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Fibre Channel Interface Manual, Rev. D 271
12.25.1 Read Combined Descriptor Header and Data mode (000b)
In this mode, a four-byte Read Buffer header followed by the data bytes are returned to the initiator in a data
transfer. The Buffer ID and Buffer Offset fields are not used by drives supported by this manual, and must be
zero.
Field definitions (listed alphabetically)
Buffer Capacity
The number of bytes available in the read buffer.
12.25.2 Read Data mode (010b)
In this mode, the buffer data only is transferred with no header. The Buffer ID and Buffer Offset fields are not
used.
12.25.3 Read Buffer descriptor mode (011b)
In this mode, a maximum of four bytes of Read Buffer descriptor information is returned. If there is no buffer
associated with the specified buffer ID, the target returns all zeros in the Read Buffer descriptor. The Buffer Off-
set field is reserved in this mode. The allocation length should be set to four or greater. The target transfers the
lesser of the allocation length or four bytes of Read Buffer descriptor. The Read Buffer descriptor is defined as
shown in the following table.
Implementor’s 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.
Field definitions (listed alphabetically)
Buffer Capacity
The total number of data bytes that are available in the disc drive’s data buffer (see 12.64.1 and 12.64.2). 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 disc drive transfers data from its data buffer. The disc drive termi-
Table 176: Read Buffer header (000b)
Bit
Byte
76543210
000000000
1 (MSB)
2Buffer Capacity
3 (LSB)
Table 177: Read Buffer descriptor mode (011b)
Bit
Byte
76543210
0
Offset Boundary
1 (MSB)
2Buffer Capacity
3 (LSB)
Table
number
176
Table
number
177