Basic
Disk
Operating System (BDOS) Functions
4-26
The
Write
Random
function writes
data
to the disk from the
current
DMA
address.
If
the disk extent
or
data
block which is the target
of
the write has
not
yet been
allocated, the allocation
is
performed before the write
operation
continues. As in
the
Read
Random
function, the
random
record
number
is
not
changed as a result
of
the write. The logical extent
number
and
current record position
of
the file control
block are set
to
correspond
to
the
random
record which
is
being written. Sequential
read
or
write operations can follow a
random
write. However, the currently
addressed record is rewritten as the sequential
operation
begins.
To
get the effect
of
a sequential write operation, advance the
random
record position following each
write. Reading
or
writing the last record
of
an
extent in
random
mode does
not
cause
an
automatic extent switch as it does in sequential mode.
To
access a file using the Write
Random
function, the base extent (extent 0) must
first be opened. This ensures
that
the FCB
is
properly initialized for subsequent
random
access operations.
If
the file
is
empty, a
Make
File function must be issued
for the base extent. Although the base extent
mayor
may
not
contain allocated
data,
this ensures
that
the file is properly recorded in the directory
and
is
visible in
DIR
requests.
A Write
Random
call returns a value in register
AL
indicating
an
error, as listed in
Table 4-5,
or
the value
00
indicating the operation was successful.
COMPUTE
FILE
SIZE
ENTRY
CL: 23H
DX: FCB
Offset
FUNCTION
35
COMPUTE
FILE
SIZE
RETURN
Random
Record
Field Set
When
computing the size
of
a file, the
DX
register addresses
an
FCB
in
random
mode
format
(bytes
rO,
rl,
and
r2 are present). The FCB contains
an
unambiguous
file
name
which
is
used in the directory scan.
Upon
return, the
random
record bytes
contain the
virtual file size which is, in effect, the record address
of
the record
following the end
of
the file. If, following a call
to
function 35, the high record byte
r2 is
01, then the file contains the maximum record
count
65536. Otherwise, bytes
rO
and
rl
constitute a 16-bit value
(rO
is
the least significant byte) which
is
the file size.
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