Fujitsu HDD modules can be accessed for reading/writing both in the service area of disk surfaces (where
they are stored permanently and regularly loaded during drive start-up when the power is switched on), and in drive RAM on its electronics board. RAM accommodates the minimum set of modules required for firmware operation.

During normal initialization, the modules from the service area are copied to drive RAM. If a HDD encounters
problems reading its service information from disk surfaces (HDA source), then the  respective values in lines
describing modules in RAM (RAM source) will display errors (except for the modules loaded to RAM from ROM by default). E.g., HS and DM modules will be initially taken from ROM; however, during the initialization process a drive replaces them with modules having the same names but copied from the service area on disk.

The importance identifier may take the following values:

A – unique for the current HDD (cannot be borrowed from another drive).
B – can be borrowed from a HDD having the same version (sometimes the same model is required).
C – can be cleared from within the utility (can be recorded using a standard template).
D – do not influence firmware start or data access.
d – influence data.
s – influence system start (adaptive data, etc.).
r – employed for self-testing/ self-recovery routine

Variations:
Ad – unique, influences data (e.g., translator module).
As – unique, influences firmware start (e.g., adaptive data).
B – can be borrowed from another drive having the same version (code overlay – 3D module).
C – erasable (logs of operation, SmartSelfScan).
D – does not influence normal drive operation.
Dd – initial translation tables.
Dr – SelfScan and similar modules.

The following modules are most typical for the service area:

DM module  – contains translator for the user data area of a HDD.
DT module  – contains the table of positioning adjustment settings for a HDD.
TS  (Track Skip) module – the table of defective tracks in a drive.
HS module  – (Head Select) – contains the total heads number and numbers of active heads.
PL module – (Primary List) – the table of defective sectors in a drive.
SN module – (Serial Number) – contains drive serial number.
FI module – (Factory Information) – contains information about manufacturing cycle progress. The utility displays
drive manufacture date only and skips other irrelevant data.
CI module – (Components Information) – contains information about parts inside the head-and-disk assembly case:
magnetic disks (MEDIA), heads (HEADS), read/write preamplifier chip (HD-IC), spindle motor (DCM).
ZP  module – (Zones Plane) – contains information about allocation of zones.
SM  module – (Security Master) – contains master password information.
SU  module – (Security User) – contains user password information.

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