SCSI Commands
Page
81
READ BUFFER
The Read Buffer command reads data from the memory on the drive and sends it to
the initiator. This command is used in conjunction with the Write Buffer command as
a diagnostic function for testing the drive buffer memory and confirming the SCSI bus
integrity. The tape is not accessed during execution of this command.
Command Descriptor Block
Byte
Bits
7
6
5
4
3
2
1
0
0
Operation Code (3Ch)
1
Ignored LUN Reserved
Mode
2
Buffer ID
3
Buffer Offset (MSB)
4
Buffer Offset
5
Buffer Offset (LSB)
6
Allocation Length (MSB)
7
Allocation Length
8
Allocation Length (LSB)
9
Control
Field Descriptions
Field
Bytes
Bits
Description
Operation
Code
0
The operation code for Read Buffer is 3Ch.
Ignored Logi-
cal Unit
1
5-7
The LUN field is residual from the SCSI-1 days and is ignored.
Mode
1
0-3
0000b = Return descriptor and data.
0001b = Return Medium Auxiliary Memory(MAM)
0010b = Return data only.
0011b = Return descriptor only.
0100b = Return Trace Buffer data.
Buffer ID
2
00h = normal access (default)
54h = specifies access to MAM
Buffer Offset
3-5
For Mode not equal 0001b: The offset from the beginning of the
specified buffer in bytes.
For Mode equal 0001b: Set to the MAM page code as described
in the tables or FFFFFFh to specify the entire 4KB MAM.
Allocation
length
6-8
Specifies the amount of data bytes to be returned. A value of 0 is
a valid entry and returns no data.
The drive stops sending data when the number of bytes specified
has been transferred or when all available data has been sent.
Reserved
All reserved bits must be 0.
Control
9
See Control Byte Format on page 18.
A Mode of 0000b requests that the data returned consist of a 4-byte descriptor
header followed by the data from the buffer. The 4-byte header is included in the
number of bytes specified by the Allocation Length. The Buffer ID and Buffer Offset
fields are reserved.
