The difference between BIOS and UEFI

BIOS (Basic Input/Output System) and UEFI (Unified Extensible Firmware Interface) are types of firmware that run when you first turn on your computer. UEFI is a more modern and capable successor to BIOS, but they both initialize hardware and load the operating system.  

Before BIOS became a standard feature of IBM-compatible PCs, each computer system had to use a custom boot ROM (read-only memory). These boot ROMs would provide a similar function to BIOS firmware, but they would be unique to each computer architecture. 

The computer BIOS is a type of firmware embedded on a computer’s motherboard that provides the basic instructions the system needs to start loading the operating system.   

BIOS firmware was first used in 1975 by the CP/M operating system and was later developed by International Business Machines Corporation (IBM) who made its own proprietary version. IBM’s implementation of BIOS was a great success, which saw it reverse-engineered by competitors who used it to create their own compatible systems. This made widespread adoption possible, and the interface soon became the de facto standard. 

A computer is simply a collection of electronics and invisible data when it is first turned on. No operating system (OS) has loaded yet, so the system requires some instructions to begin initiating the most basic functions, such as keyboard and mouse support, before the OS has loaded. BIOS contains essential code that enables the operating system boot loader to run, which then loads the rest of the OS.  

BIOS also runs a power-on self-test (POST) to test the system components to ensure they are present and working properly. Errors can be reported on screen if a monitor is connected and supported, but also through beep codes which are like Morse code for computer errors. In the original IBM specification, one short beep would mean the system was ok, while a continuous beep would mean there was a problem with the power supply, motherboard, keyboard or RAM. Other manufacturers sometimes used their own unique beep codes. 

Source: Wikipedia

In the 1990s, BIOS also allowed the user to perform a low-level configuration of their system through a BIOS Configuration Utility. This provided a simple, text-based screen that allowed the user to set up various options, such as the boot order, which would govern if the computer would first try to boot through the hard drive, optical drive or floppy disk.   

The 1990s also saw BIOS starting to show its age. It had originally been developed when RAM was measured in kilobytes and storage in megabytes, but computing power increased exponentially since the introduction of Intel’s 4004 microprocessor in 1971. 

UEFI stands for Unified Extensible Firmware Interface. Development of the original implementation was started by Intel in the late 1990s, and it was later developed further by the UEFI Forum, a joint initiative of technology companies. Crucial’s parent company, Micron, is an official adopter of the UEFI Forum.  

Two of the biggest practical advantages of UEFI are support for larger disk partitions over 2TB (using a GUID Partition Table (GPT) and support for 32-bit environments. These were essential changes needed to allow computers to make use of long-term storage devices.  

UEFI firmware is also able to store software product keys for the operating system. This feature became part of the Secure Boot initiative, designed to ensure security at all layers of the computer OS.  

As computer networks have become ubiquitous thanks to local area networks and the growth of the internet, another important feature supported by UEFI is the ability to boot a computer over a network. UEFI firmware supports common network protocols to make network boots possible and is a more robust alternative to BIOS, which has limited network-boot support.  

Despite all these changes, another key feature is that UEFI provides backward compatibility. It is possible to install operating systems that still need BIOS support on a system with UEFI by changing the setting in the firmware. However, this will disable other UEFI features in the firmware. 

BIOS and UEFI are stored in flash memory, meaning a motherboard can overwrite and upgrade the firmware. The first BIOS chips were stored in a ROM, which meant any updates required a new chip or a new motherboard.

Both versions of firmware can be found in physical chips mounted on your motherboard. Original BIOS chips could be upgraded and replaced. In modern systems, chips are soldered in place but can be flashed with new firmware.

BIOS is firmly a legacy technology now, and virtually all computers (desktops, laptops, servers, phones and tablets) have used UEFI for at least the last decade.  

If you are interested in legacy computing, retro PC gaming or happen to still be running an older computer, then an understanding of BIOS and its limitations can be helpful. For everyone else, the future of computer firmware started twenty years ago.