Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E 107
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5.1.2.4.2
Read data
In this mode, the DATA IN phase contains buffer data only with no header. The buffer ID and buffer offset
fields are not used.
5.1.2.4.3
Descriptor mode (011b)
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 descrip-
tor. 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 the Table following.
Table 5.1.2.4.3-1. READ BUFFER descriptor
Bit
7
6
5
4
3
2
1
0
Byte(s)
0
Offset Boundary
1
(MSB)
2
Buffer Capacity
3
(LSB)
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 inter-
preted as a power of two.
The value contained in the buffer field of subsequent WRITE BUFFER and READ BUFFER commands
should be a multiple of 2offset boundary as shown in table following.
Table 5.1.2.4.3-2 Buffer Offset boundary
Offset
Boundary
2Offset Boundary
Buffer Offsets
0
2
0
= 1
Byte boundaries
1
2
1
= 2
Even-byte boundaries
2
2
2
= 4
Four-byte boundaries
3
2
3
= 8
Eight-byte boundaries
4
2
4
= 16
16-byte boundaries
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⋅⋅⋅⋅⋅
⋅⋅⋅⋅⋅
⋅⋅⋅⋅⋅
⋅⋅⋅⋅⋅
⋅⋅⋅⋅⋅
FFh
Not applicable
0 is the only supported buffer offset.
The buffer capacity field shall return the size of the selected buffer in bytes
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.
