scsi command

8/7/2019 scsi command

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SCSI Command Protocol

Communication Interface

When two devices are connected across a bus or a network, their

interaction is accomplished through a communication interface (forexample, a parallel SCSI bus, a Fibre Channel arbitrated loop, or

an Ethernet network).

The communication interface allows multiple devices to share

connections, yet operate and exchange data independently.

The communication interface is comprised of the physical interfaceand the signaling protocol used during communication.


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The physical interface determines the number of devices that can

be attached to a bus or network loop, the maximum length of thecables, and the physical characteristics of the cable itself (for

example, the number of wires, shielding, and so forth).

The signaling protocol defines the electrical characteristics and

timing of signals carried by the cable, the message system

requirements, transmission speeds and maximum data transfer rates,

as well as the encoding and decoding of the individual bit patternsrepresenting commands passing between the individual devices


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Command Protocol

The format and content of the information carried over the

communication interface, as well as how each device uses and

responds to the information, is governed by a command protocol.

The command protocol determines how the host (or initiator)

interacts with the target device (for example, the tape drive) by

issuing commands, transferring data, and responding to status

information.

The command protocol also defines the individual bits in the

command data passing between the individual devices. The targetdevice responds to commands from the host by performing the

requested operation (for example, writing or reading data on

magnetic tape) and returning status information to the host.


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The Command Descriptor Block (CDB)

The standard defines the SCSI commands that are common to

more than one device model.


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CDB usage and structure

A command is communicated by sending a command descriptor

block (CDB) to the device server. For several commands, the CDB

is accompanied by a list of parameters in the Data-Out Buffer.

There are fixed length CDB formats and variable length CDB formats

If device receives CDB containing an operation code that is invalid

or not supported, it shall return CHECK CONDITION status with

the sense key set to ILLEGAL REQUEST and an additional sense

code of INVALID COMMAND OPERATION CODE.


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All fixed length CDBs shall have an OPERATION CODE field astheir first byte and a CONTROL byte as their last byte.


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Typical CDB for 6-byte commands


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Operation code

The OPERATION CODE field contains the code value identifyingthe operation being requested by the CDB.

The OPERATION CODE field has a consistently defined

meaning across all commands. The standard specifies the operation

code values used by the commands defined herein.

Service action

All CDB formats except the 6-byte format provide for a SERVICEACTION field containing a coded value identifying a function to be

performed under the more general command function specified in

the OPERATION CODE field.


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Logical block address

The logical block addresses on a logical unit or within a volumepartition shall begin with block zero and be contiguous up to the last

logical block of that logical unit or within that partition.

Transfer length

The TRANSFER LENGTH field specifies the amount of data to be

transferred, usually the number of blocks.

Parameter list length

The PARAMETER LIST LENGTH field is used to specify the

number of bytes sent from the Data-Out Buffer.


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Allocation length

The ALLOCATION LENGTH field specifies the maximum numberof bytes that an application client has allocated for returned data.

Control

The CONTROL field has a consistently defined meaning across

all commands.


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7 6 5 4 3 2 1 0

Group Code Command Code

Group Code Command Description

0 6 Byte Commands

1 10 Byte Commands

2 10 Byte Commands

3-4 Reserved5 12 Byte commands

6-7 Vendor Specific

SCSI CDB: Byte 0


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7 6 5 4 3 2 1 0

Vendor

UniqueReserved Flag Link

Flag

Indicates Target should return Linked CCw/Flag Msg

Link

Used to continue I/O process across multiple commands

SCSI CDB: Control byte


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Summary of commands implemented by all SCSI devices

This standard defines three commands that all SCSI devices shall

implement - INQUIRY, REQUEST SENSE, and TEST UNIT

READY.

These commands are used to configure the system, to test devices,and to return important information concerning errors and exception

conditions.


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Using the INQUIRY command

The INQUIRY command may be used by an application client to

determine the configuration of the logical unit.

Device servers respond with information that includes their type

and standard version and may include the vendor's identification,model number and other information.


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5

4

3

2

Logical Unit Number1

Operation Code(12h)0

01234567Byte

Bit

Reserved

Reserved

Reserved

Allocation Length : 47 (2Fh)

Reserved EVPD (0)

INQUIRY


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Using the REQUEST SENSE command

Whenever a command completes with a CHECK CONDITION

status and auto-sense data is not provided, the application client that

received the error status should issue a REQUEST SENSE command

to receive the sense data describing the cause of the condition.


