ROM (Read Only Memory)

Introduction

Every time you press your computer's power button, something remarkable happens in less than a second: the CPU — which has no programs loaded in RAM yet — somehow knows exactly what to do. How? The answer is ROM (Read Only Memory) — a special type of memory that retains its contents permanently, even without power. ROM contains the first instructions the CPU ever executes.

What Is ROM?

ROM is non-volatile memory whose contents are preserved permanently. Unlike RAM, ROM doesn't lose data when power is turned off. As the name suggests, it's primarily designed to be read from, not written to (though modern variants allow limited writing).

Why ROM Exists — The Boot Problem

Here's the fundamental problem ROM solves:

1. When you turn on a computer, RAM is empty (it's volatile — all data was lost)
2. The CPU needs instructions to know what to do
3. But instructions are normally stored in RAM, which is empty
4. Solution: Store the initial startup instructions in non-volatile ROM

These startup instructions (called BIOS or UEFI firmware) tell the CPU how to:

• Test if hardware is working (POST — Power-On Self-Test)
• Detect connected devices (drives, keyboard, monitor)
• Find the operating system on the hard drive/SSD
• Load the OS into RAM
• Hand control to the OS

Once the OS is running from RAM, ROM's job is essentially done until the next restart.

Types of ROM

MROM (Mask ROM)

• Contents are burned in during manufacturing
• Cannot be changed after production
• Cheapest for large quantities
• Used in mass-produced devices (early game cartridges, calculators)

PROM (Programmable ROM)

• Blank when manufactured
• Can be programmed ONCE by the user using a special device (PROM programmer)
• Uses tiny fuses that are permanently blown to set each bit
• Once written, cannot be changed
• Used for small production runs where custom data is needed

EPROM (Erasable Programmable ROM)

• Can be erased by exposing the chip to strong ultraviolet light for 15–20 minutes
• Has a distinctive quartz window on top of the chip (to allow UV light in)
• After erasure, can be reprogrammed
• Useful during development (test different firmware versions)
• Largely obsolete now

EEPROM (Electrically Erasable Programmable ROM)

• Can be erased and rewritten electrically (no UV light needed)
• Can erase individual bytes
• Limited write cycles (typically 100,000–1 million writes)
• Slower than Flash memory
• Used in small applications (storing device settings)

Flash Memory

• Modern evolution of EEPROM
• Can be erased and rewritten in large blocks electrically
• Much faster writes than EEPROM
• Limited but generous write cycles (often millions)
• This is what's in your computer's BIOS chip
• Also the technology behind USB drives and SSDs

ROM in Everyday Devices

ROM isn't just in computers. It's everywhere:

• Smartphones — Firmware that controls how the phone boots
• TVs — Software that runs the smart TV interface
• Cars — Engine control programs in the ECU
• Washing machines — Cycle programs and timing
• Game consoles — BIOS and system software
• Network routers — Firmware that manages network traffic
• Microwave ovens — Control programs for cooking modes

Firmware — The Software in ROM

The software stored in ROM is called firmware. It's "firm" because it sits between hardware (hard/permanent) and software (soft/changeable). Firmware is relatively permanent but can sometimes be updated.

Firmware updates (also called "flashing") are when the manufacturer releases new code to fix bugs or add features. This rewrites the Flash ROM chip with new instructions.

Examples of firmware updates:

• Motherboard BIOS update to support new CPUs
• Router firmware update to fix security vulnerability
• Smart TV update to add new streaming app
• Printer firmware update to support new features

ROM vs RAM Summary

Key Takeaways

• ROM is non-volatile memory that retains data without power
• Its primary purpose is storing boot instructions (BIOS/UEFI) that start the computer
• ROM types evolved: MROM → PROM → EPROM → EEPROM → Flash
• Flash memory is the modern standard — electrically rewritable, used in BIOS chips, USB drives, and SSDs
• Firmware is the software stored in ROM — semi-permanent programs that control hardware
• ROM exists in virtually every electronic device, not just computers