Contiguous allocation in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is contiguous allocation in operating systems?

Contiguous allocation is a method of storing files where each file occupies a set of consecutive blocks on the disk. This means the file's data is stored in one continuous area.

beginner

What is a major advantage of contiguous allocation?

It allows fast access to files because the disk head can read the file in one continuous sweep without moving to different locations.

intermediate

What is a common problem caused by contiguous allocation?

It can cause external fragmentation, where free space is broken into small pieces scattered around the disk, making it hard to find large enough continuous space for new files.

intermediate

How does contiguous allocation affect file size changes?

If a file grows beyond its allocated space, it may need to be moved to a larger continuous space, which can be inefficient and slow.

intermediate

Name one alternative to contiguous allocation to avoid its drawbacks.

Linked allocation or indexed allocation are alternatives that store file blocks non-contiguously, reducing fragmentation and allowing easier file size changes.

What does contiguous allocation require for storing a file?

A Linked list of blocks

B Random scattered blocks

C A set of consecutive disk blocks

D Indexed blocks

Contiguous allocation stores files in consecutive disk blocks to allow fast sequential access.

Which problem is most associated with contiguous allocation?

A Internal fragmentation

B External fragmentation

C Slow access speed

D File corruption

External fragmentation happens when free space is split into small pieces, making it hard to allocate large continuous blocks.

Why might a file need to be moved in contiguous allocation?

A Because it grows beyond its allocated space

B Because it is deleted

C Because it is compressed

D Because it is encrypted

If a file grows larger than its allocated continuous space, it must be moved to a bigger continuous area.

Which of these is NOT an advantage of contiguous allocation?

A No fragmentation

B Simple to implement

C Fast sequential access

D Easy to calculate file location

Contiguous allocation suffers from external fragmentation, so 'No fragmentation' is incorrect.

Which allocation method stores file blocks non-contiguously to reduce fragmentation?

A Contiguous allocation

B Direct allocation

C Sequential allocation

D Linked allocation

Linked allocation stores file blocks scattered but linked, reducing fragmentation problems.

Explain how contiguous allocation works and why it can cause fragmentation.

Think about how files are placed on disk and what happens when space is limited.

Describe one advantage and one disadvantage of contiguous allocation.

Consider speed and space management.

Practice

1. What is the main characteristic of contiguous allocation in operating systems?

easy

A. Files are stored in one continuous block of memory.

B. Files are stored in multiple scattered blocks.

C. Files are stored using linked lists.

D. Files are stored in indexed blocks.

Solution

1. Step 1: Understand file storage methods

   Contiguous allocation means storing a file in one continuous block of memory without breaks.

2. Step 2: Compare with other methods

   Other methods like linked or indexed allocation store files in scattered blocks or use pointers.

3. Final Answer:

   Files are stored in one continuous block of memory. -> Option A

4. Quick Check:

   Contiguous allocation = continuous block storage [OK]

Hint: Contiguous means continuous block storage [OK]

Common Mistakes:

• Confusing contiguous with linked allocation
• Thinking files are scattered in contiguous allocation
• Mixing indexed allocation with contiguous

2. Which of the following is the correct way to describe contiguous allocation?

easy

A. Files are divided into fixed-size blocks scattered across memory.

B. Files use pointers to link scattered blocks.

C. Files occupy a single continuous block of memory.

D. Files are stored in a tree-like index structure.

Solution

1. Step 1: Identify the definition of contiguous allocation

   Contiguous allocation means the entire file is stored in one continuous block of memory.

2. Step 2: Eliminate incorrect options

   Options A, B, and D describe other allocation methods like linked or indexed allocation.

3. Final Answer:

   Files occupy a single continuous block of memory. -> Option C

4. Quick Check:

   Continuous block = contiguous allocation [OK]

Hint: Continuous block means contiguous allocation [OK]

Common Mistakes:

• Choosing linked or indexed allocation descriptions
• Confusing fixed-size blocks with contiguous storage
• Assuming pointers are used in contiguous allocation

3. Consider a file system using contiguous allocation. If a file of size 10 blocks is stored starting at block 50, which blocks does it occupy?

medium

A. Blocks 60 to 69

B. Blocks 40 to 49

C. Blocks 51 to 60

D. Blocks 50 to 59

Solution

1. Step 1: Understand starting block and file size

   The file starts at block 50 and occupies 10 continuous blocks.

2. Step 2: Calculate occupied blocks

   Blocks occupied are from 50 up to 50 + 10 - 1 = 59.

3. Final Answer:

   Blocks 50 to 59 -> Option D

4. Quick Check:

   Start 50 + size 10 = blocks 50-59 [OK]

Hint: Start block + size - 1 = last block [OK]

Common Mistakes:

• Off-by-one errors in block range
• Starting count from 1 instead of 0
• Choosing blocks after the last block

4. A file stored using contiguous allocation needs to be extended by 5 blocks, but the next 5 blocks after the file are occupied. What is the main problem here?

medium

A. File cannot be extended because contiguous space is not available.

B. File can be extended by linking scattered blocks.

C. File size can be increased without any issue.

D. File can be moved to another location without fragmentation.

Solution

1. Step 1: Understand contiguous allocation extension

   Files must occupy continuous blocks; extension requires free blocks immediately after the file.

2. Step 2: Analyze the problem

   Since the next 5 blocks are occupied, the file cannot be extended contiguously.

3. Final Answer:

   File cannot be extended because contiguous space is not available. -> Option A

4. Quick Check:

   Extension needs free contiguous blocks [OK]

Hint: Extension needs free continuous blocks [OK]

Common Mistakes:

• Assuming scattered blocks can be linked in contiguous allocation
• Ignoring fragmentation issues
• Thinking file can grow without moving

5. You have a disk with free blocks scattered as follows: 5 blocks free at 0-4, 3 blocks free at 10-12, and 7 blocks free at 20-26. You want to store a file of size 6 blocks using contiguous allocation. Which free space can you use and why?

hard

A. Blocks 10-12, because they are in the middle of the disk.

B. Blocks 20-25, because they have enough continuous free blocks.

C. Blocks 0-4, because they are the first free blocks.

D. No space is available because all free blocks are scattered.

Solution

1. Step 1: Identify free block sizes

   Free blocks are 5 at 0-4, 3 at 10-12, and 7 at 20-26.

2. Step 2: Find a continuous free space of at least 6 blocks

   Only blocks 20-26 have 7 continuous free blocks, enough for 6 blocks.

3. Final Answer:

   Blocks 20-25, because they have enough continuous free blocks. -> Option B

4. Quick Check:

   Need ≥6 continuous blocks, only 20-26 fits [OK]

Hint: Choose free space with enough continuous blocks [OK]

Common Mistakes:

• Choosing smaller free blocks that don't fit file size
• Ignoring the need for continuous blocks
• Assuming scattered free blocks can be combined