A+ Core 1 3.5: The BIOS — Class Notes

2026-09-27 · class-notes, a-plus, comptia, messer, hardware, bios, uefi, firmware, post, boot

Full class notes for A+ Core 1 3.5: BIOS firmware, POST and boot, legacy BIOS vs UEFI, dual-BIOS flash, and settings.

Class notes · Professor Messer A+ Core 1 (220-1201) · Section 3, lesson 5

Quick reference: the short version of this lesson lives in the A+ Core 1 Section 3 resource sheets. This is a fifth lesson under objective 3.5 ("install and configure motherboards, CPUs, and add-on cards"), joining Motherboard form factors, expansion slots, connections and compatibility. Here we look at the firmware that starts the board before any operating system exists.

Learning objectives

By the end of these notes you should be able to:

Explain what the BIOS is, the other names it goes by, and where it is stored today.
Describe the POST and how it leads into the bootloader and the operating system.
Explain why boards carry a main and a backup BIOS chip.
Contrast legacy BIOS with UEFI, including how each is navigated.
Describe the kinds of settings a UEFI interface exposes, and how to change them safely.

1. What the BIOS is

Press the power button and, for the first few seconds, what fills the screen has nothing to do with the operating system that eventually loads. That early screen is the BIOS — the Basic Input/Output System. You will hear it called by several names: the system's firmware, the system BIOS, or the ROM BIOS, where ROM means Read-Only Memory.

That last name is now a historical hangover. Modern systems do not keep the BIOS in true read-only memory; it lives in flash memory on the motherboard instead, which is what makes it updatable. The BIOS's job is to initialise the system and get everything ready to hand over to the operating system.

Note (beyond this lesson): the BIOS program lives in flash, but your settings (and the real-time clock) are held in a small amount of battery-backed memory kept alive by the motherboard's CR2032 coin cell. A dead cell is the classic cause of a clock that resets and BIOS settings that revert on every power-off.

2. POST and the boot sequence

The start-up sequence you watch is the Power-On Self-Test, or POST. It is a quick diagnostic that checks the core hardware is present and sane: is there a CPU, is memory installed, are a keyboard and mouse connected? If any of those core systems fails to initialise, POST puts an error message on the screen rather than pressing on.

POST takes only a few seconds. Once it passes, control moves to the bootloader — the point at which the firmware has finished its checks and is ready to load an operating system. Depending on the machine, the OS may start immediately, or you may get a prompt asking which operating system to load. Reaching that prompt is the signal that POST is complete.

Power on
   |
   v
+----------------------+
|  BIOS / UEFI starts  |  firmware in flash on the board
+----------+-----------+
           |
           v
+----------------------+
|  POST self-test      |  CPU? RAM? keyboard/mouse?
+----------+-----------+  fail -> error message on screen
           | pass
           v
+----------------------+
|  Bootloader          |  choose an OS if prompted
+----------+-----------+
           |
           v
+----------------------+
|  Operating system    |
+----------------------+

3. Where the BIOS lives: main and backup

Because the BIOS is in flash on the motherboard, you can often find the chip physically marked on the board. Many boards carry two BIOS flash chips: a main BIOS and a backup. The reason is safety during an update. A BIOS upgrade rewrites the flash, and if that process is interrupted — a power cut, a bad image — the board could be left unbootable. A second, standby copy means you can recover to a known-good BIOS if an upgrade goes wrong.

In the real world: this is why a BIOS/UEFI update is one of the few maintenance jobs where you genuinely do not want to lose power partway. Use a reliable supply, and never interrupt a flash in progress.

4. Legacy BIOS

On older PCs you may meet a legacy (traditional) BIOS — a design that has been around for more than twenty-five years. If a machine runs an older operating system on older hardware, it is probably using legacy BIOS. The catch is that legacy BIOS was written for older, legacy hardware, so putting newer hardware into an old system can run into a legacy BIOS that simply does not support it.

