Mask ROM

Computer memory types
Volatile
RAM
DRAM SDRAM DDR GDDR HBM SRAM
Historical
Williams–Kilburn tube (1946–47) Delay line memory (1947) Mellon optical memory (1951) Selectron tube (1952) Dekatron T-RAM (2009) Z-RAM (2002–2010)
Non-volatile
ROM
Mask ROM PROM EPROM EEPROM Flash memory
NVRAM
ReRAM
Early stage NVRAM
FeRAM MRAM PCM (3D XPoint) FeFET memory
Magnetic
Magnetic tape data storage (Linear Tape-Open) Hard disk drive
Optical
Optical disc 5D optical data storage
In development
CBRAM Racetrack memory NRAM Millipede memory FJG RAM
Historical
Paper data storage (1725) Drum memory (1932) Magnetic-core memory (1949) Plated wire memory (1957) Core rope memory (1960s) Thin-film memory (1962) Disk pack (1962) Twistor memory (~1968) Bubble memory (~1970) Floppy disk (1971)

Mask ROM (MROM) is a type of read-only memory (ROM) whose contents are programmed by the integrated circuit manufacturer (rather than by the user). The terminology mask comes from integrated circuit fabrication, where regions of the chip are masked off during the process of photolithography.

It is common practice to use rewritable non-volatile memory – such as UV-EPROM or EEPROM – for the development phase of a project, and to switch to mask ROM when the code has been finalized. For example, Atmel microcontrollers come in both EEPROM and mask ROM formats.

Overview

The main advantage of mask ROM is its cost. Per bit, mask ROM is more compact than any other kind of semiconductor memory. Since the cost of an integrated circuit strongly depends on its size, mask ROM is significantly cheaper than any other kind of semiconductor memory.

However, the one-time masking cost is high and there is a long turn-around time from design to product phase. Design errors are costly: if an error in the data or code is found, the mask ROM is useless and must be replaced in order to change the code or data.

As of 2003, four companies produce most such mask ROM chips: Samsung Electronics, NEC Corporation, Oki Electric Industry, and Macronix.

Some integrated circuits contain only mask ROM. Other integrated circuits contain mask ROM as well as a variety of other devices. In particular, many microprocessors have mask ROM to store their microcode. Some microcontrollers have mask ROM to store the bootloader or all of their firmware.

See also

• Field-programmability
• Mask work