SCSI Commands: 28BREAD
Page
123
3.16.1.1
READ Command Field Descriptions
Field Bytes
Bits
Description
Operation
Code
0
The Operation Code for READ is 08h.
Ignored
Logical
Unit
1
5-7
The LUN field is vestigial from the SCSI-1 standard and is ignored.
SILI
1
1
0 = Return Check Status when an incorrect block length is detected.
1 = Suppresses invalid length error reporting when reading variable-
length blocks
Fixed
1
0
0 = Variable block length. Transfer Length contains the number of
bytes to return.
1 = Fixed block length, as specified by the MODE SELECT Block
Descriptor. Transfer Length contains the number of blocks to
return
Transfer
Length
2-4
This field specifies the number of bytes (Fixed = 0) or blocks (Fixed
= 1) to be read. A transfer length of 0 is valid and no data is
transferred.
Reserved
All reserved bits must be 0.
Control 5
See
SCSI Command Control Byte Format on page 25.
If the SILI bit is one and the Fixed bit is one, the drive returns a Check Condition
with Illegal REQUEST SENSE key with an additional sense code of Invalid Field in
CDB.
When the SILI bit is one and Fixed bit is zero and the only error encountered by the
drive is that the actual block length differs from the requested transfer length, then
the drive:
Returns a Check Condition if the actual block length is larger than the requested
transfer length and the Block Length field in the Mode Parameter block descriptor is
nonzero.
Does not return a Check Condition if the actual block length is smaller than the
requested transfer length or if the actual block is larger than the requested block and
the block length in the Mode Parameter block descriptor is zero.
If the Fixed bit is zero and Transfer Length is not zero, then a single block of the
length in Transfer Length is transferred. If the next block on tape is of this length or
shorter then it is transferred to the initiator. If the next block is longer than this
length, then only the length requested is returned and the logical position is set to
after the block. If the length of the block was the same as the Transfer Length field,
then good status is returned. If the length of the block was not the same as in
Transfer Length and the Suppress Invalid Length Indicator (SILI) bit was zero, then
Check Condition Status is returned. If the length of the block was not the same as in
Transfer Length and the Suppress Invalid Length Indicator (SILI) bit was one, then
good status is returned.
If the fixed bit is one and the Transfer Length field is not zero and the Suppress
Invalid Length Indicator (SILI) bit is zero, then a sequence of blocks of the currently
configured block length is returned, the number of blocks being indicated in the
Transfer Length field. If there is a sequence of blocks of this length on the tape, they
are returned to the initiator with good status. If a block that is longer than the
configured length is encountered before the sequence is complete, the blocks up to
that block are returned followed by the configured length from the block that was too
long and Check Condition status. If a block that is shorter than the configured length
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