When power is applied, the CPU always begins execution at a fixed address called the reset vector. On classic x86 this is 0xFFFF:0x0000 (physical address 0xFFFF0), just 16 bytes before the top of the 1 MB address space. The BIOS ROM is mapped here so the very first instruction the CPU fetches is your code.

That 16-byte gap is too small for real code, so the reset vector is always a far jump into the main body of the BIOS:

Before any timed or interrupt-driven code can run, the BIOS must configure two essential chips:

• The PIT — generates the system clock tick (IRQ 0) and drives timing.
• The PIC — routes hardware IRQs from devices to the CPU.

The POST memory test writes a pattern to each memory cell and reads it back. Bad cells get flagged and subtracted from the usable total. The BIOS also establishes the BDA (BIOS Data Area) at address 0x0400.

The IVT at address 0x0000 maps interrupt numbers to handler routines. The BIOS populates it with its own service routines — for example:

• INT 0x10 — Video services (print character, set mode)
• INT 0x13 — Disk services (read/write sectors)
• INT 0x16 — Keyboard services (read key, check buffer)
• INT 0x1A — RTC / time services

One of the quirkiest parts of x86 history: the A20 gate. Early PCs had only 20 address lines; addresses above 1 MB wrapped silently back to 0. When Intel added the 21st address line with the 286, this wrap-around behaviour was controlled by toggling a bit in the keyboard controller, of all places.

The BIOS must enable A20 before switching the CPU to Protected Mode, otherwise memory accesses above 1 MB silently fail.

After POST the BIOS checks each device in the configured boot order for a valid MBR or GPT partition table. Classic BIOS looks for the two-byte signature 0x55 0xAA at the end of the first sector:

UEFI takes a fundamentally different approach. Instead of a tiny assembly stub crammed into a ROM chip, UEFI is a full software ecosystem written in C with a defined API called Boot/Runtime Services.

• UEFI reads a GPT partitioned disk and looks for an ESP (EFI System Partition) instead of an MBR.
• Secure Boot validates every EFI binary with a cryptographic signature before running it.
• Boot entries are stored in NVRAM variables, so the firmware remembers which OS to boot without scanning every disk.
• ACPI tables expose hardware topology (CPU cores, power states, thermal sensors) to the operating system in a standardised way.

The result: UEFI boots faster, supports modern hardware, and is far more extensible — but is also significantly more complex to implement from scratch.