Read
Buffer
The READ
BUFFER
command is used in conjunction with the
WRITE
BUFFER
command
as
a diagnostic tool for testing Target memory and
the
SCSI bus integrity. This command
does not alter the media or
the
buffer. The
data
returned by the READ
BUFFER
command
contains
a 4-byte header, followed by the buffer data.
Read Buffer Command Descriptor Block
(COB)
Bit
Byte
7
I
6
I
5
I
4
I
3
I
2
I
1
I
0
00
Opcode
=
3CH
01 Logical
Unit
Number
I
Reserved
1
Mode
02
Buffer ID = 0
03-05
(MSB=03)
Buffer Offset = 0
(LSB=05)
06-08
(MSB=06)
Allocation
Length
(LSB=08)
09
Control
Byte
Mode. Modes 0
(OOOb)
and 3 (OUb) are supported. Mode 0 returns a 4-byte header followed
by the
data
bytes, and Mode 3 returns only the 4-byte header.
Buffer ID. This field
is
not supported and should be set
to
zero (0).
Buffer Offset. This field
is
not supported and should be set
to
zero (0).
Allocation Length. This
field
specifies the number of bytes
that
the
initiator has allocated for
returned data.
An
allocation length of zero specifies
that
no
data
be transferred and is not
considered an error. The target will terminate the
Data
Phase when the specified number of
bytes or when all available buffer
data
has been transferred, whichever is less.
Read Buffer Header Format
Bit
Byte
7
I
6
I
5
I
4
I
3
1
2
I
1
I
0
00
Reserved
01-03
(MSB=Ol)
A vailable
Length
(LSB=03)
Available Length. This field may contain up to 262,144 bytes
(1
's-based 256K). An Available
Length of zero indicates
that
no
data
transfer will take place.
If
the
Allocation Length of
the CDB
is
too small
to
allow all of the Available Length, the Available Length field is NOT
adjusted
to
reflect the truncation.
The
data
in the buffer may have been altered since the last
WRITE
BUFFER
command.
It
is
recommended
that
the Target be placed in reserve or
that
the
WRITE
BUFFER
command
and subsequent READ
BUFFER
command be linked
to
ensure
that
the
initiator can reliably
test
the
Target's
data
buffer.
If any command has been executed by the controller between the execution of the
WRITE
BUFFER
command and
the
READ
BUFFER
command, a status of CHECK CONDITION
will be returned. In this case, the sense information
wi11
have a Sense Key of MISCOMPARE
set. The amount of requested buffer
data
will be returned regardless of the MISCOMPARE
error
status,
but
the contents should be suspect.
A-32 SCSI Command Descriptions DRAFT
1/18/93 09:58
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