Chapter
5
I
Device Drivers
I
3-BYTE Near JUMP
to
boot code
I
8
BYTES OEM name and version
I
WORD Bytes per sector
BYTE Sectors per allocation unit
I
WORD Reserved sectors
I
BYTE Number
of
FATS
P
B WORD Number of
root
directory entries
WORD
BYTE Media descriptor
WORD Number
of
FAT sectors
Number
of
sectors in logical image
~~ ~
7
WORD Sectors per-track
WORD Number
of
heads
WORD Number
of
hidden sectors
The three words
at
the end (sectors per track, number
of
heads,
and number
of
hidden sectors) are intended
to
help the BIOS un-
derstand the media. Sectors per track may be redundant (could
be calculated from total size
of
the disk). Number
of
heads is
useful for supporting different multi-head drives which have the
same storage capacity but different numbers
of
surfaces. Number
of
hidden sectors may be used
to
support drive-partitioning
schemes.
Media Descriptor Byte
The last
two
digits
of
the FAT ID are called the media descriptor
byte. Currently, the media descriptor byte has been defined
for
a
few media types, including
5-114"
and
8"
standard disks.
Although these media bytes map directly
to
FAT ID bytes (which
are constrained to the
8
values F8H-FFH), media bytes can, in
general, be any value in the range OOH-FFH.
184
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