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

LD HL,FCBX+9 ;Point to the LRL field
LD B,(HL) ;P/u the current LRL
LD (HL),1 ; & reset to LRL=1
LD A,@BKSP ;Identify the SVC request
RST 40 ;Invoke the SVC
LD (HL),B ;Reset to original LRL
JP NZ,IOERR ;Transfer on error
If you want to add sequential data to the end of an existing file, you will need to
position to the end of the file after it is opened. Use the @PEOF SuperVisor Call request
for this purpose. The SVC will return an "End of file encountered" error if the request
is successful. Any other error code indicates a malfunction. This is one of the few
system requests that return an error code upon success so you should be careful when you
use it.
The @RREAD request is useful when reading nested files. Nested files are those where you
are accessing each consecutively but not coincidentally. In this case, the same disk file
I/O buffer can be used for each file. When you switch from one file to another, issue a
@RREAD so that the system reloads the buffer with the sector that was being accessed for
the last record read or for the last character obtained from @GET. The @RREAD request
will force a rereading of the sector identified by the NRN provided that the LRL is
either 1-255 or the file was accessed via @PUT or @GET. What do you do if you were using
LRL=256 and @READ requests while maintaining your own offset pointer. All you need do in
this case is to decrement the NRN and issue another @READ. For example, the PRO-CREATE
editor assembler available from MISOSYS uses sector I/O for reading source files. PRO-
CREATE maintains its own offset pointer as it extracts lines of code from the disk
buffer. When it detects the "*GET filespec" request for including a nested file, it saves
the current FCB in a save area and then opens the requested file using the same file
buffer. When the end of the second file is reached, PRO-CREATE restores the saved FCB of
the original file and executes the following code:
LD DE,FCB ;Point to the opened FCB
LD HL,(FCB+10) ;Obtain the current NRN,
DEC HL ; decrement by one
LD (FCB+10),HL ; and update the FCB
LD A,@READ ;Identify the SVC function
RST 40 ;Invoke the SVC
JP NZ,IOERR ;Transfer on error
The @RWRIT SuperVisor Call request would be used where you want to read a full sector
(LRL=256) into the disk file I/O buffer, alter it directly in the buffer, then
immediately write that buffer back to disk. The @RWRIT will force the NRN that was
automatically advanced by the @READ request to be decremented by one so that it repoints
to the sector corresponding to the buffer contents. It then performs the requests
necessary to write the buffer to disk. Note that @RWRIT is not to be used when the LRL is
not equal to 256 as this SuperVisor Call does not reference the user record buffer.
The @WEOF SuperVisor Call request allows you to update the end-of-file (EOF) information
in the directory while still keeping the file in an open state. Obviously, a similar
function can be performed with an @CLOSE followed by an @OPEN; however, complications can
prevail with a CLOSE-OPEN combination. Remember that the close operation restores the
filespec to the FCB but cannot reclaim the password. Therefore, if the FCB was
referencing a password protected file, the subsequent OPEN will fail unless you had saved
the original filespec somewhere in the program and restuffed the FCB prior to the second
OPEN request. Also, the CLOSE-OPEN combination updates the MOD flag and date as required,
and checks to see if it can deallocate any unused file space. This takes time. If all you
want to do is to update the EOF, use the @WEOF function.
One last function that can be performed by the file access routines is the allocation of
disk space to a file. A file can be pre-allocated by the CREATE library command but that
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