Free space management in Operating Systems - Step-by-Step Execution

Concept Flow - Free space management

Start: Disk with free and used blocks

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Choose free space management method

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Bit Vector

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Mark free/used

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Allocate space

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Update free space info

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End

This flow shows how an operating system manages free disk space by choosing a method, marking or linking free blocks, allocating space, and updating the free space information.

Execution Sample

Operating Systems

Disk blocks: [F, F, U, F, U, U, F]
Bit Vector: 1=used, 0=free
Allocate 2 blocks
Update bit vector

This example shows how a bit vector tracks free (0) and used (1) blocks and updates when allocating space.

Analysis Table

Step	Disk Blocks	Bit Vector	Action	Result
1	[F, F, U, F, U, U, F]	[0, 0, 1, 0, 1, 1, 0]	Initial state	Free blocks at positions 0,1,3,6
2	[F, F, U, F, U, U, F]	[1, 1, 1, 0, 1, 1, 0]	Allocate 2 blocks at positions 0,1	Blocks 0 and 1 marked used
3	[U, U, U, F, U, U, F]	[1, 1, 1, 0, 1, 1, 0]	Update disk blocks	Disk blocks updated to used at 0,1
4	[U, U, U, F, U, U, F]	[1, 1, 1, 0, 1, 1, 0]	Next allocation would start at position 3	Ready for next allocation
5	[U, U, U, F, U, U, F]	[1, 1, 1, 1, 1, 1, 0]	Allocate 1 block at position 3	Block 3 marked used
6	[U, U, U, U, U, U, F]	[1, 1, 1, 1, 1, 1, 0]	Update disk blocks	Disk blocks updated to used at 3
7	[U, U, U, U, U, U, F]	[1, 1, 1, 1, 1, 1, 0]	No more blocks allocated	Free block remains at position 6

💡 No more free blocks available or allocation request fulfilled

State Tracker

Variable	Start	After Step 2	After Step 5	Final
Disk Blocks	[F, F, U, F, U, U, F]	[F, F, U, F, U, U, F]	[U, U, U, F, U, U, F]	[U, U, U, U, U, U, F]
Bit Vector	[0, 0, 1, 0, 1, 1, 0]	[1, 1, 1, 0, 1, 1, 0]	[1, 1, 1, 1, 1, 1, 0]	[1, 1, 1, 1, 1, 1, 0]

Key Insights - 3 Insights

Why does the bit vector change from 0 to 1 when a block is allocated?

Because 0 means the block is free and 1 means it is used. When allocating, the system marks the block as used by changing 0 to 1, as shown in execution_table step 2.

Why does the disk block status update after changing the bit vector?

The bit vector is a map of free/used blocks. After marking blocks as used in the bit vector, the actual disk blocks are updated to reflect this, as seen in execution_table step 3.

What happens if there are no free blocks left?

Allocation cannot proceed because no free blocks are available. The exit_note explains that allocation stops when no free blocks remain.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the bit vector after allocating 2 blocks?

A [0, 0, 1, 0, 1, 1, 0]

B [1, 1, 1, 0, 1, 1, 0]

C [1, 0, 1, 0, 1, 1, 0]

D [0, 1, 1, 0, 1, 1, 0]

💡 Hint

Check the 'Bit Vector' column in execution_table row for step 2.

At which step does the disk block at position 3 become used?

A Step 3

B Step 5

C Step 6

D Step 4

💡 Hint

Look at the 'Disk Blocks' and 'Action' columns in execution_table for when position 3 changes.

If we allocate one more block after step 6, what will the bit vector look like?

A [1, 1, 1, 1, 1, 1, 1]

B [1, 1, 1, 1, 1, 1, 0]

C [0, 1, 1, 1, 1, 1, 1]

D [1, 1, 1, 0, 1, 1, 1]

💡 Hint

After allocating the last free block at position 6, bit vector changes from 0 to 1 at that position.

Concept Snapshot

Free space management tracks which disk blocks are free or used.
Common methods: bit vector, linked list, grouping.
Bit vector uses 0 for free, 1 for used blocks.
Allocation marks blocks used and updates tracking.
When no free blocks remain, allocation stops.

Full Transcript

Free space management in operating systems involves tracking which parts of the disk are free or used. The system chooses a method like bit vector, linked list, or grouping to keep this information. For example, a bit vector uses 0 to mark free blocks and 1 for used blocks. When allocating space, the system changes 0 to 1 in the bit vector and updates the disk blocks accordingly. This process repeats until no free blocks remain, at which point allocation stops.