Disk Support - Tandy 600 Programmer's Reference Manual

Disk
Support
The
Disk
Support
functions
have been designed
to
provide compatibility with
MS-DOS
style
disks.
Several of the
terms
used
in
the descriptions of
the functions are
taken from
MS-DOS,
and
it
is
recommended
that
the
BIOS implementor be
familiar
with
the
MS-DOS
2.0
Adaptation
Guide-
Several
of
the disk support function
calls refer to
a
BPB. The
BPB
used by
the
HH
O/S
is
identical to
that
used by
MS-DOS
2.X
and
described
in
the
MS-DOS
2.0
Adaptation Guide.
Even
if
the
BPB
on the boot sector
was
not the
same
as
one on
the
BIOS-ROM,
the
BIOS
will
not return
an
error
because the
Tandy 600
is
capable
of
reading another
logical
formatted
disk.
The
description of the structure of
a
BPB
is
repeated
here:
WORD
Sector
size
in
bytes
must be
a
multiple
of
64
BYTE
Sectors per allocation
unit
must be a power
of
2
(1
,
2, 4, etc.)
WORD
Number
of
reserved sectors
may
be
BYTE Number
of
FATs
WORD
Number
of
directory entries
WORD
Total
number
of
sectors includes reserved sectors
BYTE
Media descriptor byte
must be unique
for
each unique
BPB
WORD
Number
of
sectors
occupied by a
single
FAT
The
following interrupts
provide the disk support functions:
INT 57h, Function
-
Request Disk Device
Entry:
AH
=
(function
code)
Exit:
AL=; Standard
BIOS
error
code
This function
will
be
called
before
any
disk
requests are
made.
Its
purpose
is
to
mark
the disk
device as
in
use.
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