When a computer is booting, BIOS is loaded to the memory by

Concept — Firmware is program code stored permanently inside a hardware chip so that the processor can execute it before any operating system, disk driver, or network stack exists in the machine. Such code has to survive a power cycle and be readable at the very instant power is switched on, so it is held in non-volatile read-only memory rather than in volatile working memory.

Application — At power-on the processor starts fetching instructions from a fixed reset address that the chipset maps onto the motherboard's firmware chip, so the boot-time BIOS routines are read out of ROM.

1. Power is applied and the processor begins execution at its fixed reset vector.

2. That reset address is mapped to the motherboard's read-only memory chip, so the first instructions fetched come from ROM.

3. The BIOS code held in ROM runs the power-on self-test and initialises the CPU, the memory controller, and the basic input/output devices.

4. BIOS is then commonly copied (shadowed) into faster read/write memory for execution speed, after which it loads the boot sector and hands control to the operating-system loader.

Cross-check — the other listed items, by their own nature:

• RAM is volatile, so at the moment of power-on it holds nothing to execute; it is the destination BIOS is shadowed into, not the store BIOS comes from.

• CD-ROM is removable optical media reached through a drive controller and driver routines that the BIOS firmware itself has to initialise first.

• TCP is a transport-layer network protocol for moving byte streams between hosts, so it is not a memory device at all.

Result — while a computer is booting, BIOS is supplied from ROM.