SCSI command

In SCSI computer storage, computers and storage devices use a client-server model of communication. The computer is a client which requests the storage device to perform a service, e.g., to read or write data. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with Fibre Channel, iSCSI, Serial Attached SCSI, and other transport layers.

In the SCSI protocol, the initiator sends a SCSI command information unit to the target device. Data information units may then be transferred between the computer and device. Finally, the device sends a response information unit to the computer.

SCSI commands are sent in a command descriptor block (CDB), which consists of a one byte operation code (opcode) followed by five or more bytes containing command-specific parameters. Upon receiving and processing the CDB the device will return a status code byte and other information.

The rest of this article contains a list of SCSI commands, sortable in opcode or description alphabetical order. In the published SCSI standards, commands are designated as "mandatory," "optional" or "vendor-unique." Only the mandatory commands are required of all devices. There are links to detailed descriptions for the more common SCSI commands. Some opcodes produce different, though usually comparable, effects in different device types; for example, opcode 0x01 recalibrates a disk drive by seeking back to physical sector zero, but rewinds the medium in a tape drive.

SCSI command lengths

Originally the most significant 3 bits of a SCSI opcode specified the length of the CDB. However, when variable-length CDBs were created this correspondence was changed, and the entire opcode must be examined to determine the CDB length. The lengths are as follows:

Opcode (hex )	Length
00h to 1Fh	6
20h to 5Fh	10
60h to 7Dh	reserved
7Eh	variable (more than 16 bytes)
7Fh	extended (variable length; may contain one or more CDBs)
80h to 9Fh	16
A0h to BFh	12
C0h to FFh	vendor specific

List of SCSI commands

When a command is defined in multiple CDB sizes, the length of the CDB is given in parentheses after the command name, e.g., READ(6) and READ(10).

Code	Command
00	TEST UNIT READY
01	REWIND
03	REQUEST SENSE
04	FORMAT
05	READ BLOCK LIMITS
07	REASSIGN BLOCKS
07	INITIALIZE ELEMENT STATUS
08	READ(6)
0A	WRITE(6)
0B	SEEK(6)
0F	READ REVERSE(6)
10	WRITE FILEMARKS(6)
11	SPACE(6)
12	INQUIRY
13	VERIFY(6)
14	RECOVER BUFFERED DATA
15	MODE SELECT(6)
16	RESERVE(6)
17	RELEASE(6)
18	COPY
19	ERASE (6)
1A	MODE SENSE (6)
1B	START STOP UNIT
1B	LOAD UNLOAD
1C	RECEIVE DIAGNOSTIC RESULTS
1D	SEND DIAGNOSTIC
1E	PREVENT ALLOW MEDIUM REMOVAL
23	READ FORMAT CAPACITIES
25	READ CAPACITY(10)
28	READ(10)
29	READ GENERATION
2A	WRITE(10)
2B	SEEK(10)
2B	LOCATE(10)
2C	ERASE(10)
2D	READ UPDATED BLOCK
2E	WRITE AND VERIFY(10)
2F	VERIFY(10)
33	SET LIMITS(10)
34	PRE-FETCH(10)
34	READ POSITION
35	SYNCHRONIZE CACHE(10)
36	LOCK UNLOCK CACHE(10)
37	READ DEFECT DATA(10)
37	INITIALIZE ELEMENT STATUS WITH RANGE
38	MEDIUM SCAN
39	COMPARE
3A	COPY AND VERIFY
3B	WRITE BUFFER
3C	READ BUFFER
3D	UPDATE BLOCK
3E	READ LONG(10)
3F	WRITE LONG(10)
40	CHANGE DEFINITION
41	WRITE SAME(10)
42	UNMAP
43	READ TOC/PMA/ATIP
44	REPORT DENSITY SUPPORT
45	PLAY AUDIO(10)
46	GET CONFIGURATION
47	PLAY AUDIO MSF
48	SANITIZE
4A	GET EVENT STATUS NOTIFICATION
4B	PAUSE/RESUME
4C	LOG SELECT
4D	LOG SENSE
50	XDWRITE(10)
51	XPWRITE(10)
51	READ DISC INFORMATION
52	XDREAD(10)
53	XDWRITEREAD(10)
54	SEND OPC INFORMATION
55	MODE SELECT(10)
56	RESERVE(10)
57	RELEASE(10)
58	REPAIR TRACK
5A	MODE SENSE(10)
5B	CLOSE TRACK/SESSION
5C	READ BUFFER CAPACITY
5D	SEND CUE SHEET
5E	PERSISTENT RESERVE IN
5F	PERSISTENT RESERVE OUT
7E	extended CDB
7F	variable length CDB
80	XDWRITE EXTENDED(16)
80	WRITE FILEMARKS(16)
81	READ REVERSE(16)
83	Third-party Copy OUT commands
84	Third-party Copy IN commands
85	ATA PASS-THROUGH(16)
86	ACCESS CONTROL IN
87	ACCESS CONTROL OUT
88	READ(16)
89	COMPARE AND WRITE
8A	WRITE(16)
8B	ORWRITE
8C	READ ATTRIBUTE
8D	WRITE ATTRIBUTE
8E	WRITE AND VERIFY(16)
8F	VERIFY(16)
90	PRE-FETCH(16)
91	SYNCHRONIZE CACHE(16)
91	SPACE(16)
92	LOCK UNLOCK CACHE(16)
92	LOCATE(16)
93	WRITE SAME(16)
93	ERASE(16)
9D	SERVICE ACTION BIDIRECTIONAL
9E	SERVICE ACTION IN(16)
9F	SERVICE ACTION OUT(16)
A0	REPORT LUNS
A1	ATA PASS-THROUGH(12)
A2	SECURITY PROTOCOL IN
A3	MAINTENANCE IN
A4	MAINTENANCE OUT
A4	REPORT KEY
A5	MOVE MEDIUM
A5	PLAY AUDIO 12
A6	EXCHANGE MEDIUM
A7	MOVE MEDIUM ATTACHED
A8	READ(12)
A9	SERVICE ACTION OUT(12)
AA	WRITE(12)
AB	SERVICE ACTION IN(12)
AC	ERASE(12)
AD	READ DVD STRUCTURE
AE	WRITE AND VERIFY(12)
AF	VERIFY(12)
B0	SEARCH DATA HIGH(12)
B1	SEARCH DATA EQUAL(12)
B2	SEARCH DATA LOW(12)
B3	SET LIMITS(12)
B4	READ ELEMENT STATUS ATTACHED
B5	SECURITY PROTOCOL OUT
B6	SEND VOLUME TAG
B7	READ DEFECT DATA(12)
B8	READ ELEMENT STATUS
B9	READ CD MSF
BA	REDUNDANCY GROUP (IN)
BB	REDUNDANCY GROUP (OUT)
BC	SPARE (IN)
BD	SPARE (OUT)
BE	VOLUME SET (IN)
BF	VOLUME SET (OUT)

External links

• Summary of SCSI command operation codes
• "SCSI Commands Reference Manual" (PDF). Seagate.