BIOS firmware is stored in nonvolatile memory, meaning it persists even when your computer is completely powered off. This is essential because BIOS is the first code your computer runs when you press the power button. If it were stored in volatile memory like RAM, it would vanish every time you shut down, and your computer would have no way to start up again.
What Nonvolatile Actually Means
Memory comes in two broad categories. Volatile memory, like the RAM in your computer, only holds data while it has power. The moment you unplug your machine, everything in RAM disappears. Nonvolatile memory is the opposite: once data is written to it, the chip retains that data with no power at all. Your hard drive, SSD, and the chip holding your BIOS are all nonvolatile.
BIOS has to be nonvolatile because it contains the instructions your processor needs before it can load an operating system. Without those instructions sitting ready on the motherboard the instant power flows, nothing else can happen.
The Chip That Stores Your BIOS
Early PCs stored BIOS on true read-only memory (ROM) chips that were programmed at the factory and could never be changed. That gave way to EPROM chips, which could be erased with ultraviolet light and reprogrammed, though in practice most were sealed in opaque plastic and treated as permanent. Next came EEPROM (electrically erasable programmable read-only memory), which allowed the chip to be rewritten electrically without removing it from the board.
Today, virtually all motherboards use flash memory for BIOS storage. Flash is a descendant of EEPROM designed for faster speeds and higher storage density. It’s the same fundamental technology found in USB drives and SSDs, though the BIOS chip is far smaller, typically just a few megabytes. As of 2020, flash costs significantly less than traditional byte-programmable EEPROM and dominates anywhere a system needs solid-state nonvolatile storage. Because it’s rewritable, flash is what makes BIOS updates possible: the manufacturer releases new firmware, and a utility writes it directly to that chip.
Where the Confusion Comes From: CMOS RAM
If BIOS itself is nonvolatile, why do people talk about a battery on the motherboard keeping BIOS settings alive? The answer involves a second, smaller piece of memory historically called CMOS RAM. This tiny chip doesn’t store the BIOS code. It stores your personal configuration: boot order, clock speed settings, hardware preferences, and the system clock.
CMOS RAM is technically volatile. It uses a low-power SRAM chip that would lose its contents without electricity. The small coin-cell battery on your motherboard (usually a CR2032) keeps this chip powered around the clock, making it effectively nonvolatile in day-to-day use. That’s why it’s sometimes called NVRAM, or nonvolatile RAM, even though it depends on a battery rather than its own physics to hold data.
When that battery dies, the CMOS RAM loses everything it was holding. Your system clock resets, your boot drive preference disappears, and all BIOS settings revert to factory defaults. The BIOS firmware itself, sitting safely in flash memory, is completely unaffected. Your computer can still boot. It just won’t remember how you had things configured.
What a Dead CMOS Battery Looks Like
The classic symptom is your computer forgetting the date and time after every shutdown. But a failing CMOS battery can cause stranger problems: random shutdowns, boot loops, USB ports not being detected, Wi-Fi settings flickering on and off, or the system failing to recognize which drive holds your operating system. In some cases, a corrupted CMOS chip can even cause memory test failures that disappear once you swap in a fresh battery.
These issues happen because many components rely on the configuration data stored in CMOS RAM to initialize correctly. When that data is missing or garbled, hardware that is physically fine can misbehave in unpredictable ways.
How Modern Systems Handle This
Modern motherboards running UEFI (the successor to traditional BIOS) have started moving away from battery-dependent CMOS RAM. Many now store configuration settings in a small reserved section of the same flash chip that holds the firmware itself, making those settings truly nonvolatile without any battery. On these systems, the CR2032 battery’s only remaining job is powering the real-time clock so your computer keeps accurate time while unplugged.
The practical result is that on a newer motherboard, a dead battery might only reset your clock. On an older board, it can wipe all your custom settings and cause the cascade of odd symptoms described above. Either way, the BIOS firmware code remains safe in flash. The battery has never protected that.
Quick Summary of What Lives Where
- BIOS/UEFI firmware code: stored in flash memory (nonvolatile, no battery needed, survives indefinitely without power)
- BIOS settings (older systems): stored in CMOS RAM (volatile chip kept alive by a CR2032 battery)
- BIOS settings (newer systems): increasingly stored in a section of flash memory (nonvolatile, no battery needed)
- System clock: maintained by the CMOS battery on both old and new systems
So when someone asks whether BIOS is volatile or nonvolatile, the firmware itself is definitively nonvolatile. The settings you configure inside BIOS may or may not be, depending on how old your motherboard is and whether it still relies on battery-backed CMOS RAM or has moved everything into flash.