SCSI Write Commands

There are several different SCSI Write commands defined.

Write (6)

The Write (6) command is only 6 bytes long and is a legacy from early SCSI days. Its use is restricted to 21-bit LBAs. The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 0Ah
1	Reserved			LBA
2	LBA
3	LBA
4	Transfer length
5	Control

Write (10)

The Write (10) command was introduced to cope with 32-bit LBAs. It also included four other enhancements. Disable Page Out (DPO) allows the initiator to warn the target that the data being written is unlikely to be read back soon and so is not worth keeping in the target's data cache. Force Unit Access (FUA) tells the target to immediately send the data to the media surface and to not buffer it through a cache. The EBP bit tells the target that it may skip the erase process when writing data. The RelAdr bit is used to indicate that the LBA value is relative (only used with linked commands). The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 2Ah
1	WRPROTECT			DPO	FUA	Reserved	FUA_NV	Obsolete
2–5	LBA
6	Reserved			Group Number
7–8	Transfer length
9	Control

Write (12)

The Write (12) command is similar to the Write(10) command except that it has a larger Transfer Length field. The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = AAh
1	WRPROTECT			DPO	FUA	Obsolete	FUA_NV	Obsolete
2–5	LBA
6–9	Transfer length
10	Restricted for MMC-6	Reserved		Group Number
11	Control

Write (16)

The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 8Ah
1	WRProtect			DPO	FUA	Reserved	FUA_NV	Obsolete
2–9	LBA
10-13	Transfer Length
14	Restricted for MMC-6	Reserved		Group Number
15	Control

Write (32)

The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 7Fh
1	Control
2–5	Reserved
6	Reserved			Group Number
7	Additional CDB Length (18h)
8-9	Service Action (000Bh)
10	WRProtect			DPO	FUA	Reserved	FUA_NV	Reserved
12–19	LBA
20–23	Expected Initial Logical Block Reference Tag
24–25	Expected Logical Block Application Tag
26–27	Logical Block Application Tag Mask
28–31	Transfer Length

Write Long (10)

The Write Long command allows for Error Correcting Code (ECC). The data sent from the initiator will include ECC fields as well as real data. The Byte Transfer Length field tells the target how much data will need to be returned when the data is subsequently retrieved with a Read Long command. The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 3Fh
1	COR_DIS	WR_UNCOR	PBLOCK	Reserved				Obsolete
2	LBA
3
4
5
6	Reserved
7	Byte Transfer Length
8
9	Control

Write Long (16)

The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 9Fh
1	COR_DIS	WR_UNCOR	PBLOCK	Service Action (11h)
2–9	LBA
10	Reserved
11
12	Byte Transfer Length
13
14	Reserved
15	Control