Definition
Nonvolatile semiconductor memory whose contents are set during manufacturing or programming and primarily read during operation.
What it is
ROM sets the bits output by each address during manufacturing or programming, and retains its contents when power is removed. The earliest semiconductor ROMs used diode or MOS arrays to represent fixed connections, with decoders selecting the output by address.
Why it emerged and what problems it solved
Computers and calculators needed stable storage for programs, characters, and lookup tables, but mechanical or board-level matrices were large and complex to assemble. ROM chips compressed these fixed contents into a single package that could connect directly to the processor bus.
Why it matters and what it enabled
ROM allowed microprocessor systems to execute a defined program automatically after power-up and supported microprogrammed control and specialized functions. It also highlighted the tradeoff between low-cost fixed contents and inflexible modification, encouraging the evolution of programmable nonvolatile memory.
Principles and impact
Problems solved
- Stored fixed programs, character tables, and lookup tables in compact chips.
- Allowed processor systems to retrieve defined control contents automatically after power-up.
Principles
- The presence or absence of devices or connections in a memory array represents fixed 0s and 1s.
- Row and column decoding selects an address, and output buffers supply the selected bits to the system bus.
Improvements
- More compact than board-level diode matrices, with lower cost per bit and greater reliability.
- Could share manufacturing and packaging systems with MOS logic and processors.
Significance
- Became a storage medium for programs, microprograms, and character data in microprocessor systems.
- Provided a basis for the later development of PROM, EPROM, and flash memory.
History and applications
People
General Microelectronics engineering team、Intel memory engineering team
Organizations
General Microelectronics、Intel
Events
MOS ROM arrays appeared in 1965、The Intel 4001 became the companion ROM for MCS-4 in 1971
Applications
Microprocessor program storage、Character generators and lookup tables、Microprogram control stores
Limitations
- Changing the contents of mask ROM requires replacement of the manufacturing masks, making iteration inflexible.
- Programmable variants involve tradeoffs in the number of write cycles, erasure methods, and cost.
Upstream and downstream relations
Upstream
Downstream
Sources: 1965: Semiconductor Read-Only-Memory ArraysIntel's Microprocessor