Input/output management in the operating system

Device drivers and the BIOS

• A device driver is software that lets the operating system communicate with a specific hardware device. It translates general OS requests ('print this page') into the device's own commands. Applications talk to the OS; the OS talks to the driver; the driver talks to the device.
• The BIOS (Basic Input/Output System), or its modern replacement UEFI, is firmware that runs when the computer is switched on. It runs a power-on self-test (POST), initialises basic hardware and loads the operating system's boot loader. It also offers basic low-level device access during start-up.

Buffering and caching

• A buffer is an area of memory that temporarily holds data being transferred between devices that work at different speeds. The CPU fills a print buffer quickly and gets on with other work while the printer empties it slowly. Keyboard and video streaming buffers work the same way.
• A cache holds copies of frequently or recently used data in faster storage so it can be reused without the slow fetch. A disk cache in RAM avoids rereading the disk.
• Difference: a buffer smooths a one-off flow of data; a cache keeps data in the hope it will be needed again.

Interrupts

An interrupt is a signal from a device or program telling the processor that something needs attention (key pressed, printer out of paper, data transfer complete, timer).

1. At the end of each fetch-decode-execute cycle the CPU checks for interrupts.

2. If one has higher priority than the current task, the CPU saves the contents of its registers (including the program counter) on the stack.

3. It runs the interrupt service routine (ISR) for that device.

4. It restores the saved registers and carries on where it left off.

Interrupts are better than polling (repeatedly asking each device) because the CPU does useful work until a device actually needs it.

Why I/O management matters

Benefits: programs are device-independent; slow devices do not hold up the CPU; many devices can share the system safely. Drawbacks: drivers add a layer that can contain bugs or security holes and must be updated; buffering and caching use memory; badly prioritised interrupts can delay important tasks.

A typical diagram: user -> application -> operating system -> device driver -> device.

Worked example

A user prints a 50-page document and keeps typing. Explain how the operating system makes this possible.

1. The application passes the print job to the OS, which uses the printer driver to convert it into the printer's commands.
2. The data is placed in a buffer, so the CPU can return to the word processor straight away.
3. When the printer has used up a chunk of the buffer it sends an interrupt; the CPU saves its state, runs the ISR to send more data, then resumes the user's typing.

Answer: Driver translation, buffering and interrupts let printing happen in the background.

Check yourself

1. What is the role of a device driver?
   • It stores frequently used data in fast memory
   • It checks the hardware when the computer starts
   • It translates operating system requests into commands a specific device understands
   • It decides which program uses the CPU next
   Show answer

   It translates operating system requests into commands a specific device understands. Each device type needs a driver so the OS can control it.

2. What does the BIOS do when a computer is switched on?
   • Tests and initialises the hardware, then loads the operating system's boot loader
   • Runs the user's applications
   • Deletes temporary files
   • Encrypts the hard disk
   Show answer

   Tests and initialises the hardware, then loads the operating system's boot loader. POST, then hand over to the boot loader.

3. Why is a buffer used when sending data to a printer?
   • It permanently stores every document printed
   • It makes the printer print faster
   • The CPU can transfer data quickly into memory and continue other work while the slow printer empties it
   • It converts data into the printer's language
   Show answer

   The CPU can transfer data quickly into memory and continue other work while the slow printer empties it. A buffer bridges the speed gap.

4. How does a cache differ from a buffer?
   • A cache is slower than the storage it serves
   • A cache keeps copies of data likely to be reused; a buffer temporarily holds data in transit
   • A buffer is only used for the processor's registers
   • A cache is only used by printers
   Show answer

   A cache keeps copies of data likely to be reused; a buffer temporarily holds data in transit. Caching is about reuse; buffering is about speed matching.

5. What does the processor do first when it accepts an interrupt?
   • Deletes the current program
   • Restarts the computer
   • Runs the BIOS
   • Saves the contents of its registers, including the program counter
   Show answer

   Saves the contents of its registers, including the program counter. Saving state lets it resume the interrupted task afterwards.

6. Why are interrupts usually better than polling?
   • Interrupts never need any processor time
   • The CPU does useful work instead of repeatedly checking devices that do not need attention
   • Polling cannot be used with keyboards
   • Interrupts stop all other programs
   Show answer

   The CPU does useful work instead of repeatedly checking devices that do not need attention. Polling wastes cycles asking 'are you ready?'.

7. In an I/O diagram, which order shows a print request travelling from the user to the printer?
   • User, printer, device driver, application, operating system
   • User, device driver, application, printer, operating system
   • User, operating system, printer, application, device driver
   • User, application, operating system, device driver, printer
   Show answer

   User, application, operating system, device driver, printer. The request passes down through the layers.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary and Advanced Level in Computer Science specification (2026), Issue 1
• Pearson Edexcel IAL Computer Science sample assessment materials (Unit 3 paper, resource booklet and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.