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4.1 Control/Sense Commands
b. Block descriptor
The logical attributes of the data format on the disk media are described in the
8-byte long "Block descriptor."
− Data block count
This field specifies the total number of logical data blocks (the block
length is specified in "Data block length") allocated in the user space on
the disk media. The maximum logical block address is a value with 1
subtracted from the value in this field.
If only the "Block descriptor" in this command is changed accessible data
block count can be limited without formatting if "Data block count" value
is within a "Cylinder count" value of page 4.
If zero is specified for the "Data block count," or when the block
descriptor is not specified, the HDD retains its current capacity, if the
block size has not changed. If the "Data block count" is set to zero and
the Data block size has changed, the HDD shall set to its maximum
capacity when new block size to be effect.
− Data block length
This field specifies the byte length of a logical data block on the disk
media.
The logical data block length and physical data block length are the same
in the HDD. The data block length can be specified at 512 to 528 bytes
(4-byte boundary).
If less than 512 bytes is specified in this field, the HDD performs
rounding processing (there is a rounding processing report), setting the
data block length at 512 bytes. If more than 528 bytes is specified,
rounding processing is performed (there is a rounding processing report),
setting the data block length at 528 bytes. If the block length is not 4-
byte boundary, rounding processing is performed to change 4-byte
boundary.
If zero is specified in this field, the value specified in bytes 12 and 13
(Data byte length/sector) of Page 3 (Format Parameter) is set. At that
time, if Page 3 is not specified at the same time, the current value in Page
3 is used.
If the value set in this field differs from the value specified in bytes 12
and 13 (Data byte length/sector) of Page 3 (Format Parameter), the data
block length in the block descriptor is set.
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