BIOS and UEFI are firmware systems that prepare a computer before Windows starts. They initialise hardware, run startup checks, locate a bootable device, and hand control to the operating system.

Understanding them is essential for CompTIA A+ study and real-world IT troubleshooting.

What Is BIOS?

BIOS stands for Basic Input/Output System.

It is the traditional firmware used on older computers. BIOS performs tasks such as:

• Initialising hardware
• Running POST
• Detecting storage devices
• Checking the boot order
• Starting the operating system

BIOS commonly works with the MBR partition style.

What Is UEFI?

UEFI stands for Unified Extensible Firmware Interface.

It is the modern replacement for BIOS and provides:

• Faster startup
• Secure Boot
• Support for large drives
• Better hardware compatibility
• Graphical menus and mouse support
• Support for GPT partitions

Most modern computers use UEFI.

BIOS vs UEFI

Feature	BIOS	UEFI
Typical systems	Older computers	Modern computers
Partition style	MBR	GPT
Secure Boot	Not supported	Supported
Drive support	Commonly limited to about 2 TB	Supports drives larger than 2 TB
Interface	Usually keyboard-based	Often graphical
Startup speed	Generally slower	Generally faster

MBR and GPT

BIOS usually boots from an MBR drive.

MBR supports:

• Drives up to about 2 TB
• Four primary partitions

UEFI usually boots from a GPT drive.

GPT provides:

• Support for larger drives
• More partitions
• Backup partition data
• Better reliability

Secure Boot

Secure Boot is a UEFI security feature.

It checks whether startup software is trusted before allowing it to run. This helps block unauthorised boot loaders and certain types of pre-boot malware.

Secure Boot may sometimes need to be disabled temporarily when using older operating systems or unsigned diagnostic tools.

Boot Order

The boot order tells BIOS or UEFI which device to check first.

Example:

1. Windows Boot Manager
2. NVMe SSD
3. USB Drive
4. Network Boot

An incorrect boot order can cause errors such as:

No Boot Device Found

Common Firmware Settings

BIOS and UEFI commonly include settings for:

• Boot order
• Secure Boot
• TPM
• CPU virtualisation
• Storage controller mode
• System date and time
• Firmware passwords
• Hardware diagnostics

Changing storage mode or boot mode incorrectly can prevent Windows from loading.

CMOS Battery

The CMOS battery helps preserve firmware settings when the computer is disconnected from power.

A weak battery may cause:

• Incorrect date and time
• Reset boot order
• Firmware settings not being saved
• CMOS checksum errors

Common Problems

Symptom	Possible Cause
SSD not detected	Loose connection, failed SSD, disabled controller
No Boot Device Found	Incorrect boot order, failed drive, damaged boot files
Windows Boot Manager missing	Corrupted EFI partition or wrong boot mode
Secure Boot violation	Untrusted or unsigned boot software
Date and time reset	Weak CMOS battery

Troubleshooting Workflow

1. Confirm the computer powers on
2. Check whether POST completes
3. Enter BIOS or UEFI
4. Confirm the SSD is detected
5. Check the boot order
6. Confirm UEFI or Legacy mode
7. Review Secure Boot settings
8. Test the storage device
9. Repair Windows boot files if required

Make one change at a time and test the result before changing another setting.

CompTIA A+ Key Points

Remember:

• BIOS is older firmware.
• UEFI is the modern replacement.
• BIOS commonly uses MBR.
• UEFI commonly uses GPT.
• Secure Boot is a UEFI feature.
• POST checks hardware before Windows starts.
• Boot order determines which device starts first.
• A weak CMOS battery can reset firmware settings.
• Firmware updates must not be interrupted.

Conclusion

BIOS and UEFI connect the computer's hardware to the operating system startup process.

Knowing how they work helps IT Support technicians diagnose storage problems, boot failures, incorrect firmware settings, and Windows startup issues more quickly.