A type of non-volatile storage medium that permanently or semi-permanently stores data on computers and other electronic devices. Unlike RAM, data stored in ROM is not lost when the power is turned off and is typically difficult or slow to rewrite.
Core Characteristics
- Non-Volatility: ROM retains its data without a constant power supply, making it essential for a device’s initial “bootstrapping” process.
- Primary Function: It stores the Firmware—the low-level software that communicates directly with hardware (such as the BIOS or UEFI).
- Read-Centric: While it can be read at high speeds, writing to modern ROMs (such as Flash memory) is significantly slower than reading.
Types of ROM
| Type | Full Name | Description |
| Mask ROM | Masked Read-Only Memory | Data is hard-wired during manufacturing. It cannot be changed or erased. |
| PROM | Programmable ROM | Manufactured blank; can be programmed once by the user using a high-voltage “ROM burner.” |
| EPROM | Erasable Programmable ROM | Can be erased by exposing the chip to strong Ultraviolet (UV) light through a transparent quartz window. |
| EEPROM | Electrically Erasable PROM | Can be erased and rewritten electronically. This is what most modern BIOS/UEFI chips use. |
| Flash Memory | Flash ROM | A high-performance version of EEPROM that erases and writes data in large “blocks” (used in SSDs and USB drives). |
Key Terminology
- Firmware: The specific software stored on a ROM chip that provides control, monitoring, and data manipulation of engineered products.
- Flashing: The process of overwriting the data on an EEPROM or Flash ROM chip with a newer version of firmware.
- Bootloader: The specific code stored in ROM that tells the CPU how to start the operating system.
- Shadowing: A technique where the contents of a slow ROM chip are copied into faster RAM during startup to improve system performance.
ROM vs. RAM
| Feature | ROM | RAM |
| Persistence | Non-volatile (Permanent) | Volatile (Temporary) |
| Usage | Startup instructions & Firmware | Active applications & Data |
| Speed | Slower | Much Faster |
| Capacity | Typically small (MBs) | Large (GBs) |
Common Use Cases
- Microcontrollers: Storing the logic for household appliances like washing machines and microwaves.
- Gaming Consoles: Cartridges for older systems (like the NES or Genesis) were essentially large ROM chips.
- Smartphones: Storing the Boot ROM, which is the first code the processor executes to verify the operating system’s security.
- Embedded Systems: Industrial sensors that require a fixed set of instructions that must never be accidentally deleted.
Technical Note: In modern computing, strictly read-only memory is rare. Most devices use Flash EEPROM, which allows for field updates while maintaining the non-volatile characteristics of traditional ROM.