Read Buffer - X'3C'

The Read Buffer command is supported by the tape library. Table 1 shows the command format.

Table 1. Read Buffer Command
Byte Bit 7


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


0 Operation Code (X'3C')
1 LUN (Obsolete) Reserved Mode
2 Buffer ID
3 Buffer Offset
6 Allocation Length
9 Vendor Specific (B'00') Reserved (B'0000') Flag B'0' Link B'0'

The following tape library-specific parameters apply:

  • Mode: The following value applies.
      B'010' Data mode. Returns data contained in the buffer specified by the Buffer ID.
  • Buffer ID: The following read buffer ID is supported:
    Buffer ID Description
    X'11' HTTP response to the most recent REST over SCSI (RoS) command that was sent via SCSI Write Buffer (Buffer ID of X'10') on the same control path. Each Read Buffer command response has a maximum Transfer Length of 1 MiB. In order to determine the remaining length of an HTTP response that has been truncated to 1 MiB, use the value returned for the "Content-Length:" field in the header of the HTTP response (at Buffer Offset of 0). For examples, see Examples of POST/GET commands.
  • Buffer Offset: For mode B'010', this field is vendor-specific and equates to an offset of 1 MiB (X'100000') for each unit of Buffer Offset.
  • Allocation Length: The maximum number of bytes to be transferred. The device transfers the number of bytes specified in the Allocation Length field or the number of bytes in the header and buffer being read, whichever is less. This is not an error.