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

Concept Flow - Contiguous allocation

Start: Request for file storage

↓

Search for contiguous free space

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Allocate space

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Store file data

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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.