The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

also inhibits any deallocation of unused space. The following routine will allocate file
space without any restriction on deallocation to a file opened with LRL equal to 256.
Register pair DE is expected to be pointing to the file's FCB. The file's size is passed
in register pair BC as the number of 256-byte records. A successful allocation will be
indicated by the setting of the Z flag.
WRERN LD A,B ;If space = 0, don't
OR C ; do any allocation
RET Z
DEC BC ;Adjust for 0 offset
LD A,@POSN ;Position to the "size"
RST 40
LD A,@WRITE ;Write a dummy sector
RST 40
JR NZ,WRERN1 ;Branch on error
LD A,@REW ;Now rewind the file
RST 40
LD HL,0 ;Set ERN record to 0
LD (FCB1+12),HL
RET
WRERN1 CP 27 ;Disk Full?
RET NZ ;Back on some other error
LD A,@REMOV ;Remove what can't fit
RST 40
LD A,27 ;Back with error code
OR A ; and NZ flag
RET
Examine the functions of the file access routines listed in chapter 7. They will relate
the scope of access permitted by the operating system. More complex levels of access such
as ISAM, or random access of variable length records can be supported by building
appropriate routines from the provided record I/O and character I/O routines. The
following section will provide details on each field of the File Control Block. Most
applications will not have to bother with the contents of the FCB. If you feel the need,
go to it.
6.4 The FILE CONTROL BLOCK (FCB)
The File Control Block (FCB) is a 32-byte region that is used by the system to interface
with a file that has been "opened" for access . Its contents are extremely dynamic. As
records are written to or read from the disk file, specific fields in the FCB are
modified. It is extremely important that during the time period that a file is open, you
avoid changing the contents of the FCB unless you are sure that its alteration will in no
way effect the integrity of the file.
The FCB initially contains the specification of the file that is to be opened for access.
Upon a successful "open", the system will replace the specification with data derived
from the file's directory entry. The file specification (without any password field) will
be returned to the FCB when the file is closed. The information contained in each field
of the FCB is as follows:
6.4.1 TYPE code of the control block -
This byte contains certain attributes of the control block. It correlates to the TYPE
byte of the Device Control Block, especially in light of the fact that both the DCB and
the FCB can be associated with a device specification (the FCB by the nature of a ROUTE
to a file). The TYPE byte uses each bit as a flag per the following specifications:
Bit 7 If set to a "1", it will indicate that the file is in an open condition; if
set to a "0", the file is assumed closed. This bit can be tested to
determine the "open" or "closed" status of an FCB and is used by the
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