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REQUEST SENSE (03H)

5

4

3

2



01234567Byte

Bit

Reserved

Reserved

Reserved

Allocation Length (12h)

Reserved


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SENSE DATA FORMAT

54

3

2

10

01234567Byte

Bit

Sense KeyReserve

Segment number (00h)

12

11

1098

6

7

Valid Error code ( 70h / 71h )

FileMK ILIEOM

18 ~ N17

1615

1413

Information byte (MSB)

Information byte

Information byte

Information byte (LSB)

Additional Sense Data Length

Command-Specification Information (MSB)Command-Specification Information

Command-Specification Information

Command-Specification Information (LSB)

Additional Sense Codes

Additional Sense Code QualifierField Replaceable Unit Code

SKSV Sense-Key Specific (MSB)

Sense-Key Specific

Sense-Key Specific (LSB)

Additional Sense Bytes


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Using the TEST UNIT READY command

The TEST UNIT READY command allows an application client to

poll a logical unit until it is ready without the need to allocate space

for returned data.

The TEST UNIT READY command may be used to check themedia status of logical units with removable media. Device servers

should respond promptly to indicate the current status of the

SCSI device, delays to achieve GOOD status may adversely affectinitiator performance.


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TEST UNIT READY (00H)

5

4

3

2



01234567Byte

Bit

Reserved

Reserved

Reserved

Reserved

Reserved


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Command for all Device Types

Z5EhPERSISTENT RESERVE IN

ZA7hMOVE MEDIUM ATTACHED aZ5AhMODE SENSE(10)

Z1AhMODE SENSE(6)

Z55hMODE SELECT(10)

Z15hMODE SELECT(6)

O4DhLOG SENSE

O4ChLOG SELECT

M12hINQUIRYO83hEXTENDED COPY

TypeOperation

code

Command name


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Z16hRESERVE(6)

Z1DhSEND DIAGNOSTIC

OA4h/06h bSET DEVICE IDENTIFIER

M00hTEST UNIT READY

Z3BhWRITE BUFFER

Z03hREQUEST SENSEZ56hRESERVE(10)

TypeOperation

code

Command name

Key: M = Command implementation is mandatory

O = Command implementation is optional

X = Command implementation is defined in a previous

standard

Z = Command implementation is device type specific.


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a The MOVE MEDIUM ATTACHED and READ ELEMENT STATUS

ATTACHED operation codes shown here should be used by devices withattached medium changers.b This command is defined by a combination of operation code and service

action. The operation code value is shown preceding the slash and the service

action value is shown after the slash.


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Force Reserve [0xE4]

- Force Reservation, overriding normal SCSI protocol

Format Unit [0x04]

- Formats disk, according to Mode Parameters

Prefetch [0x34]- Read disk, place data into cache

Read (6,10) [0x08, 0x28]

- Read disk, transfer data to Initiator

Read Capacity [0x25]

- Returns size of disk, in # LBAs

SCSI Disk Commands


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Reassign Blocks [0x07]

- Remaps specified LBA(s)

Reserve, Release [0x16, 0x17]

- Locks Disk for exclusive access by this Initiator

Rezero Unit [0x01]- Seeks to LBA 0

Start/Stop Unit [0x1B]

- Command to spin up/down disk

Synchronize Cache [0x35]

- Flush cache to disk, cache remains enabled

SCSI Disk Commands


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Verify [0x2F]

- Verify LBA(s) on disk with passed data

Write (6,10) [0x0A, 0x2A]

- Write specified LBA(s) with passed data

Write and Verify [0x2E]- Treated the same as a Write command

Write Same [0x41]

- Writes passed sector to multiple LBA(s) on disk

SCSI Disk Commands


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Status Codes

TASK COMPLETETASK ABORTED40h

TASK COMPLETEACA ACTIVE30h

TASK COMPLETETASK SET FULL28h

-Obsolete22h

TASK COMPLETERESERVATION CONFLICT18h

LINKED COMMAND COMPLETEINTERMEDIATE-CONDITIONMET

14h

LINKED COMMAND COMPLETEINTERMEDIATE10h

TASK COMPLETEBUSY08h

TASK COMPLETECONDITION MET04h

TASK COMPLETECHECK CONDITION02h

TASK COMPLETEGOOD00h

Service ResponseStatusStatusCode


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Status shall be sent from the device server to the application client

whenever a command ends with a service response of TASK

COMPLETE or LINKED COMMAND COMPLETE.

GOOD. This status indicates that the device server has successfully

completed the task.

