User's Guide

Page 162 DDS-4 SCSI Interface Manual, Rev. B
WRITE (0Ah)
The WRITE command transfers one or more data blocks from the Initiator to the
drive. If the Transfer Length is zero, no data is transferred, and the current position
of the tape is not changed. This condition is not considered an error.
WRITE Command Descriptor Block
The following table shows the layout of the Command Descriptor Block.
Byte
Bits
7 6 5 4 3 2 1 0
0 0 0 0 0 1 0 1 0
1 0 0 0 0 0 0 0 Fixed
2 MSB—Transfer Length
3 Transfer Length
4 Transfer Length—LSB
5 vendor unique* 0 0 0 0 Flag Link
* This vendor-unique field is reserved and must be set to zero.
Each partition on a tape has an Early Warning Point, EWP. Approximately 10
megabytes can be written to tape after the EWP before reaching the Virtual End of
Partition, VEOP. This allows most write commands that encounter the EWP to
successfully complete and not return a residual of unwritten blocks.
When the EWP is encountered during a write command the drive attempts to
complete the current write command and return a check condition status, EOM bit
set, sense key equal to 0. If VEOP is encountered during a write command the
sense key is equal to 0Dh, volume overflow.
All write commands issued when the tape is between the EWP and VEOP will
complete with check condition and a sense key with EOM set. If volume overflow is
reported the sense will have the valid bit set and the information field will contain the
residual count of all blocks not written to tape.
Command Descriptor Block field description
The following table defines the fields in the CDB.
Field
Name Bytes Bits Description
Fixed
Block
Size bit
1 0 If this bit is 0, the transfer length contains the number of bytes in the
block. IF this bit is 1, blocks are of fixed size, as specified by the MODE
SELECT Block Descriptor..
Transfer
Length
2-4 0-7 These fields specify the number of bytes or blocks to be written at the
current tape position, if Fixed=0.
Completion Status
The Completion Status for the WRITE command is shown in the following table.