Read Combined Descriptor Header And Data Mode (0000B); Read Data Mode (0010B); Read Buffer Descriptor Mode (0011B) - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
5.20.1

Read Combined Descriptor Header and Data mode (0000b)

In this mode, a four byte Read Buffer header followed by the data bytes are returned to the initiator during the
Data In phase. The Buffer ID and the Buffer Offset fields are not used by drives supported by this manual, and
must be zero (see Table 39).
Table 121: Read Buffer header (0000b)
Bit 7
Byte
0 0
1 (MSB)
2
3
[1] The Buffer Capacity field specifies the total number of data bytes that are available in the drive's data
buffer (see Sections 5.51.1 and 5.51.2). This number is not reduced to reflect the allocation length nor is it
reduced to reflect the actual number of bytes written using the Write Buffer command. Following the Read
Buffer header, the drive shall transfer data from its data buffer. The drive terminates the Data In phase
when allocation length bytes of header plus data have been transferred or when all available header and
buffer data have been transferred to the initiator, whichever is less.
5.20.2

Read Data mode (0010b)

In this mode, the Data In phase contains buffer data only with no header. The Buffer ID and Buffer Offset fields
are not used.
5.20.3

Read Buffer Descriptor mode (0011b)

In this mode, a maximum of four bytes of Read Buffer descriptor information are returned. If there is no buffer
associated with the specified buffer ID, the target shall return all zeros in the Read Buffer descriptor. The Buffer
Offset field is reserved in this mode. The Allocation Length should be set to four or greater. The target shall
transfer the lesser of the Allocation Length or four bytes of Read Buffer descriptor. The Read Buffer descriptor
is defined as shown in Table 122.
IMPLEMENTORS NOTE: In a multi-tasking system, a buffer may be altered between the Write Buffer and
Read Buffer commands by another task. Buffer testing applications may wish to insure that only a single task is
active. Use of reservations (to all logical units on the device) may also be helpful in avoiding buffer alteration
between these two commands.
Table 122: Read Buffer Descriptor mode (0011b)
Bit 7
Byte
0 Offset Boundary [1]
1 (MSB)
2
3
[1] The Offset Boundary field returns the boundary alignment with the selected buffer for subsequent Write
Buffer and Read Buffer commands. The value contained in the offset boundary field shall be interpreted
as a power of two.
6 5
0 0
Buffer Capacity [1]
6 5
Buffer Capacity [2]
4 3
0 0
4 3
2 1
0 0
(LSB)
2 1
(LSB)
165
0
0
0