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

of a variable length record. Some systems use CARRIAGE RETURN followed by LINE FEED
(X'0A'). Some languages use a one-byte or two-byte length indicator within the record to
indicate the actual length of the record. Program files that are directly executable are,
in fact, variable length record files which use a one-byte length field within each
record. These "load module" files even include a record TYPE character which permits the
specification of different records for different purposes within the same file.
Some files may not even be able to be conceptualized as containing fixed or variable
length records. You might consider a word processing text file as not falling into the
above classification although each paragraph may, in fact, be a "record". Other files may
be variable length but include an index which points to the beginning of each record or
group of records. The records are accessed sequentially after the record pointer is
extracted from the index. This type of access is usually called Indexed Sequential Access
Method (ISAM). Both the operating system's library files and the Partitioned Data Set
files supported under the PRO-PaDS utility are ISAM files. The bottom line is for you to
determine the type of access you want to employ after exploring the nature of your data
and understanding how the system accesses disk files.
There are three methods which are used in application programs to access disk files. The
first method is to consider the file as a stream of characters. This access method uses
the GET and PUT character I/O SuperVisor Call functions and was discussed in chapter 3,
DEVICE INPUT/OUTPUT INTERFACING. The second method is where your file contains physically
consistent fixed length records. In this case, it is probably practical to consider
RECORD I/O. The third method is to use 256-byte records and perform your own blocking or
deblocking as required.
The following sections describe the methods used to control and access files. The last
section completely describes the fields in the File Control Block which is used in all
interfacing of disk files.
6.2 CONTROLLING DISK FILES
When a file is to be opened for access, the application program initially provides the
file specification to the DOS by placing it in the File Control Block (FCB) which will be
used for the file. The program then invokes the OPEN function. The DOS, in turn, searches
the disk drive(s) for the file's directory entry. Once found, it replaces the filespec in
the FCB with information needed by the file access routines. The system then manages the
FCB contents according to the demands of the file access requests. The following sections
will illustrate some of these control functions.
6.2.1 Getting Filespecs
From where does a program obtain the filespec? You are already familiar with the DOS
commands that appear to get the filespec from the command line. Let's take a look at this
method. You will learn from the chapter on SuperVisor Calls that when the system
transfers control to a program, register pair HL contains a pointer to the first nonblank
character on the command line which terminated the name of the executing program.
Let us assume that our program will use a command line syntax as follows:
PROGRAM-NAME FILE-SPECIFICATION (PARAMETERS)
The command-line pointer will be pointing to the first character of the file
specification. For the moment, let's make the filespec entry mandatory. We can then code
the routine to fetch the filespec as follows:
ENTRY LD DE,FCB1 ;Point to FCB
LD A,@FSPEC ;Identify the SVC
RST 40 ;Invoke the SVC
JP NZ,SPCERR ;Transfer on error
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