External Memory Systems

1
External Memory Systems

• Lecture 16
• Ref Ch 6

2
Outline

• Magnetic Disk
• Characteristics
• Floppy Disk
• Winchester Disk
• Multiple Platter

3
Disk Data Layout
4
Disk Layout
5
ST506 format
Gap1
Gap1
Id
Gap2
Data
Gap3
Id
Gap2
Data
Gap3
Sync Byte
Track
Sync Byte
Head
Sector
CRC
Data
CRC
6
Fixed and Movable Heads
7
Multiple Platter

• One head per side
• Heads are joined and aligned
• Aligned tracks on each platter form cylinders
• Data is striped by cylinder
• reduces head movement
• Increases speed (transfer rate)

8
Multiple Platter
9
Cylinders
10
Floppy Disk

• 8, 5.25, 3.5
• Small capacity
• Up to 1.44Mbyte (2.88M never popular)
• Slow
• Universal
• Cheap

11
Performance (Speed)

• Seek time
• Moving head to correct track
• (Rotational) latency
• Waiting for data (sector) to rotate under head
• Access time Seek Latency
• Transfer time

12
Performance