Command Protocol

The commands are grouped into different classes according to the protocols followed for command execution. The command classes with their associated protocols are defined below.

Please refer to Serial ATA Revision 3.0 about each protocol.

For all commands, the host must first check if BSY=1, and should proceed no further unless and until BSY=0. For all commands, the host must also wait for RDY=1 before proceeding.

A device must maintain either BSY=1 or DRQ=1 at all times until the command is completed. The INTRQ signal is used by the device to signal most, but not all, times when the BSY bit is changed from 1 to 0 during command execution.

A command shall only be interrupted with a COMRESET or software reset. The result of writing to the Command register while BSY=1 or DRQ=1 is unpredictable and may result in data corruption. A command should only be interrupted by a reset at times when the host thinks there may be a problem, such as a device that is no longer responding.

Interrupts are cleared when the host reads the Status Register, issues a reset, or writes to the Command Register.

10.1 PIO Data In commands

These commands are:

 Device Configuration Identify

 Identify Device

 Read Buffer

 Read Log Ext

 Read Multiple

 Read Multiple Ext

 Read Sector(s)

 Read Sector(s) Ext

 Read Stream Ext

 SMART Read Attribute Values

 SMART Read Attribute Thresholds

 SMART Read Log Sector

HGST hard disk drive specifications

10.2 PIO Data Out commands

These commands are:

 Device Configuration Set

 Download Microcode

 Format Track

 Security Disable Password

 Security Erase Unit

 Security Set Password

 Security Unlock

 Set Max Set Password command

 Set Max Unlock command

 SMART Write Log Sector

 Write Buffer

 Write Log Ext

 Write Multiple

 Write Multiple Ext

 Write Multiple FUA Ext

 Write Sector(s)

 Write Sector(s) Ext

 Write Stream Ext

Execution includes the transfer of one or more 512 byte sectors of data from the host to the device. In case of 4K bytes/sector format (512 bytes/sector emulation) drives, the start LBA and the sector length of write commands (Write Multiple, Write Multiple Ext, Write Multiple FUA Ext, Write Sector(s), Write Sector(s) Ext, Write Stream Ext) should be aligned to multiple of 8 as much as possible to avoid the potential performance loss due to the internal read operation.

10.3 Non-Data commands

These commands are:

 Check Power Mode

 Configure Stream

 Device Configuration Freeze Lock

 Device Configuration Restore

 Execute Device Diagnostic

 Flush Cache

 Flush Cache Ext

 Idle

 Idle Immediate

 Initialize Device Parameters

 NOP

 Read Native Max Address

 Read Native Max Address Ext

 Read Verify Sector(s)

 Read Verify Sector(s) Ext

 Recalibrate

 Security Erase Prepare

 Security Freeze Lock

 Seek

 Set Features

 Set Max Address

 Set Max Address Ext

 Set Max Lock command

 Set Max Freeze Lock command

 Set Multiple Mode

 SMART Disable Operations

 SMART Enable/Disable Attribute Autosave

 SMART Enable Operations

 SMART Execute Off-line Data Collection

 SMART Return Status

 SMART Save Attribute Values

 SMART Enable/Disable Automatic Off-Line

 Standby

 Standby Immediate

 Write Uncorrectable Ext

Execution of these commands involves no data transfer.

10.4 DMA Data In commands and DMA Data Out commands

These commands are:

 Read DMA

 Read DMA Ext

 Read Stream DMA Ext

 Write DMA

 Write DMA Ext

 Write DMA FUA Ext

 Write Stream DMA Ext

Execution of this class of command includes the transfer of one or more blocks of data between the device and the host using DMA transfer. In case of 4K bytes/sector format (512 bytes/sector emulation) drives, the start LBA and the sector length of write commands (Write DMA, Write DMA Ext, Write DMA FUA Ext, and Write Stream DMA Ext) should be aligned to multiple of 8 as much as possible to avoid the potential performance loss due to the internal read operation.

10.5 First-party DMA commands

These commands are:

 Read FPDMA Queued

 Write FPDMA Queued

Execution of this class of commands includes command queuing and the transfer of one or more blocks of data between the device and the host. The protocol is described in the section 13.6 “Native Command Queuing” of “Serial ATA Revision 3.0”.

Host knowledge of I/O priority may be transmitted to the device as part of the command. There are two priority classes for NCQ command as high priority, the host is requesting a better quality of service for that command than the commands issued with normal priority.

The classes are forms of soft priority. The device may choose to complete a normal priority command before an outstanding high priority command, although preference shall be given to the high priority commands. The priority class is indicated in bit 7 (Priority Information) in the Sector Count register for NCQ commands (READ FPDMA QUEUED and WRITE FPDMA QUEUED). This bit can indicate either the normal priority or high priority class. If a command is marked by the host as high priority, the device shall attempt to provide better quality of service for the command. It is not required that devices process all high priority requests before satisfying normal priority requests. In case of 4K bytes/sector format (512 bytes/sector emulation) drives, the start LBA and the sector length of Write FPDMA Queued command should be aligned to multiple of 8 as much as possible to avoid the potential performance loss due to the internal read operation.

HGST hard disk drive specifications