Hard drives are mechanical devices that are used by computers to hold information (data). The data is magnetically recorded on specially coated disks called platters. The platters are divided into cylinders that start from the outside of the platter and go towards the inside (imagine the platter's rim being the first cylinder, then all the other cylinders are are just smaller circles one inside the other traveling towards the center of the platter). More than one platter are usually stacked with a small spacing between them so that a mechanical read/write head can move between them. Each Cylinder is broken down into sectors which are like pie wedges going across the circular path of the cylinder.

A head is swiveled in such a way that it can move back and forth across the platter(s). The time is takes for the head to find a cylinder (track) is usually reported as the access time in milliseconds.

The rotation of the platter is measured in revolutions per minute (rpm) and a higher value usually translates to better performance. Typical speeds range from 3600 rpm to 10000 rpm.

There are several different type of interfaces that can be used including IDE, EIDE and SCSI.

IDE (Integrated Drive Electronics) is the most common hard drive standard. It is also known as ATA.

IDE and EIDE ( Enhanced IDE) interfaces come in standard, ATA, Fast ATA-2, ATA-3 Ultra DMA, ATA66.

Hard drives can and also have a built-in memory cache buffer usually between 128k and 512k bytes.

IDE or EIDE interfaces can use the Processor Input/Output (PIO) mode or the Direct Memory Access (DMA) mode which are defined in the ATA-2 specification as follows:

* ATA-2 is Fast-ATA, EIDE
* Ultra-ATA is DMA/33

* Mode 1 DMA 13.3 MB/sec
* Mode 2 DMA 16.6 MB/sec
* Mode 3 PIO 11.1 MB/sec
* Mode 4 PIO 16.6 MB/sec

IDE and EIDE style hard drives connect to the computer usually to a built-in connector on the motherboard. As many as 2 drives can be connected to a single 2 headed cable. The hard drives have to be designated as either Master or Slave. Small jumper pins on the drive are used for that purpose.

A computer can have a total of 2 IDE/EIDE hard drive cables connected. The first cable, motherboard connector, and associated hard drives are referred to as primary. The second set is called the secondary. Each of the cables has its own designation of Master and Slave. In the case of 4 drives the connections would be

1) Primary channel has a Master drive and a Slave Drive
2) Secondary channel has a Master and a Slave Drive

The IDE/EIDE ATAPI interface also supports CD-ROM drives and allows them to be mixed with hard drives on the same cable. Most CD-ROM drives are now of this EIDE type design and share the hard drive cable.

The Small Computer Systems Interface (SCSI) interface can also be used to connect computer hard drives. More information on the SCSI interface will be added shortly.

Size Limitations -

IDE drives have BIOS and INT13 combined limitations. There is a limitation at the 504MB size, the 2GB size, the 4GB and 8.4GB sizes. This issue is discussed in detail at the firmware.com internet site.

Hard Drive Connectors -

Note : In AT type computers, the floppy and hard drive cables have different twists and should not be interchanged.

Practical Tip:
MFM and RLL hard drive cables have five twisted lines. Floppy cable has seven twisted lines.

Floppy cables have the twists in the lower pin numbers.

SCSI cables can have 25 or 50 pin connectors
The bus it is attached to must be terminated at two ends. Terminators are found close to the connector slot on the SCSI device.
Terminators can a set of three resistors or on newer devices the termination might be built-in and switched ON or OFF by using switches.

IDE uses a 40-pin cable with a maximum length of 18 inches. There are several designs including one or two connectors and the connector can be placed on one side of the ribbon cable or the other. The placement of the connector on the ribbon would obviously allow the cable to travel either upward from the connected drive or downward.
The distance between the connectors on a two-connector cab can also vary between designs making it at times too short to reach 2 drives simultaneously.

The older style RLL and MFM drives use two cables to connect to the controller .
The hard drive has a thinner control cable and a wider data cable . If you are using two drives, remove the terminator that is on the drive in the middle of the chain



Hard Drive : Type of Heads

* Magneto Resistive
* Thin Film

Placement of recording sectors

* Standard Density Recording
Each track has 16 sectors
* ZDR -

Method of calculating Megabyte Sizes

* Manufacturer's Ratings = 1,000,000 is one Megabyte
* Microsoft Chkdsk.exe = 1,048,576 is one Megabyte

Programs that allow you to break the 540MB hard drive limit on older computers

* Micro House EZ Drive 8.01 (usually comes free with Western Digital drives)
* OnTrack Disk Manager -

Method of low-level Formatting Older non-IDE Hard Drives ( Never low-level format IDE drives)

* at a DOS prompt run DEBUG
then
G=C800:5

Hard Drive Glossary & Links

* differences between MFM, RLL, IDE, EIDE, ATA, ESDI, SCSI discussion @ indiana.edu

* ATA - Advanced Technology Attachment or AT Attachment.
* ATA-66 -
* DASP (Drive Active/Slave Present)
A connection signal used by ATA-2 Hard Drives on pin 39 which helps report to the Master drive the presence of the slave drive.
* IDE - Integrated Drive Electronics
* EIDE - Enhanced Integrated Drive Electronics
* ZDR - Zone Density Recording -
A method of recording hard drive sectors, where the outer tracks have more sectors then the inside tracks.
* UDMA - UltraATA also known as UltraDMA and Bus Mastering -