BIOS, device drivers and virtual machines

Overview

Recall questions on what the BIOS does at start-up, the power-on self test, device drivers and why they are device and operating system specific, and virtual machines that run intermediate code or host a guest operating system.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does BIOS stand for and what is it?

   Model answer

   Basic Input Output System. It is firmware - software permanently held in a read-only or flash memory chip on the motherboard - and it is the first code the processor executes when the machine is switched on, before any operating system exists in memory.

2. 2

   Describe what the BIOS does when a computer is switched on.

   Model answer

   The processor's program counter is set to the BIOS start address, so the BIOS runs first. It carries out the power-on self test, checking that essential hardware such as the processor, memory, keyboard and graphics are present and working, and reports faults by beep codes or on-screen messages. It then initialises hardware and reads its configuration settings to find the boot order, locates the boot device, loads the bootstrap loader from it into RAM, and hands control over so the operating system can load itself.

3. 3

   What is the power-on self test?

   Model answer

   A sequence of hardware checks run by the BIOS at start-up, before the operating system loads, that verifies that essential components such as memory, the processor, the graphics adapter and the keyboard are present and responding. If a check fails, the process stops and the fault is signalled, often by a pattern of beeps because the display may not yet be usable.

4. 4

   Why are BIOS settings held in a small memory chip powered by a battery?

   Model answer

   Settings such as the boot order, the system clock and hardware options must survive being switched off, but must also be changeable by the user, so read-only memory alone will not do. A small amount of volatile CMOS memory with a backup cell keeps them. If that cell goes flat the settings and the clock revert to defaults, which is why an old machine may lose the date and try to boot from the wrong device.

5. 5

   What is a device driver?

   Model answer

   A piece of software that allows the operating system to communicate with a particular piece of hardware. It translates the general instructions the operating system issues - for example a request to print a page or read a block - into the specific signals and command codes that that particular model of device understands, and passes the device's replies back.

6. 6

   Why is a device driver specific both to the device and to the operating system?

   Model answer

   It is specific to the device because different models use different internal command sets, registers and protocols, even when they do the same job. It is specific to the operating system because it must plug into that operating system's own interfaces and run within its kernel conventions. This is why a printer needs one driver for one operating system and a different one for another, and why hardware can be left unusable when a manufacturer stops writing drivers for new operating system versions.

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