TABLE OF CONTENTS The Command layer protocol defines the sequences of Transport layer actions that are performed in order to execute ATA commands. Because the command protocol is defined in the existing ATA standards and the Serial ATA interface provides compatibility with ATA, the Serial ATA Command layer definition is largely a restatement of the existing ATA command protocol using Serial ATA terminology and equivalent transactions in place of their parallel ATA counterparts. The Command layer protocol for the host is not even defined in the Serial ATA specification, because the requirements of software compatibility already describe the behavior of the host completely. The Command layer is therefore described in terms of the actions taken by the device when executing the various classes of commands. Thus, the Command layer is the only layer in the specification that is defined only for the device end of the interface, whereas all other layers are defined for both the device and the host. Figure 13.1 indicates the layer of the architecture covered in this chapter.
The parallel ATA standard defines 13 command protocols according to a set of defined command classes or categories. This classification avoids having to define the command protocol for each and every command that appears in the standard, which contains over 65 commands. The Serial ATA Command layer protocol definition follows the parallel ATA example and essentially defines the Serial ATA equivalents for each of the ATA command protocols.
The following 13 command protocols...