A legacy BIOS presents a text-based screen. You navigate it from the keyboard — arrow keys to move, and Enter, Space or various function keys to select and change values — with the available choices listed along the bottom of each screen so you always know your options.

Correction: the transcript describes the legacy screen as one you move around with "mouse keys". Legacy BIOS is keyboard-navigated and has no mouse support — mouse control is a UEFI feature (below). The keyboard keys the transcript then lists (Enter, Space, function keys) are the correct ones.

Note (beyond this lesson): legacy BIOS pairs with MBR partitioning, which brings the familiar limits — up to four primary partitions and a ~2 TB boot-disk ceiling. UEFI pairs with GPT, which lifts both. That partition detail belongs with the Operating systems material, but it is the practical reason legacy vs UEFI matters at install time.

5. UEFI

Modern hardware needs a modern firmware, and on a recent computer that is a UEFI BIOS. UEFI stands for Unified Extensible Firmware Interface, a standard created by Intel and now used on essentially all modern systems.

Because UEFI is a standard, its functionality is broadly consistent across manufacturers — the options and features look and behave much the same whoever built the machine. It also brings a friendlier interface: graphical screens you can drive with a mouse, not just the keyboard.

The settings a UEFI interface exposes are the ones you would expect for bringing a machine up to the point of loading an OS:

An overview of the CPU and the firmware version itself.
Connected devices, especially storage, audio and network.
Advanced settings for CPU features — notably virtualisation support.
Power options, security, startup/boot order, and more.

Note (beyond this lesson): UEFI grew out of Intel's earlier EFI and is now maintained by the multi-vendor UEFI Forum, which is why the experience is so similar across brands. UEFI also enables Secure Boot, which pairs with a TPM in the modern trusted-boot stack.

5.1 Worked example — enabling virtualisation safely

A user needs to run virtual machines, and the hypervisor complains that hardware virtualisation is off.

Document first. Note the current relevant settings (or photograph the screens) so you can put anything back.
Enter firmware setup at power-on (the key varies — Del, F2 and F10 are common; the POST screen usually tells you).
Advanced -> CPU features: enable the virtualisation option (Intel VT-x or AMD-V, depending on the platform).
Change one thing at a time. Do not tweak unrelated settings "while you're in there" — several are critical to stable operation.
Save and exit, then confirm the hypervisor now sees hardware virtualisation. If something misbehaves, your notes let you revert.

The discipline the lesson stresses: some firmware settings are critical to reliable operation, so back up and document before you change anything, and change only what you understand.

6. Changing BIOS settings safely

To restate the caution plainly: several BIOS/UEFI settings are critical to whether the system runs reliably at all, so they are not things to change on a hunch. Before any BIOS update or configuration change, make backups and keep documentation of the previous configuration, so you have a route back if the new settings cause trouble.

7. Security perspective

Firmware runs before and beneath the operating system, which makes the BIOS both a powerful control point and a high-value target:

Firmware persistence outlives a reinstall. Malware that reaches the firmware (a bootkit or UEFI implant) survives wiping the disk and reinstalling the OS, because it does not live on the disk. This is exactly why signed firmware, Secure Boot and measured boot matter, and why only vendor-signed BIOS images should ever be flashed. The dual-BIOS design also gives you a recovery path if an implant or a bad flash bricks the main chip.
Lock the boot path. An attacker with a few minutes at the machine (the "evil maid") can boot a live USB and bypass the installed OS entirely. Set a supervisor/UEFI password, restrict the boot order so the machine will not boot removable media by default, and keep Secure Boot enabled. Combined with full-disk encryption, this closes the easy offline-access routes.
Change control is a security control. Because critical settings live here, "back up and document before changing" is not just good hygiene — an undocumented firmware change (disabling Secure Boot, opening the boot order, turning off a security feature) can quietly weaken a machine. Track firmware version and settings as part of the build baseline.
Keep firmware patched. CPU microcode and platform vulnerabilities are fixed through BIOS/UEFI updates from the board vendor; an unpatched firmware is an unpatched attack surface below the OS.

