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

Next, it would be very useful if the disk file I/O buffer already contained relative
sector number 30. The Next Record Number (NRN) is the relative sector number. However,
before we can make use of the NRN, we have to make sure that the buffer currently
contains the sector identified by the NRN. To determine this, @POSN first checks the
"buffer current" flag. If the buffer contains the sector identified by the NRN, @POSN
then checks if the NRN and the sector number needed to satisfy the position of record 99
are in agreement. If the file buffer currently holds the needed sector, it immediately
transfers to a routine which checks on end-of-file conditions and returns to the caller.
If the buffer does not contain the needed sector, then the NRN must be changed to the
relative sector needed. But first the system must check to see if it has to write the
buffer contents back to the disk file. This determination is based on whether the buffer
is current and contains changed data which has not yet been written to disk (perhaps the
result of a previous record written that did not span two disk sectors and thus did not
require any physical writing).
When the @READ request is passed to the system, again the system must first check if the
disk file I/O buffer contains any data which is updated but not yet written to disk. The
@READ routine does not know that an @POSN request immediately preceded it. Then, since
the LRL is less than 256, the @READ routine passes a series of character read requests
for as many characters as that identified by the LRL. Each character is placed into a
consecutive location of the user record buffer, which in this case is UREC1. The
character read requests are virtually identical to those requested by an @GET SuperVisor
Call request as both are performed by the same routines. Finally, the system adjusts the
Next Record Number and OFFSET pointers so that the next @READ references the next
consecutive record.
We now have to look at what happens when a character read is requested. First, the system
checks to see if the end of the file has been reached so it can return the "End of file
encountered" error code. Next, it checks to see if the byte is contained in the current
disk file buffer (i.e. if the buffer is current). If the buffer is not current, the
sector identified by the NRN must be read from disk. Before the system even wants to
calculate what sector that represents, it has to ensure that the requesting user has READ
permission to the file. This it can do by examining the access level stored in the FCB.
When it concludes that proper access is available, it proceeds to calculate the logical
cylinder and sector that the file's NRN relative sector represents. If you thought the
process was complex up to this point, hang on to your hat! The relative sector (remember
number 30?) is converted to a relative granule number and relative sector offset in that
granule. In this case, we will assume that the file is stored on a 5-1/4" floppy disk ette
formatted in double density with six sectors per granule. The system obtains the sectors
per granule data from the Drive Control Table (DCT) for the drive containing the file.
This means that the relative granule needed is granule number 5 (30 divided by 6). Since
the remainder of the calculation is zero, the relative sector offset in that granule is
number 0 which is the first sector of the granule.
The system then examines the EXTENT fields of the FCB to determine what extent contains
data covering relative granule number 5. To do this, the system uses the cumulative
granule figures contained in the EXTENT fields. After determining that the granule is in
one of the existing extents, the system can calculate the needed cylinder and relative
sector in that cylinder by the following process. A few numbers may help this
explanation. Say the file has two extents. The first extent contains three granules
(numbered 0-2), while the second extent contains twelve granules (numbered 3-14) and
starts on the third granule of cylinder 25. Figure 6-3 illustrates part of the second
extent by cylinder and granule. First subtract off the number of granules contained in
all extents previous to the desired extent and add the result to the starting granule
number of the extent (5-3+2=4). Next, divide that result by the number of granules per
cylinder derived from DCT information and keep the remainder (4/3=1 remainder 1). The
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