Digital Equipment RW504-ZA, Service Manual

Page: 167 / 220

The DDS contains information on how the disk is organized into user and spare
groups. There are three important parameters; the variables g, n, and m are used
in the ISO standard, and are here for consistency:
• g - number of groups
• n - number of sectors in a user group
• m - number of sectors in a spare group
User data is stored initially in the sectors of the user group, while the spare
groups are reserved sectors for the linear replacement sparing algorithm. The
values of g, n, and m are generally chosen so that they maximize the number of
spare sectors allowed, and maximize the size of the user area. (The ISO standard
for 650-Mbyte media allows for a maximum of 2048 spare sectors total from the
PDL and th SDL, while the ECMA standard for 1.3-Gbyte allows for 2057 or
1077, depending on the sector size of 1024 or 512 bytes per sector respectively.)
For 1.3-Gbyte the value for g must be 1 or 16.
In general for 650-Mbyte: g * (n + m) <= (size of User Area)
In general for 1.3-Gbyte: g = 1 or 16, (n, m or n0 thru n15 and m0 thru m15 are
predefined based on g).
For more details consult the ISO or ECMA standard.
The PDL contains a list of defective sectors determined by the manufacturer or
by a certification of the User Area, i.e. during a SCSI Format Unit Command.
Defective sectors in the PDL are managed according to the slip sparing algorithm
described in this chapter.
The SDL contains a list of defective sectors and corresponding replacement
sectors determined during disk use, after certification. Defect/replacement
entries in the SDL are managed according to the replacement sparing algorithm
described in this chapter.
The Slipping Area is a portion of the user zone used by the slip sparing algorithm.
Defects found during certification are excluded from use. The user accessible
space is slipped by a corresponding number of sectors into the slip area. This
area is large enough to account for a maximum of 2048 slip spares. Any unused
sectors in the slipping area are unavailable for user data.
Note
The slipping area applies only to 650-Mbyte media.
Theory of Operation 6–19