micro computers architecture

1. DISKS

2. INTRODUCTION
• Third Most Important Component.
• System Performance ~ Disk Access Frequency
• Used Extensively for Virtual Memory.
• Disk Critical Applications.
1. Databases.
2. Graphics Oriented Progress.
3. Network Servers.
4. Program Development.
• Topics Covered in This Chapter.
1. Hard Disk Installation.
2. Drive Specifications.
3. ST-506.
4. IDE.
5. ESDI.
6. SCSI.

3. HARD DRIVE INSTALLATION
• The Two Basic Components
1. Hard Disk - (Control Operating Speed & Storage Capacity).
2. Disk Controller - (Controls Speed of Data Transfer).
• Special Purpose ROM's 8-bit ISA and 16-bit EISA/MCA
Buses.
• INTERFACE - Connection Between Two Pieces of
Electronic Equipment.
• Types of Interfaces.
1. ST - 506/ST - 412
2. SCSI (Small Computer System Interface).
3. IDE (Interface Design Enhancement).
• ST - 506 & IDE Are for ISA Bus ISA Based Systems

4. DRIVE SPECIFICATIONS
• Magnetic Disk Spinning at High Speeds.
• Read/write Heads Moving in & Out.
• Vacuum Controlled Operation.
• Tracks, Cylinders & Sectors
• Access Time Is
1. Seek Time.
2. Search Time.
3. Data Transfer Time.
• The Average Latency.
• Three Important Factors
1. Cylinder Sized.
2. Avg. Access Time.
3. Avg. Latency.

5. ST - 506 (NOW OBSOLETE)
• The MFM & RLL
• Characteristics
1. Introduced With Original IBM PCs & ATs.
2. Very Low Capacity (20 - 80 Mb).
• Difference Between MFM & RLL - Data Coding MFM >>
0.75 Flux Reversals/bit.
RLL >> 0.25 Flux Reversals/bit
• Data Transfer Rates.
MFM >> 0.6 M Bytes/sec.
RLL >> 0.9 M Bytes/sec.
• The St - 506 Interface.

6. IDE
– Drive Characteristics
1. Fast
2. Large capacities (upto 500MB
3. Easy to install
4. Only two drives with one ISA Bus
5. Cost affective
– Controller integrated into the drive
– Data transfer rate improvement
Theoretical >>6.5MB/S
Practical >> 1.2MB/S
– Requires mentioning in BIOS setup
– May use IDE card in absence of IDE connector

7. ESDI
– Drive Characteristics
1. High speed
2. Very large capacity (500MB & Beyond)
3. ISA, MCA & EISA bus compatible
4. Cabling & software similar to ST-506
5. Very High Data transfer Rate (4MB/S)
6. Requires definition in BIOS setup

8. SCSI
– Drive Characteristics
1. General Purpose interface
2. Multiple device connections
3. Large capacities (500MB & Beyond)
– May also be used for connecting tape drives, optical drives, CD-ROMs
etc.
– Standardized Bus using 50-way ribbon cable to chain-connect devices
– Two types of external connectors
1. 50-pin connector by Hewlett packard
2. 25-pin connector by apple
– Different Imlementation methods
1. SCSI 1
2. Fast SCSI
3. Wide SCSI
4. SCSI 2

9. SCSI.....Continued
– Each device identification by
1. ID number
2. Logical Unit numbers
– Difficult detection of SCSI drive by BIOS
– SCSI ROM BIOS on controller
– Low level formatting & advanced diagmostic programs do
not sure
– Advantage : Single controller for many devices
– Plug & play approach
– May have 5-10MB/S data transfer rate in future.

10. ISSUES IN DRIVE SELECTION
• Sector Interleave
• Track Buffer
• Caching Controllers (Elevator Algorithms)
• Disk Mirroring
• Software Caching (Smart Disk)
• RAM Disk
• Disk Fragmentation