Basic
I/O
System (BIOS) Functions
SET
TRACK
NUMBER
ENTRY
cx: Track Number
FUNCTION
10
SETTRK
RETURN
This function sets a
track
number. Register
CX
contains the track number for
subsequent disk accesses
on
the currently selected drive. Programs
can
seek the
selected
track
at
this time,
or
delay the seek until the next read
or
write actually
occurs. Register
CX
can take values in the range 0-76 (for floppy diskettes)
and
0-720 (for
hard
disk) corresponding
to
valid track numbers. A call
to
this function
with the value
00
in
CX
is
equivalent
to
a call to function
8.
SET SECTOR NUMBER
ENTRY
CX: Sector Number
FUNCTION
11
SETSEC
RETURN
This function sets a sector number. Register CX contains the translated sector
number
(see function
16)
for subsequent disk accesses
on
the currently selected
drive.
Programs can send this information
to
the controller at this point,
or
delay
sector selection until a read
or
write operation occurs. The BIOS uses a combination
of
track number
and
sector number
to
determine the physical location
of
data
on
a
diskette.
•
For
single-sided, single-density diskettes
(lD),
there are
26
sectors (num-
bered 1-26) per track,
and
one track per cylinder.
•
For
double-sided, double-density diskettes (2D), there are 26 sectors
(numbered 1-26) per track,
and
two tracks (numbered 0
and
1)
per cylinder.
Track
1,
sector 1 refers
to
cylinder 0, sector
52.
•
For
hard
disks, there are 64 logical sectors (numbered 1-64)
per
track,
and
8
tracks per cylinder.
See Chapter 7 for more complete addressing information.
5-13
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