Write Buffer - X'3B'

Table 1 shows the format of the Write Buffer command for the tape library.

Table 1. Write Buffer Command
Byte Bit 7


Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0


0 Operation Code (X'3B')
1 LUN (Obsolete) Reserved Mode
2 Buffer ID
3 Buffer Offset
6 Parameter List Length
9 Vendor Specific (B'00') Reserved (B'0000') Flag (B'0') Link (B'0')
Mode: Supported modes are:
  Mode 2 Data Mode
  Mode 4 Download Microcode
  Mode 5 Download Microcode and Save
  Mode 6 Download Microcode With Offsets
  Mode 7 Download Microcode With Offsets and Save

The following 3584 Tape Library-specific parameters apply for Modes 4–7:

  • Buffer ID: X'00' for microcode.
  • Buffer Offset: This field is vendor-specific and equates to an offset of 1 MiB (X'100000') for each unit of Buffer Offset.
  • Parameter List Length: The number of bytes to be transferred. This must be X'100000' for all but the last Write Buffer command in the sequence.
The following 3584 Tape Library-specific parameters apply for Data Mode 2:
  • Buffer ID:
    The following write buffer ID is supported:
    Buffer ID Description


    REST over SCSI (RoS) command.

    Use SCSI Read Buffer with Buffer ID of X'11' to receive the HTTP Response to this RoS command. The HTTP Response is maintained separately for each control path in the library. Thus, when performing control path failover of a SCSI Write Buffer for a POST task, the application must first send GET (all) tasks to check if the original POST task was accepted or not. For control path failover of all other RoS commands, the application can blindly retry the SCSI Write Buffer command. For examples, see Examples of POST/GET commands.

    All RoS commands are allowed during SCSI Not Ready conditions such as door open. For the POST of a task, the task will be created but will report a state of failed if the library is in a SCSI Not Ready condition at any time during the execution of that task. For additional details on SCSI Not Ready conditions, see Supported SCSI commands and Sense Key 2 (Not Ready).

  • Buffer Offset: For Data Mode 2, the Buffer Offset field is always 0.
  • Parameter List Length: The number of Data Mode bytes to be transferred with the current Write Buffer command.