Contiguous allocation in Operating Systems - Step-by-Step Execution

Concept Flow - Contiguous allocation

Start: Request for file storage

↓

Search for contiguous free space

↓

Allocate space

↓

Store file data

↓

File stored successfully

The system looks for a single continuous block of free space to store a file. If found, it allocates and stores the file; if not, allocation fails or fragmentation is handled.

Execution Sample

Operating Systems

Disk blocks: [Free, Free, Free, Used, Free, Free]
File size: 3 blocks
Search for 3 contiguous free blocks
Allocate blocks 0,1,2
Store file data

This example shows how the system finds 3 continuous free blocks at the start of disk and allocates them to store a file.

Analysis Table

Step	Disk Blocks State	Action	Result
1	[Free, Free, Free, Used, Free, Free]	Check blocks 0-2	All free, suitable space found
2	[Free, Free, Free, Used, Free, Free]	Allocate blocks 0,1,2	Blocks 0,1,2 marked as Used
3	[Used, Used, Used, Used, Free, Free]	Store file data in allocated blocks	File stored successfully
4	[Used, Used, Used, Used, Free, Free]	End	Allocation complete

💡 Allocation stops after finding and using 3 contiguous free blocks at start

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Disk Blocks	[Free, Free, Free, Used, Free, Free]	[Free, Free, Free, Used, Free, Free]	[Used, Used, Used, Used, Free, Free]	[Used, Used, Used, Used, Free, Free]	[Used, Used, Used, Used, Free, Free]
Allocated Blocks	None	Blocks 0-2 found	Blocks 0-2 allocated	Blocks 0-2 storing data	Blocks 0-2 allocated and used

Key Insights - 3 Insights

Why must the free space be contiguous for allocation?

Because contiguous allocation requires the file to occupy a single continuous block of disk space, as shown in step 1 and 2 of the execution_table where blocks 0-2 are checked and allocated together.

What happens if there is no contiguous free space large enough?

Allocation fails or the system must handle fragmentation, as indicated by the 'No' branch in the concept_flow where no suitable continuous space is found.

Why does the disk block 3 remain 'Used' after allocation?

Because it was already used before allocation and is not part of the newly allocated contiguous space, as seen in the disk blocks state in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. Which blocks are allocated to the file?

A Blocks 1, 2, and 3

B Blocks 0, 1, and 2

C Blocks 3, 4, and 5

D Blocks 4 and 5 only

💡 Hint

Refer to the 'Action' and 'Result' columns at Step 2 in the execution_table.

At which step does the file data get stored in the allocated blocks?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Action' column for storing file data in the execution_table.

If the file size was 4 blocks instead of 3, what would happen according to the concept_flow?

A Allocation would fail due to no 4 contiguous free blocks

B Allocation would succeed using blocks 0-3

C Allocation would use non-contiguous blocks

D File size would be reduced automatically

💡 Hint

Look at the 'Found? Yes/No' decision in the concept_flow for contiguous space availability.

Concept Snapshot

Contiguous allocation stores files in one continuous block of disk space.
The system searches for a free space large enough to hold the entire file.
If found, it allocates that space and stores the file.
If not, allocation fails or fragmentation must be handled.
Simple and fast access but can cause external fragmentation.

Full Transcript

Contiguous allocation means storing a file in a single continuous block of disk space. The system starts by checking the disk for a free space large enough to hold the whole file. If it finds such a space, it allocates those blocks and stores the file data there. If it cannot find enough continuous free blocks, the allocation fails or the system must handle fragmentation. This method is simple and allows fast access but can lead to external fragmentation over time.

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