1. How is a disk Physically Organised ?
2. Give one name for Logical Unit of Transfer, how is it made up ?
3. A disk has 768 cylinders, 8 platters, 512 sectors per track and 512 bytes per sector. It rotates at 7200rpm, its average seek time is 9ms and there are 16 sectors per cluster.
   1. What is its capacity in Gb
   2. How long would it take to read a cluster on average
4. A video card is advertised as having a resolution of 1792 x 1280 and 16 million colours.
   1. What does the 1792 x 1280 refer to ?
   2. How much video RAM would you expect to have with it ?
5. Another disk has 256 tracks, 8 heads, 512b/sector and 128 sectors per track. It has an average seek time of 12ms and spins at 4800rpm. It has an 8 sector cluster.
   1. What is its capacity ?
   2. How long would it take to read a cluster on average
6. How much video RAM is required for a 640 x 480 x 65536 colour display ?

Answers

1

Disk is organised as follows:

• Platters containing two Surfaces supporting one Head each side
• Concentric Tracks on each Platter Surface
• The same Track on all Platters is called a Cylinder
• Each Track is made up of Sectors
• Sectors are made up of Bytes
• Sectors are grouped together and accessed as Clusters
• A Cluster is the minimum Allocation Unit

2

A Logical Unit of Transfer is called a Cluster and is comprised of a number of Sectors (2^n). A Cluster is the minimum Allocation Unit of a disk.

3

768 cylinders = 768 tracks
8 platters = 16 heads (2 per platter, one each side)
512 sectors per track
512 bytes per sector
16 sectors per cluster
9ms seek
7200rpm

a)
Size can be calulated from: (Tracks x Heads x Sectors x Bytes) / Gbytes:

Size = (768 x 8 x 2 x 512 x 512) / (1024 x 1024 x 1024)
Size = (1 x 24 x 1 x 1) / (2 x 2 x 2)
Size = 3Gb

b)
512 Sectors per Track
16 Sectors per Cluster
Therefore 32 Clusters per Track (per Revolution!)
Spin Speed 7200rpm = 60/7200 seconds per rev
You are not required to cancel right down to the lowest form. This is quite good enough.

Time = (60 x 1000 x 16) / (7200 x 512)
Time = 100/384ms
Time = 25/96 milliseconds(3.84ms)
R = L + S + T , Where R = Read, L = Latency, S = Seek, T = Transfer
= 25/6 + 9/1 + 25/96 = 13.4 ms

4

The 1792 is the number of Horizoltal Pixels across the display.
The 1280 is the number of Vertical Pixels across the display

To create an image with 1792 x 1280 x 16 million colours will require 24 bits (16 million is 16(4 bits) x 1024(10 bits) x 1024(10 bits) = 24(bits)) x 1792 x 1280 divided by 8 bits to get back into bytes, divided by 1024 x 1024 to get into Mbytes.

(1792 x 1280 x 24) / 8 = 105/16Mb (6.5625Mb)
Just over 6.5Mb

5

256 Tracks
8 Heads
128 Sectors per Track
512 Bytes per Sector
a)
Size can be calulated from: (Tracks x Heads x Sectors x Bytes) / Mbytes:

Size = (256 x 8 x 128 x 512) / (1024 x 1024) = 128
Size = 128Mb

b)
8 Sectors per Cluster
4800rpm
Read = Latency + Transfer + Seek
Latency = 1/2 revolution = (60 x 1000) / (4800 x 2) ms
Transfer = (60 x 8 x 1000) / (4800 x 128) ms
Seek = 12 ms Therefore = (25 + 384 + 200) / 32 = 19.03 ms
Read a Cluster = 19 ms

6

640 x 480 x 65536 colours
=(640 x 480 x 16(bits))/8
RAM = 600kbytes

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