IDE and Mass Storage - Computer Busses - page 137

6.5 Drive specifications

Access time is the time taken for a disk to locate data. Typical access times for modern disk
drives range from 10 to 30 ms. The average access time is the time for the head to travel half
way across the platters. Once the head has located the correct sector then there may be an-
other wait until it locates the start of the data within the sector. If it is positioned at a point
after the start of the data, it requires another rotation of the disk to locate the data. This aver-
age wait, or latency time, is usually taken as half of a revolution of the disk. If the disk spins
at 3600 rpm then the latency is 8.33 ms.
The main parameters which affect the drive specification are the data transfer rate and the
average access time. The transfer rate is dependent upon the interface for the controller/disk
drive and system/controller and the access time is dependent upon the disk design.

6.6 Hard disk and CD-ROM interfaces

There are two main interfaces involved with a hard disks (and CD-ROMs). One connects the
disk controller to the system (system-controller interface) and the other connects the disk
controller to the disk drive (disk-controller interface).
The controller can be interfaced by standards such as ISA, EISA, MCA, VL-Local bus or
PCI bus. For the interface between the disk drive and the controller then standards such as
ST-506, ESDI, SCSI or IDE can be used. ST-506 was developed by Seagate Technologies
and is used in many older machines with hard disks of a capacity less than 40 MB. The en-
hanced small disk interface (ESDI) is capable of transferring data between itself and the
processor at rates approaching 10 MB/s.
The small computer system interface (SCSI) allows up to seven different disk drives or
other interfaces to be connected to the system through the same interface controller. SCSI is
a common interface for large capacity disk drives and is illustrated in Figure 6.3.
The most popular type of PC disk interface is the integrated drive electronics (IDE) stan-
dard. It has the advantage of incorporating the disk controller in the disk drive, and attaches
directly to the motherboard through an interface cable. This cable allows many disk drives to
be connected to a system without worrying about bus or controller conflicts. The IDE inter-
face is also capable of driving other I/O devices besides a hard disk. It also normally contains
at least 32K of disk cache memory. Common access times for an IDE are often less than 16
ms, where as access times for a floppy disk are about 200 ms. With a good disk cache system
the access time can reduce to less than 1 ms. A comparison of the maximum data rates is
given in Table 6.2.

Table 6.2 Capacity of different disk types

Interface Maximum data rate (MB/s) Interface Maximum data rate (MB/s)

ST-506 0.6 E-IDE 16.6

ESDI 1.25 SCSI 4 .0

IDE 8.3 SCSI-II 10.0