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Lesson 5 of 5Hardware, Software and Operating Systems
Unit 2 · Lesson 5 of 5

Operating Systems, File Systems and Maintenance

What the OS is actually managing behind every click, how files are organized on a drive, and what real maintenance involves.

4 min read9 practice questionsAsk AI about this →

Key things to remember

  1. 1The OS manages processes, memory, files, devices and security.
  2. 2On a single core, multitasking is rapid switching, not simultaneous execution.
  3. 3FAT32 caps a single file at about 4 GB; exFAT and NTFS do not.
  4. 4Deleting a file removes its directory entry and frees its blocks — the data stays until overwritten.
  5. 5Never defragment an SSD: it gains nothing and burns write cycles.
  6. 63-2-1 backups: three copies, two media types, one off-site — and test a restore.

An operating system is easiest to understand as a resource manager with a monopoly. There is one CPU and a dozen programs that want it. There is a finite amount of RAM and no program that would refuse more. There is one printer and several documents. The OS decides who gets what, when, and for how long — and it stops any one program from taking everything or interfering with another.

Each layer only has to talk to the one below it.Application softwarebrowser, IDE, gameSystem software / utilitiesantivirus, backup, defragOperating system kernelprocesses · memory · files · I/ODevice driverstranslate for each piece of hardwareHardwareCPU, RAM, disk, network cardAn app never writes to a disk directly — it asks the OS, which asks the driver, which talks to the controller.
Fig 1Each layer only has to talk to the one below it.

The five jobs of an operating system

  • Process management — deciding which process gets CPU time. With more processes than cores, the scheduler switches between them so rapidly that they appear simultaneous.
  • Memory management — allocating RAM to each process, keeping them isolated so one cannot read or corrupt another's memory, and paging to disk when RAM runs out.
  • File management — organizing storage into files and folders, tracking where each file physically lives, and enforcing permissions.
  • Device management — talking to hardware through drivers, so an application can say "print this" without knowing anything about the printer.
  • User interface and security — providing a GUI or command line, and authenticating users so permissions can be enforced.
Multitasking is an illusion (mostly)
A single core runs one instruction stream at a time. The scheduler gives each process a few milliseconds, saves its state, and switches — thousands of times per second. Only a multi-core CPU achieves genuine simultaneous execution, and only up to the number of cores it has.

File systems

File systemUsed byNotable property
NTFSWindowsPermissions, journaling, encryption, very large files
exFATCross-platform removable mediaWorks on Windows, macOS and Linux; no permissions
FAT32Old USB sticks, some camerasMaximum single file size of 4 GB — the limit you actually hit
APFSmacOSOptimized for SSDs, snapshots, strong encryption
ext4Most Linux distributionsJournaled, robust, the default nearly everywhere

A file system's job is to turn a flat array of numbered blocks into a tree of named files. It keeps a directory structure, and for each file records which blocks hold its contents. This is why deleting a file is instant on any size of file: the OS marks the blocks as free and removes the directory entry, without touching the data itself. The contents remain on the drive until something else writes over them, which is the entire basis of file recovery — and the reason a "deleted" file is not gone in any security sense.

FAT32's 4 GB limit
If a large video refuses to copy onto a USB stick that has plenty of free space, the stick is almost certainly FAT32. Its file size field is 32 bits, capping any single file at bytes, about 4 GB. Reformat as exFAT — after copying off anything you want to keep, since formatting erases the drive.

Real maintenance

TaskWhyFrequency
Install OS and application updatesMost exploited vulnerabilities already have a patch availableAutomatic, or monthly
Verify backups by restoring oneAn untested backup is a guess, not a backupQuarterly
Check drive health (SMART)Drives usually warn before they fail outrightMonthly
Remove unnecessary startup programsStartup items are the main cause of slow bootsWhen boot time grows
Clean dust from fans and heatsinksBlocked airflow causes thermal throttling and shortens lifeEvery 6–12 months
Defragment (HDD only)Reduces head movement for fragmented filesAutomatic; never on an SSD
Never defragment an SSD
Defragmenting rearranges data so files sit in contiguous blocks, which only helps a drive that physically moves a head. An SSD reaches any block in the same time, so defragmenting achieves nothing — while consuming a large number of the finite write cycles the drive has left. Windows already knows this and runs TRIM instead.

Backups that actually work

The usual guidance is the 3-2-1 rule: three copies of the data, on two different types of media, with one copy off-site. The off-site copy is the part people skip, and it is the one that survives theft, fire and flood. A backup that has never been restored from is untested — the only way to know it works is to actually retrieve a file from it.

AFull backup
  • Copies everything every time
  • Slowest to create, largest storage cost
  • Fastest and simplest to restore — one set of files
BIncremental backup
  • Copies only what changed since the last backup
  • Fast to create, small storage cost
  • Slower to restore: you need the full backup plus every increment since

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A 5 GB video will not copy to a 64 GB USB stick that reports 58 GB free. Diagnose the problem and give a fix that does not lose the data already on the stick.
Example 2
A company keeps nightly backups to a network drive in the same office. Ransomware encrypts every machine including the network drive. Identify what failed and rewrite their strategy using 3-2-1.

Where marks get lost

  • Defragmenting an SSD — no benefit, real wear cost.
  • Believing a deleted file is gone. The blocks are marked free but the data stays until overwritten.
  • Assuming a USB stick with free space can hold any file. FAT32 caps single files at 4 GB.
  • Calling a copy on the same drive a backup. A drive failure destroys both the original and the copy.
  • Trusting a backup that has never been restored from.
  • Thinking a single-core CPU runs programs genuinely simultaneously. The scheduler switches between them very fast.

Flashcards 9

The five jobs of an operating system
How does a single core run several programs at once?
FAT32's most important limitation
What actually happens when you delete a file?
+ 5 more

Practice

1
A 6 GB file will not copy to a USB stick with 40 GB free. The most likely cause is:
2
Which maintenance task should NEVER be performed on a solid state drive?
3
What is the OS component that decides which process gets CPU time next?
4
Why does deleting a 20 GB file take the same fraction of a second as deleting a 20 KB file?
5
Order these layers from closest to the hardware to closest to the user.
  1. 1Device drivers
  2. 2Application software
  3. 3Operating system kernel
  4. 4Hardware
6
A backup strategy copies files nightly to a second partition on the same physical drive. What is the main weakness?
7
Match each file system to its defining characteristic.
NTFS
exFAT
FAT32
ext4
8
Which backup type is fastest to create but slowest to restore from?
9
A file system uses 4 KB blocks. How many blocks does a 10 000 byte file occupy? (Blocks cannot be shared between files.)
blocks

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