Basic Disk Operating System (BDOS) Functions
FILE
CONTROL
BLOCK
FORMAT
The
format
of
the File
Control
Block (FCB) is
dr
fl
f2 / / f8 t 1 t2 t3 ex s I s2 rc
dO
/ / dn cr
rO
r1
r2
10010
11021
...
1081091101111121131141151161
...
1311321331341351
where:
dr
Drive code (0-16)
o = use default drive for file
1
=
auto
disk select Drive A
2
=
auto
disk select Drive B
16
=
auto
disk select drive P
fl
...
f8
File
name
in
ASCII
uppercase, with high
bit
= 0
tl,t2,t3
File type in
ASCII
uppercase, with high
bit
= 0
tl',t2'
and
t3'
denote
the
high bit
of
these positions
tl'
= 1 -
Read/Only
file,
t2'
= 1 - SYS file,
no
DIR
list
ex
Current
extent
number,
normally
set
to
00 by
the
user,
but
in
the
range 0-31
during
file
I/O
s 1 Reserved for internal system use
s2 Reserved for internal system use; set
to
0
on
call
to
OPEN,
MAKE,
SEARCH
rc Record
count
for extent 'ex', takes values 0 -
128
dO
...
dn
Filled in by
CP/M,
reserved for system use
cr
Current
record
to
read
or
write
in
a sequential file
operation,
normally set
to
0 by the user
rO,rl,r2
Optional
random
record
number
in
the
range 0 - 65535, with over-
flow
to
r2
rO,rl constitute a 16-bit value with low byte
rO
and
high byte
rl
Users
of
earlier versions
of
CP/M
should
note
that
both
CP
1M
Version 2
and
CP
IM-86
perform
directory
operations
in a reserved
area
of
memory
that
does
not
affect write
buffer
content,
except in
the
case
of
Search
for
First
(function
17)
and
Search
for
Next
(function
18) where the directory
record
is copied
to
the
current
DMA
address.
4-11
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