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

travels the turnpike. In this manner, all sectors of a cylinder are accessed before the
disk drive has to step to the next cylinder.
It is not always possible to allocate space consecutively. For instance, say we want to
add a granule to an existing file but the next granule consecutive to the last granule of
the file has already been allocated to another file. Our file must then be fractured into
more than one piece. We term each piece of the file an EXTENT. The system's file access
routines logically connect each EXTENT so to a program accessing the file, it appears as
if the file exists as one continuous allocation of space.
The disk directory stores all the allocation data on each file contained on the disk.
Allocation data on a particular file is stored in a directory entry record. Each record
can hold the allocation information on up to four extents. The first record is termed the
File's Primary Directory Entry or FPDE while all succeeding directory records are
considered to be the File's Extended Directory Entries or FXDE records. In order to
access the file data, the system's file access routines must utilize the information
contained in the file's FPDE.
It is impractical to have to read the FPDE each time another sector of data is
transferred. Therefore, the scheme employed is to access the directory once in a process
to obtain all of the file's access information and place the information into a memory
area termed a File Control Block (FCB). The actual process is termed "opening the file".
The reverse process, that of updating the directory entry once the access of a file is
complete is termed "closing the file". The DOS provides SuperVisor Call requests to
perform the OPEN and CLOSE functions. These type of requests are called "file control"
functions since they give you the means of controlling the disk file. Other types of
requests are associated with accessing the data in a file and are thus called "file
access" requests. INTERFACING VIA SUPERVISOR CALLS, chapter 7, describes each access and
control SuperVisor Call.
Data is generally collected into units called RECORDS. These may be fixed-length records
with each record being exactly the same length or they may be variable length records
where the length of the record varies from record to record. Fixed-length records can be
accessed sequentially (i.e. starting from record zero and continuing to the last record
of the file). This type of access is termed RECORD I/O. The DOS supports fixed length
records from one to 255 characters in length by automatically handling the blocking and
deblocking of records into and out of the disk file I/O buffer. Since the DOS
standardizes disk file I/O buffer sizes at 256 characters each, record lengths of 256 are
handled directly without recourse to the blocking and deblocking used on shorter records
and these records can also be transferred to and from the disk more quickly. Record sizes
larger than 256 can be used in an application program; however, the blocking and
deblocking of records must be performed entirely within the application while, in
general, the application will use 256-character records to and from the system.
Henceforth, any reference to the term RECORD will consider to be associated with a record
which ranges from 1 to 256 characters in length.
Fixed length records can also be accessed directly by record number (which is customarily
called RANDOM ACCESS). The DOS provides SuperVisor Call requests to position the record
pointer maintained in the File Control Block to the record of choice. The application can
then address the record via READ or WRITE SuperVisor Call access requests. Additional
SVCs provide other functions associated with the access of a file.
The structure of variable length records is highly dependent on the programming language
used to code the program. Most high-level languages (BASIC, FORTRAN, etc) provide
variable length file structures which may not be equivalent across each language. One
common structure which is supported by more than one language is to use a character or
character combination to represent the end of the record. The BASIC language operating
under Version 6 uses the ASCII code X'0D' which is a CARRIAGE RETURN to indicate the end
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