FCFS Scheduling Algorithm

Overview

• FCFS stands for First Come First Serve
• It is a scheduling algorithm
• Jobs are executed in the order they arrive

Example Problem

• Context: 200 cylinders (0 to 199)
• Disk queue requests: 98, 183, 37, 122, 14, 124, 65, 67
• Initial read/write head position: Cylinder 53
• Objective: Find total head movements

Steps to Calculate Total Head Movements

1. Initial Position: 53
2. Move to 98:
   • Head movement: (98 - 53 = 45)
3. Move to 183:
   • Head movement: (183 - 98 = 85)
4. Move to 37:
   • Head movement: (183 - 37 = 146)
5. Move to 122:
   • Head movement: (122 - 37 = 85)
6. Move to 14:
   • Head movement: (122 - 14 = 108)
7. Move to 124:
   • Head movement: (124 - 14 = 110)
8. Move to 65:
   • Head movement: (124 - 65 = 59)
9. Move to 67:
   • Head movement: (67 - 65 = 2)

Total Head Movements Calculation

• Head movements:
  • 45 (53 to 98)
  • 85 (98 to 183)
  • 146 (183 to 37)
  • 85 (37 to 122)
  • 108 (122 to 14)
  • 110 (14 to 124)
  • 59 (124 to 65)
  • 2 (65 to 67)
• Total: 640

Summary

• FCFS scheduling executes jobs in the sequence they arrive.
• For the given disk queue, 640 total head movements are required based on FCFS scheduling.