ROM (Read-Only Memory)

Overview

ROM (read-only memory) is memory whose contents are fixed when the chip is manufactured and cannot be changed afterwards, typically by the pattern of one of the final mask layers. Because each bit can be as simple as a connection that is either made or not made, ROM is among the densest and cheapest forms of memory per bit, and it keeps its contents without power. Today, on-chip ROM mostly holds code and data that must never change, above all a processor's first boot code, along with fixed tables.

How it works

A ROM is a grid of word lines, selected by the address, crossing bit lines that carry the data out. At each crossing a transistor or diode is either connected or left unconnected, and that choice is the stored bit. Reading a word activates one word line and senses which bit lines respond. In a mask ROM the connections are made by a late masking step in wafer fabrication, so the same design can be customised with different contents by changing a single mask.

The first semiconductor ROMs appeared in 1965, and mask ROMs were later used in very high volumes, for example in video game cartridges. Their weakness was that every new code meant a new mask and a long lead time, which led to user-programmable variants: PROM, EPROM, EEPROM and finally flash. Those programmable memories often keep 'ROM' in their names, but strictly speaking ROM means memory fixed at manufacture.

On a modern system-on-chip, ROM is a small block next to SRAM and non-volatile memory. Its classic job is the reset code: on Espressif's ESP32, for example, the reset vector code sits in mask ROM, cannot be modified, and loads the next stage of boot software from external flash. Because ROM cannot be patched in the field, any error in it stays in every chip built from that mask, so ROM code is kept small and is reviewed and verified with particular care before tapeout.

Examples

• The first-stage bootloader of Espressif's ESP32, held in mask ROM and run immediately after reset.
• Sylvania's 256-bit bipolar ROM of 1965, built for Honeywell and among the first semiconductor ROM chips.
• Mask ROMs in video game cartridges; production of Nintendo's first Super Mario Brothers game alone exceeded 40 million units.
• One-time programmable memory IP from Synopsys, used instead of ROM where each chip needs its own data, such as keys or trim values.

Why it matters

ROM contents are fixed in silicon, so bootloader and secure boot engineers, firmware engineers, embedded memory designers and verification teams treat them with particular care: a mistake means a mask change or a permanent workaround. For high-volume products, ROM also remains a low-cost way to store fixed code and tables on chip.

In the chip lifecycle, the boot flow and ROM size are set at architecture, the ROM code is written and verified alongside the hardware, its contents are frozen at tapeout as part of the mask set, and any later change waits for a mask revision.

It matters most to: Bootloader and Secure Boot Engineer, Embedded Memory and SRAM Compiler Design Engineer, Embedded Memory Engineer, Firmware Engineer

Common confusions

Not the same as boot ROM

A boot ROM is one particular use of ROM, holding the first code a processor runs, and has its own page. ROM is the general memory type.

Not the same as EEPROM or flash

EEPROM and flash are non-volatile memories that can be erased and rewritten in the field, despite the 'ROM' in EEPROM's name. True ROM cannot be changed after manufacture.

Not the same as OTP memory

One-time programmable memory is written once after manufacture, so each chip can hold different data. ROM contents are fixed by the mask and identical in every chip.

Sources

1. 1965: Semiconductor Read-Only-Memory Chips Appear (opens in a new tab), Computer History Museum, 2026-10-22
2. Application Startup Flow (ESP-IDF Programming Guide, ESP32) (opens in a new tab), Espressif Systems, 2026-10-22
3. Foundation IP: Logic Library, Memory Compiler, OTP, NVM (opens in a new tab), Synopsys, 2026-10-22
4. Read-only memory (Wikipedia) (opens in a new tab), Wikimedia Foundation, 2026-10-22

Related terms

• Boot ROM
• OTP
• eFuse
• Secure boot

Where it appears on the Hub

Used on 7 pages of the Hub. In each group, the pages that mention it most come first.