CHECK CONDITION. This status indicates that sense data has

been delivered in the buffer defined by the Sense Data argument

BUSY. This status indicates that the logical unit is busy.

TASK ABORTED. This status shall be returned when a task isaborted by another SCSI initiator port


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Bus Free

Arbitration/80 (7)

Select w/ATN/81 (7,0)

Message Out/80 (Identify)

Command/08 (Read) 00 34 22 01 00

Data In/00 01 02 03 04 05 06 07...

Status/00 (Good)Message In/00 (Command Complete)

Bus Free

Sample SCSI Command


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Bus Free

Arbitration/80 (7)


Message Out/C0 (Identify)

Command/08 (Read) 00 34 22 01 00

Message In/04 (Disconnect)

Bus FreeArbitration/01 (0)

Reselection/81 (7,0)

Message In/80 (Identify)

Data In/00 01 02 03 04 05 06 07...Status/00 (Good)

Message In/00 (Command Complete)

Bus Free

Sample SCSI Command with Disconnect


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Bus Free

Arbitration/80 (7)


Message Out/C0 (Identify)Command/08 (Read) 00 34 22 01 00

Message In/04 (Disconnect)

Bus Free

Arbitration/01 (0)

Reselection/81 (7,0)

Message In/80 (Identify)

Data In/00 01 02 03 04* 05+ 06+ 07+...

Message Out/05 (Initiator Detected Error)

Message In/03 (Restore Pointers)Data In/00 01 02 03 04 05 06 07...

Status/00 (Good)

Message In/00 (Command Complete)

Bus Free

Sample SCSI Command with Parity Error


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Bus FreeArbitration/80 (7)Select w/ATN/81 (7,0)Message Out w/ATN /C0 (Identify)Message Out w/ATN /20 (Simple Tag)Message Out/00 (Tag ID)Command/08 (Read) 00 34 22 01 00Message In/04 (Disconnect)

Bus FreeArbitration/01 (0)Reselection/81 (7,0)Message In/80 (Identify)Message In/20 (Simple Tag)

Message In/00 (Tag ID)Data In/00 01 02 03 04 05 06 07...Status/00 (Good)Message In/00 (Command Complete)

Bus Free

Sample SCSI Queued Command


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Bus Free

Arbitration/80 (7)


Message Out/80 (Identify)

Command/08 (Read) 00 34 22 01 01

Data In/00 01 02 03 04 05 06 07...

Status/10 (Intermediate/Good)Message In/0A (Linked Command Complete)

Command/08 (Read) 03 22 68 01 00

Data In/00 01 02 03 04 05 06 07...

Status/00 (Good)Message In/00 (Command Complete)

Bus Free

Sample SCSI Linked Command


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A special data packet specifying error status and location(s)

Separate copy maintained by Target for each Initiator

Retrieved by Request Sense command

Must be retrieved immediately after a CHECK CONDITION,

or sense data is lost

All queued commands are suspended after CHECK CONDITION,until Sense Data is retrieved (Contingent Allegiance)

SCSI Sense Data


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0 Valid Error Code (0x70 or 0x71)

1 Segment Number

2 FM EOM ILI (R) Sense Key3-6 Information Bytes

7 Additional Sense Length

8-11 Command Specific Information12 Additional Sense Code

13 Additional Sense Code Qualifier

14 Field Replaceable Unit Code15-17 Sense Key Specific

18-xx Additional Sense Bytes

SCSI Sense Data Format


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0x0: No Sense 0x8: Blank Check

0x1: Recovered Error 0x9: Vendor Specific

0x2: Not Ready 0xA: Copy Aborted

0x3: Medium Error 0xB: Aborted Command0x4: Hardware Error 0xC: Equal

0x5: Illegal Request 0xD: Volume Overflow

0x6: Unit Attention 0xE: Miscompare

0x7: Data Protect 0xF: Reserved

Sense Data Shorthand: SKey/ASC/ASCQ

3/11/01: Medium Error, Read Retries Exhausted5/20/00: Illegal Command, Invalid Operation Code

D/47/00: Aborted Command, SCSI Parity Error

SCSI Sense Keys


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First sense packet after any of the following:

- Power-on

- Reset

- Mode/Inquiry parameters changed

- Runtime firmware change

- Commands cleared by another initiator

Reports to initiator that operating mode of the Target has changed

from the last command sent to the Target.

Reported on all SCSI Commands except Inquiry.

Command it is reported with is not executed.

Unit Attention Condition

scsi command

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