openskills.info
Course catalog
Course Preview

Firmware, BIOS, and UEFI

Firmware is software stored with hardware that prepares a computer to run. BIOS is the older PC firmware interface, while UEFI is its modern successor with defined boot services, variables, executable formats, and security features.

itComputer architecture and hardware

Firmware, BIOS, and UEFI

Firmware is software that starts before the operating system and establishes the machine state that later software depends on. It runs from nonvolatile storage associated with the platform or a device. On a PC, platform firmware initializes processors, memory, chipsets, and selected devices. It then chooses the next executable in the boot chain.

BIOS and UEFI describe two different firmware interfaces. The original PC BIOS exposes 16-bit services and historically starts boot code from a selected device. UEFI defines data structures, executable images, protocols, variables, boot services, and runtime services. Modern setup screens may still be labeled “BIOS” even when the implementation is UEFI.

Firmware is broader than either term. A storage controller, network adapter, embedded controller, or management controller can run its own firmware. This course concentrates on PC platform firmware and the handoff to an operating-system loader.

The startup path

power or reset
    ↓
processor reset vector and immutable or protected startup code
    ↓
early silicon and memory initialization
    ↓
device discovery and platform configuration
    ↓
firmware boot manager
    ↓
EFI application or legacy boot code
    ↓
operating-system loader

The processor begins at an architecture-defined reset state. Platform-specific code establishes enough execution context to continue. Memory is not necessarily usable at first, so early firmware works within severe resource constraints. Later phases initialize DRAM, enumerate buses, publish hardware-description tables, and prepare interfaces for the operating system.

The UEFI Platform Initialization specification describes a common phase model. The Security phase begins from reset. Pre-EFI Initialization establishes temporary execution resources and memory. The Driver Execution Environment initializes the platform and dispatches drivers. Boot Device Selection applies boot policy and reaches a UEFI boot option. Implementations can differ internally while still presenting the UEFI interface at the operating-system boundary.

Continue the course

This section is part of the paid course.

See pricing to subscribe, or log in if you already have access.

Where this skill leads

Relevant careers

See how this topic contributes to broader role-level skill maps.

Sources