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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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