sys/i386/doc/ata/ata-3

3.  General Description

The application environment for the AT Attachment is any computer which uses
an AT Bus or 40-pin ATA interface.

The AT Bus is a widely used and implemented interface for which a variety of
peripherals have been manufactured. As a means of reducing size and cost, a
class of products has emerged which embed the controller functionality in the
drive. These new products utilize the AT Bus fixed disk interface protocol,
and a subset of the AT bus. Because of their compatibility with existing AT
hardware and software this interface quickly became a de facto industry
standard.

The purpose of the ATA standard is to define the de facto implementations.

Software in the Operating System dispatches I/O (Input/Output) requests via
the AT Bus to peripherals which respond to direct commands.

3.1  Structure

This standard relies upon specifications of the mechanical and electrical
characteristics of the AT Bus and a subset of the AT Bus specifically
developed for the direct attachment of peripherals.

Also defined are the methods by which commands are directed to peripherals,
the contents of registers and the method of data transfers.