Summary

The BIOS (Basic Input/Output System) is the motherboard firmware that initialises the system; also called system/ROM BIOS, now stored in flash, not ROM.
POST checks core hardware (CPU, memory, keyboard/mouse) and shows an error on failure; on success the bootloader loads (or lets you choose) the OS.
Many boards have a main and backup BIOS so a failed update can be recovered.
Legacy BIOS is old, text-based and keyboard-navigated, and may not support newer hardware. UEFI (Unified Extensible Firmware Interface, from Intel) is the modern standard: consistent across vendors, graphical/mouse, with settings for devices, CPU/virtualisation, power, security and boot.
Some settings are critical — back up and document before changing anything.

Glossary

Term	Meaning
BIOS	Basic Input/Output System; the firmware that starts the board.
Firmware	Low-level software stored on the hardware itself.
ROM BIOS	Old name; the BIOS is now in flash, not read-only memory.
Flash memory	Rewritable storage on the board holding the BIOS.
POST	Power-On Self-Test; the start-up hardware check.
Bootloader	The stage that loads (or offers a choice of) the OS.
Dual BIOS	Main plus backup BIOS chips for safe recovery.
Legacy BIOS	Traditional text-based, keyboard-navigated firmware.
UEFI	Unified Extensible Firmware Interface; the modern standard.
Secure Boot	UEFI feature that only lets signed boot code run.
Virtualisation setting	Advanced CPU option (VT-x / AMD-V) enabling hypervisors.
Boot order	The sequence of devices the firmware tries to boot from.
Supervisor password	A firmware password protecting BIOS settings.
CR2032	The coin cell backing BIOS settings and the clock.
Baseline	The documented known-good firmware version and settings.

Review questions

What does BIOS stand for, and give two other names for it.
Where is the BIOS stored on a modern motherboard, and why is "ROM BIOS" now a misnomer?
What does POST check, and what happens if a core component fails?
What is the bootloader, and what does reaching it tell you?
Why do some motherboards have two BIOS chips?
Roughly how old is the legacy BIOS design, and what is its main limitation with new hardware?
How is a legacy BIOS navigated?
What does UEFI stand for, and who created the standard?
Give two advantages UEFI has over legacy BIOS.
Name four categories of setting you would find in a UEFI interface.
What should you always do before making a BIOS change or update, and why?
Scenario: after a disk wipe and clean OS reinstall, a machine is still infected. What class of problem should you suspect, and why?

Answer key

Basic Input/Output System; also firmware, system BIOS or ROM BIOS. Same component, different names.
In flash memory on the motherboard; it is no longer read-only, so it can be updated. The ROM name is historical.
It checks core hardware (CPU, memory, keyboard/mouse); a failure shows an error message instead of continuing. A go/no-go start-up test.
The stage that loads the OS (sometimes offering a choice); reaching it means POST has passed. Hand-off point to the OS.
A main and a backup, so a failed or interrupted BIOS update can be recovered. Safety during flashing.
More than twenty-five years old; it was built for legacy hardware and may not support newer hardware. Age is the constraint.
From the keyboard — arrow keys plus Enter, Space and function keys, with choices listed at the bottom (no mouse). Text-based navigation.
Unified Extensible Firmware Interface; created by Intel. The modern standard.
Consistency across manufacturers and a graphical, mouse-driven interface (it also enables Secure Boot) — any two. Standardised and friendlier.
Device info, CPU/virtualisation (advanced), power, security, boot/startup — any four. Plus a CPU/firmware overview.
Back up and document the current configuration, in case you need to revert a change that harms reliability. Critical settings live here.
Firmware-level malware (a bootkit / UEFI implant), because it lives below the OS and survives a disk wipe. Reinstalling the OS does not clear it.