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

5.3 THE HASH INDEX TABLE (HIT)
The Hash Index Table is the key to addressing any file in the directory. It is designed
so as to pinpoint the location of a file's primary directory entry with a minimum of disk
accesses. A minimum quantity of disk accesses is useful to keep system overhead low while
at the same time providing for rapid file access.
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Figure 5-4: File NAME/EXT buffer
When an application requests the system to open a file, the system must locate that
File's Primary Directory Entry (FPDE) record which contains the disk storage data needed
to address the file. The procedure that the system uses to locate a file's FPDE is to
first take the file name and extension and construct an 11-byte field with the file name
left justified and padded with blanks so as to fill out eight positions. The file
extension is then inserted, padded with blanks, and will occupy the three least
significant bytes of the 11-byte field. The resulting string is illustrated in figure 5-
4. This field is then processed through a hashing algorithm which produces a single byte
value in the range X'01' through X'FF' (a hash value of X'00' is reserved to indicate a
spare HIT position). The following code may be used to obtain the one-byte hash code for
an 11-character NAME/EXT buffer.
HASHSPEC LD HL,SPECPTR ;Point to the 8-char buffer
LD A,0D4H ;Specify filename hash function
RST 40 ;Issue the RST instruction
The one-byte hash code is returned in the accumulator. Registers AF, B and HL are
altered. The operating system will not return to the address following the RST 40
instruction when the SVC function code is an internal system request code (i.e. has bit-7
set) but will return to the previous caller. Thus, it is necessary to CALL this routine.
Each file's hash code is stored in the Hash Index Table (HIT) at a position which is
associated with the FPDE record containing the file's access information. After the OPEN
routine obtains the hash code for the file identified in the file specification, it
searches the HIT for a matching hash code. Since more than one 11-byte string can hash to
identical codes, the opportunity for a "collision" exists (a collision is where two or
more file names result in the same hash code). For this reason, the search algorithm will
sequentially scan the HIT for a matching code entry and when found, will then read the
FPDE record corresponding to the matching HIT position. OPEN will then compare the file
name/ext stored in the FPDE record with that provided in the file specification. If both
match, the file's FPDE directory record has been found. If the two fields do not match,
the HIT entry was a collision and the algorithm continues its search from where it left
off. If a match to the hash code is not found in the HIT, the file does not exist on that
disk drive. If the user passed a drive specification (drivespec) as part of the file
specification, a "File not found" error will be returned. If no drivespec was passed, the
system will search all drives in logical number order starting with drive 0. If the @INIT
SuperVisor Call was used to open the file, the system will first use @OPEN to determine
the possible existence of the file. If @OPEN advises that the file has not been found,
then @INIT will create the file by obtaining a spare HIT position then constructing the
corresponding FPDE.
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