent or semi-permanent data. Unlike RAM, ROM is non-volatile; even after you turn off your computer, the contents of ROM will remain.

Almost every computer comes with a small amount of ROM containing the boot firmware. This consists of a few kilobytes of code that tell the computer what to do when it starts up, for example running hardware diagnostics and loading the operating system into RAM. On a PC, the boot firmware is called the BIOS.

Originally, ROM was read-only. To update the programs in ROM, one had to remove and physically replace the ROM chips. Contemporary versions of ROM allow some limited rewriting, allowing upgrade of firmware such as the BIOS by using installation software. Rewritable ROM chips include PROMs (programmable read-only memory), EPROMs (erasable read-only memory), EEPROMs (electrically erasable programmable read-only memory), and a common variation of EEPROMs called flash memory.

Where is the ROM Chip?

ROM chips are installed on a computer’s motherboard. The model of the board usually determines the type of ROM needed to control a computer’s boot routine. Electrical current is not needed to maintain the prerecorded data instructions on the ROM chip.

The instructions cannot be overwritten and only special equipment can be used to rewrite or erase data.

Early days

During the 1980s, most personal computers used ROM to store their entire operating system. This was a problem for manufacturers, because they had to build their systems according to ROM coding. In addition, ROM chips that contained errors or were defective during manufacturing had to be removed from computers and replaced with new ones.

Now

Modern computers do not have their entire operating systems stored on ROM, but still rely on three ROM programs: BIOS (Basic Input-Output System), POST (Power On Self Test) and Setup. BIOS tell the computer how to execute the hard disk, locate the operating system and load it into RAM (random-access memory). After the operating system has loaded, the computer will be able to understand input, display output, run software and access data.

ROM on Cards

Adapter cards contain a small amount of ROM with BIOS routines that load startup drivers during the boot process. For example, video cards, SCSI (Small Computer Systems Interface) cards, Enhanced IDE controller cards and some network cards contain ROM, drivers and programs, which are executed by the motherboard during the boot routine.

ROM Variations

There are several types of ROM, and each memory chip is classified by how data is prerecorded or erased. The ROMs have common characteristics because they are all nonvolatile.

Types

PROM (Programmable Read Only Memory) is usually manufactured as blank chips, and a device called a PROM programmer allows data to be written onto the chip. These chips can only be programmed once.EPROM (Erasable Programmable ROM) chips usually contain data for a computer’s user settings and the date. The chips are designed with a controller, which allows the data stored on the chip to be changed by exposing it to ultraviolet light. EEPROM (Electrically Erasable Programmable Read Only Memory) data instructions can be changed when electric rays are focused on chip. Unlike EPROM, EEPROM chips do not have to be removed from a computer when new data needs to be stored on the chip.Flash ROM data can be changed by using a super-voltage charge to erase a block of data. This technology allows new BIOS information to be stored in the ROM to update it. Flash memory is a type of EEPROM chip.

Examples of flash memory: