✨ VIPSolutions

Let's solve each part **step-by-step**: **Given:** - Queue: **95, 180, 34, 119, 11, 123, 62, 64** - Initial head position: **50** - Disk size: **0 to 199** --- ## (a) **First Come First Serve (FCFS)** **Order:** 95 → 180 → 34 → 119 → 11 → 123 → 62 → 64 **Initial Head:** 50 ### **Head Movements:** - 50 → 95 = **45** - 95 → 180 = **85** - 180 → 34 = **146** - 34 → 119 = **85** - 119 → 11 = **108** - 11 → 123 = **112** - 123 → 62 = **61** - 62 → 64 = **2** **Total = 45 + 85 + 146 + 85 + 108 + 112 + 61 + 2 = 644** --- ## (b) **Shortest Seek Time First (SSTF)** **Always go to the nearest request.** **Requests:** 95, 180, 34, 119, 11, 123, 62, 64 **Current Head:** 50 ### **Stepwise Solution:** 1. **From 50:** Closest is **62** (|62-50|=12), then 64 (14), 34 (16), 95 (45), 11 (39), 119 (69), 123 (73), 180 (130) - Go to **62**: Move = 12 - Remaining: 95, 180, 34, 119, 11, 123, 64 2. **From 62:** Closest is **64** (2) - Go to **64**: Move = 2 - Remaining: 95, 180, 34, 119, 11, 123 3. **From 64:** Closest is **95** (31), 34 (30) - Go to **34**: Move = 30 - Remaining: 95, 180, 119, 11, 123 4. **From 34:** Closest is **11** (23) - Go to **11**: Move = 23 - Remaining: 95, 180, 119, 123 5. **From 11:** Closest is **95** (84) - Go to **95**: Move = 84 - Remaining: 180, 119, 123 6. **From 95:** Closest is **119** (24) - Go to **119**: Move = 24 - Remaining: 180, 123 7. **From 119:** Closest is **123** (4) - Go to **123**: Move = 4 - Remaining: 180 8. **From 123:** Go to **180**: Move = 57 **Total = 12 + 2 + 30 + 23 + 84 + 24 + 4 + 57 = 236** --- ## (c) **SCAN (Elevator Algorithm)** - Head moves in one direction (let's assume upwards), services all requests until end, then reverses. **Requests (sorted):** 11, 34, 62, 64, 95, 119, 123, 180 **Initial Head:** 50 ### **Moving towards higher tracks:** - Service: 62, 64, 95, 119, 123, 180, then reverse to 34, 11 **Stepwise:** - 50 → 62 = 12 - 62 → 64 = 2 - 64 → 95 = 31 - 95 → 119 = 24 - 119 → 123 = 4 - 123 → 180 = 57 **(Now reverse direction)** - 180 → 34 = 146 - 34 → 11 = 23 **Total = 12 + 2 + 31 + 24 + 4 + 57 + 146 + 23 = 299** --- ## (d) **C-SCAN (Circular SCAN)** - Head moves in one direction, services all requests, goes to the end, jumps to beginning, continues. - **Assume moving upwards.** **Order:** 62, 64, 95, 119, 123, 180, then jump to 0, then 11, 34 ### **Stepwise:** - 50 → 62 = 12 - 62 → 64 = 2 - 64 → 95 = 31 - 95 → 119 = 24 - 119 → 123 = 4 - 123 → 180 = 57 - 180 → 199 = 19 (go to end) - 199 → 0 = 199 (jump to start) - 0 → 11 = 11 - 11 → 34 = 23 **Total = 12 + 2 + 31 + 24 + 4 + 57 + 19 + 199 + 11 + 23 = 382** --- ## **Summary Table** | Algorithm | Total Head Movement | |-----------|--------------------| | FCFS | **644** | | SSTF | **236** | | SCAN | **299** | | C-SCAN | **382** | ### **Which is best?** - **SSTF** results in the **least head movement (236)**, hence the **faster seek time**. --- ## **Final Answers:** ### a) FCFS: **644** ### b) SSTF: **236** ### c) SCAN: **299** ### d) C-SCAN: **382** ### **SSTF gives the least head movement and fastest seek time.**