Operating Systems Memory Management and Bash Scripting

Memory Allocation Strategies

🔹 First Fit

👉 Take the first block that fits.

Example:

Blocks: 120, 450, 300, 700, 200
Processes: 250, 100, 500, 180

Step by step:

• 250 → goes into 450 → left: 200
• 100 → goes into 120 → left: 20
• 500 → goes into 700 → left: 200
• 180 → goes into 300 → left: 120

Final memory:
20, 200, 120, 200, 200

🔹 Best Fit

👉 Use the smallest block that fits.

Same example:

• 250 → 300 → left 50
• 100 → 120 → left 20
• 500 → 700 → left 200
• 180 → 200 → left 20

Final:
20, 450, 50, 200, 20

👉 Note: Causes more fragmentation (many small pieces).

🔹 Worst Fit

👉 Use the largest block.

Same example:

• 250 → 700 → left 450
• 100 → 450 → left 350
• 500 → ❌ no block large enough
• 180 → 350 → left 170

🔹 Next Fit

👉 Same as First Fit, but continue from the last position.

✔️ Do NOT restart from the beginning.

🧩 Key Concepts

• Best Fit: More fragmentation.
• Worst Fit: Keeps large blocks available.
• General Rule: Always subtract memory after allocation.

📦 Address Translation

Formulas

• Page = Address ÷ Page Size
• Offset = Address % Page Size

Example:

Page size = 1024
Address = 2500

• Page = 2
• Offset = 452

📋 Page Table

Steps:

1. Get page and offset.
2. Find frame in table.
3. Use formula:

Physical Address = (frame × page size) + offset

Example:

Page table: Page 2 → Frame 8

Physical: (8 × 1024) + 452 = 8644

⚡ TLB (Translation Lookaside Buffer)

• TLB is a fast cache for address translation.
• Hit: Fast access.
• Miss: Slower (requires memory access).
• ✔️ Improves overall performance.

⏱️ EMAT (Effective Memory Access Time)

Formula: EMAT = h(TLB + M) + (1 − h)(TLB + 2M)

Example:

• Memory (M) = 100 ns
• TLB = 20 ns
• Hit rate (h) = 80%

EMAT = 0.8(20 + 100) + 0.2(20 + 200) = 96 + 44 = 140 ns

📚 Multilevel Page Table

• Used because single-level tables are too large.
• Saves memory space.
• Slower due to multiple lookup steps.

🔁 Page Replacement Algorithms

• FIFO: Oldest page removed.
• LRU: Least recently used page removed.
• Clock: Circular version of FIFO.
• NFU: Counts usage frequency.
• Aging: Improved version of NFU.
• Working Set: Tracks active pages.

🔢 FIFO Example

Reference: 1, 2, 3, 1, 4, 5 | Frames = 3

Total faults = 5

🖥️ Bash Scripting Essentials

Even / Odd Check

if (( $1 % 2 == 0 )); then echo "Even"; else echo "Odd"; fi

Sum 1 to N

sum=0; for ((i=1; i<= $1; i++)); do sum=$((sum + i)); done; echo $sum

File Exists

if [ -f "$1" ]; then wc -l "$1"; else echo "File not found"; fi

Count .txt Files

ls *.txt 2>/dev/null | wc -l

🚨 Common Exam Mistakes

• Forgetting the offset.
• Using the wrong EMAT formula.
• Restarting Next Fit from the beginning.
• Not subtracting memory after allocation.
• Incorrect FIFO order.

🔥 Final Strategy

👉 Allocation: Go process by process and always update memory.

👉 Address: Divide for page, Modulo for offset.

👉 FIFO: Always draw the table.

👉 Bash: Keep logic simple and use standard syntax.