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

The position of a file's hash code entry in the Hash Index Table is called the Directory
Entry Code (DEC) for the file. All files will have at least one DEC. A contiguous block
of granules allocated to a file is termed an EXTENT. The FPDE record contains fields to
hold the data on four extents. Files that use more than four extents because they are
either large (an extent can address a maximum of 32 contiguous granules) or fractured
into non-contiguous space require extra directory records to hold the additional extents.
These additional records are termed the File's Extended Directory Entries (FXDE) which
also have four extent fields each. A Directory Entry Code is also used to associate an
FXDE with a HIT entry. Thus, a file will have DECs for each FXDE record and use up more
than one filename slot in the HIT. Therefore, to maximize the quantity of file slots
available, you should keep your files below five extents wherever possible.
The FPDE and FXDE records are contained in the remaining sectors of the directory
cylinder. The Directory Entry Codes are mapped to the FPDE/FXDE records by each DEC's
position in the Hash Index Table. Conceptualize the HIT as eight rows of 32-byte fields
as shown in figure 5-5. Each row will be mapped to one of the directory entry records in
a directory sector. The first HIT row to the first directory entry record, the second HIT
row to the second directory entry record, and so forth. Each column of the HIT field (the
0-31) is mapped to a directory entry sector. The first column is mapped to the first
directory entry sector in the directory cylinder (not including the GAT and HIT).
Therefore, the first column corresponds to sector number 2, the second column to sector
number 3, and so forth. The maximum quantity of HIT columns actually used will be
governed by the disk formatting according to the formula: N = (number of sectors per
track times the number of sides) minus two.
In the 5-1/4" double density single-sided configuration, there exist eighteen sectors per
cylinder - of which two are reserved for the GAT and HIT. Since only sixteen directory
entry sectors are possible, only the first sixteen positions of each HIT field are used.
Other formats will use more or less columns of the HIT, depending on the quantity of
sectors per cylinder in the formatting scheme.
This arrangement works nicely when dealt with in assembly language for interfacing.
Consider the DEC value of X'84'. If this value is loaded into the accumulator, a simple:
AND 1FH ;Strip off row and
ADD A,2 ; calculate sector
will extract the sector number of the directory cylinder containing the file's directory
entry. If that same value of X'84' was operated on by:
AND 0E0H ;Strip off sector and keep row
the resultant value will be the low-order starting byte of the directory entry record
assuming that the directory sector was read into a buffer starting at a page boundary.
This procedure makes for easy access to the directory record. The system provides two
routines, @DIRRD and @DIRWR, that will read/write the correct directory entry sector
corresponding to a DEC. The directory I/O uses the system buffer and a pointer in the HL
register pair is automatically positioned to the proper FPDE (the buffer is on a page
boundary for physical I/O). @DIRWR performs verification after write!
The following figure may help to visualize the correlation of the Hash Index Table to the
directory entry records. Each byte value shown represents the position in the HIT and is,
in fact, the Directory Entry Code value. The actual contents of each byte will be either
an X'00' indicating a spare DEC, or the one-byte hash code of the file occupying the
corresponding directory entry record.
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