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C141-E064-03EN 3 - 11
3.2 Logical Data Block Addressing
Independently of the physical structure of the disk drive, the IDD adopts the logical data block
addressing as a data access method on the disk medium. The IDD relates a logical data block
address to each physical sector at formatting. Data on the disk medium is accessed in logical
data block units. The INIT specifies the data to be accessed using the logical data block
address of that data.
The logical data block addressing is a function whereby individual data blocks are given
addresses of serial binaries in each drive.
(1) Block address of user space
The logical data block address number is consecutively assigned to all of the data blocks in the
user space starting with 0 to the first data block.
The IDD treats sector 0, track 0, cylinder 0 as the first logical data block. The data block is
allocated in ascending order of addresses in the following sequence (refer to Figure 3.5):
1) Numbers are assigned in ascending order to all sectors in the same track.
2) By following step 1), numbers are assigned in ascending order of tracks to all sectors in
each track in the same cylinder except the last track.
3) By following step 1), numbers are assigned to all sectors in the last track except the spare
sectors.
4) After completing steps 1) through 3) for the same cylinder, this allocation is repeated from
track 0 in the next cylinder and on to the last cylinder (cylinder p-q in Figure 3.1) except
for the alternate cylinders in ascending order of cylinder numbers.
When the logical data block is allocated, some sectors (track skew and cylinder skew) shown
in Figure 3.5 are provided to avoid waiting for one turn involving head and cylinder switching
at the location where the track or the cylinder is physically switched.
See Subsection 3.3.2 for defective/alternate block treatment and the logical data block
allocation method in case of defective sectors exist on the disk.
(2) Alternate area
Alternate areas in the user space (spare sectors in the cylinder and alternate cylinders) are not
included in the above logical data block addresses. Access to sectors which are allocated as an
alternate block in the alternate area is made automatically by means of IDD sector slip
treatment or alternate block treatment (explained in Subsection 3.3.2), so the user does not
have to worry about accessing the alternate area. The user cannot access with specifying the
data block on the alternate area explicitly.