ANSI/INCITS TR 38:2007 pdf download.INCITS Technical Report for Information Technology一 SMART Command Transport (SCT).
4 Command Transport
There are several phases Involved in the Issuing and executing of SCT commands. These phases are:
1. Capability Identification
2. Command Transport
3. Data Transfer
4. Status
Capability Identification is performed by issuing IDENTIFY DEVICE and checking to make sure the command is supported. Command Transport occurs when a 51 2-byte data packet is created and then written to SMART or extended log page EOh. The 512•byte data packet contains a srngle command as documented later in this report. lithe command was a data transfer command then reading or writing log page Elh can transfer the data. If the data is larger than a single log page (larger than 255 sectors), the page is read or written multiple times. Finally. command status can be read at anytime by reading log page EOh. It the command involves data transfer, the host will need to check status betore data is transferred to ensure that the &ive is ready, as well as when the command is complete. to confirm that the data was transferred successfully, When the command is complete, the host may need to check status a third time to determine if the command succeeded, failed, or partially succeeded.
SCT commands are executed hd*e regular ATA commands, therefore they take precedence over any background function (such as SMART EXECUTE OFFLINE IMMEDIATE) the device may be performing when the SCT command is issued. Some SCT commands indicate ATA command completion and return ATA status while the SCT command is still executing.
4.1 Capability Identification
The ATA IDENTIFY DEVICE command is used to determine if SCT is enabled and which SCT Action Codes are supported.
For commands that access the mea, the drive will auto advance the sector poanter by the number of sectors transferred, and report in the LBA Mid and LBA High registers the number of sectors remainang to be transferred (if both registers are zero then the command is done, proceed to Step 4). In other words, the host has complete control over the number of sectors to transfer at a time. Note, it the number of sectors to be transferred is greater or equal to FFFFh, the drive will post FFFFH in the LBA Mid and High registers and the value will remain at FFFFh until the number of sectors remairwng drops below FFFFh. The exact number to be transferred is reported by the SCT Status command. Upon receiving the last block of data, the drive mll perform the specified operation. In the case of very large amounts of data, such as LBA Segment Access, some data may be processed fwritteil to the disk) prior to receiving all of the data from the host.
4.5.1.4 Step 4 – Final StatusJSCT Command Completion
Read the SCT status response (see Table 9, Table 10, and Table 11) to determine how the command complefed. If the command has not completed (by reporting FFFFh in Table 11 byte 14) then wait f or some period of time and repeat Step 4 until the command is complete.
For SCT commands that require transfer of data to the drive (such as a write command). typically the command is not complete until the last block of data has been transferred to the drive.
4.6 DrIve AddressIng Methods
Standard ATA commands employ either LBA or Logical CHS addressing using both 28-bit and 48-bit capability. SCT commands only support 48•bit addressing.
4.6.1 LogIcal Block Address
For LBA access, all user sectors on the drive are numbered in a one-cmensional sequence from 0 to the maximum number of user sectors minus one. ATA Commands support 28-bits of LBA addressing and ATA Extended commands support 48-bits of LBA addressing. All SCT commands support 48-bits of IBA address. In this method, all detective cylinders, heads and sectors are mapped out by defect management, rendering them inaccessible.
4.7 SCT Command Nesting and intermingling with Standard commands
In general, standard ATA commands can be intermingled with SCT Commands but SCT commands cannot be nested. SCT commands that do not require a follow•on data transfer operation never have an issue with being intermixed with any ATA commands or each other. SCT commands that do require data transfer, on the other hand. may not be nested; that is, if a key command that requires a data transfer is issued, all data transfer – to or from the host must complete before another SCT command is issued. In most cases, however, ATA read/write commands may be inserted in between SCT data transfers, that is, between complete SMART Read Log/Write Log commands. Furthermore, any reset (power-on, software or hardware) will cause the SCT command to be aborted.
4.8 Resets
If an SCT command is executing, any reset including Soft Reset (SRST), Hard Reset, COMRESET, and Power- On Reset (PaR) all cause the command to be terminated. This could result in partial command execution or data loss. There Is no indication once the drive becomes ready that the previous command was terminated.
POR and COMRESET clear the SCT Status Response fields (Extended Status Cede, Action Code, and Function Code), all other resets preserve the SCT Status Response fields except extended status cede which is cleared to zero.
