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Command Processing
110 C141-C013
2.3 Command Queuing Function
The HDD is equipped with the command queuing function. Through command
queuing, the HDD can receive multiple commands in advance and can execute
them.
Through the tagged queuing function, the HDD can receive multiple commands
from the same INIT or from different INITs until the command queue is full. The
number of commands that can be received by the HDD is 128 per HDD, without
relation to the INIT. When the HDD receives a new command, if the command
queue is full, the new command is responded to with the TASK SET FULL status.
The HDD controls the command queue, but it is possible for the INIT to delete
commands from the queue by adding a command.
When the HDD adds a command to the queue, an INIT can specify the TASK
ATTRIBUTE of the CMND frame to give the HDD instructions about the
execution order of commands or the command to be executed next. There are
three TASK ATTRIBUTEs: ORDERED, HEAD OF QUEUE, and SIMPLE.
When ORDERED is specified, the HDD executes commands in the order they are
received. All the commands received with SIMPLE specified before a command
received with ORDERED specified are, regardless of the INIT, executed before
the command received with ORDERED specified. All the commands received
with SIMPLE specified after a command received with ORDERED specified are
executed after the command received with ORDERED specified.
Commands received with a HEAD OF QUEUE are registered at the top of the
queue for waiting execution. The HDD does not interrupt the current command
execution and executes them after completion of current command execution.
When commands with the HEAD OF QUEUE are received continuously, the
HDD first executes the command which was received last.
If "0000" or "0001" is specified by the Queue algorithm modifier in the control
mode page, the HDD performs reordering processing of commands with respect to
commands issued together with the SIMPLE. Reordering processing is performed
with the purpose of reducing the total processing time in command processing.
• Conditions for Reordering:
If commands which are objects of reordering are issued by an INIT which
permits reordering together with a SIMPLE.
• Commands which are Objects of Reordering:
READ, READ EXTENDED, WRITE, WRITE EXTENDED
If the INIT permits command reordering processing, the HDD changes the
command processing order. It is necessary for the INIT to control concerning the
validity of data.