Contiguous allocation in Operating Systems - Mini Project: Build & Apply

Understanding Contiguous Allocation in Operating Systems

📖 Scenario: You are learning how operating systems manage files on a disk using contiguous allocation. This method stores each file in one continuous block of disk space.Imagine a simple disk divided into blocks, and files stored in these blocks without gaps.

🎯 Goal: Build a simple representation of disk blocks and files using contiguous allocation. You will create a data structure to represent files and their starting block and length, then write logic to check if a new file can fit contiguously on the disk.

📋 What You'll Learn

Create a dictionary called disk_files with file names as keys and a tuple of (start_block, length) as values

Create a variable called disk_size representing total disk blocks

Write a function can_allocate_contiguous that checks if a new file can fit contiguously given a start block and length

Add a final check to see if a new file named 'FileC' starting at block 5 with length 3 can be allocated

💡 Why This Matters

🌍 Real World

Contiguous allocation is a basic method used in file systems to store files efficiently and quickly access them.

💼 Career

Understanding file allocation methods helps in roles related to system administration, software development, and operating system design.

1

Create the initial disk files dictionary

Create a dictionary called disk_files with these exact entries: 'FileA': (0, 4) and 'FileB': (4, 3) representing files and their start block and length on the disk.

Operating Systems

# Create the disk_files dictionary with FileA and FileB
# Your code here

Hint

Use a dictionary with file names as keys and tuples for (start_block, length) as values.

2

Set the total disk size

Create a variable called disk_size and set it to 10 representing the total number of blocks on the disk.

Operating Systems

disk_files = {'FileA': (0, 4), 'FileB': (4, 3)}
# Set disk_size to 10
# Your code here

Hint

Just assign the number 10 to the variable disk_size.

3

Write the allocation check function

Write a function called can_allocate_contiguous that takes start_block and length as parameters and returns True if the file fits contiguously on the disk without overlapping existing files, otherwise False. Use the disk_files and disk_size variables.

Operating Systems

disk_files = {'FileA': (0, 4), 'FileB': (4, 3)}
disk_size = 10
# Define can_allocate_contiguous function
# Your code here

Hint

Check if the requested block range overlaps with any existing file's block range.

4

Check allocation for a new file

Add a variable called file_c_can_allocate that stores the result of calling can_allocate_contiguous with start_block=5 and length=3 to check if 'FileC' can be allocated contiguously.

Operating Systems

disk_files = {'FileA': (0, 4), 'FileB': (4, 3)}
disk_size = 10

def can_allocate_contiguous(start_block, length):
    if start_block  disk_size:
        return False
    for _, (s, l) in disk_files.items():
        if not (start_block + length = s + l):
            return False
    return True

# Check if FileC can be allocated starting at block 5 with length 3
# Your code here

Hint

Call the function with the given start block and length, and assign the result to file_c_can_allocate.

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