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

02 02 00 2C
This will represent the TRANSFER record (TYPE=02). Again, we have a LENGTH byte which
shows a 2-byte data field. The data field contains the transfer address or entry point to
the program in standard low-order, high-order sequence. The system uses this address as
an entry to the program after successfully loading it into memory. This address is also
what is returned in register pair HL by the @LOAD SuperVisor Call.
So far we have discussed the HEADER, the COPYRIGHT, the LOAD, and the TRANSFER records.
These are the four common record types you will find in most load module files. We also
observe that our discussion of program load modules was limited to a single program per
file. Another kind of file is one that contains many program modules (or data modules) as
sub-files. Since the file is divided into sub-files, it is considered a "partitioned data
set" abbreviated as "PDS". The PDS contains a directory of its sub-files with each subfile
being termed a MEMBER of the PDS and having an entry in the directory. The system
loader supports a particular kind of PDS used to contain the library overlays: SYS6/SYS,
SYS7/SYS, and SYS8/SYS (LIB A, B, and C respectively).
Let's take a look at one of these libraries. List the first record of SYS6/SYS via the
command: LIST SYS6/SYS.LSIDOS (H). Look at the area just past the copyright message. You
will see something like this:
08 06 21 00 24 00 00 CB 08 06 61 ...
The TYPE code of X'08' indicates an ISAM DIRECTORY ENTRY record. The LENGTH byte denotes
a DATA area of six bytes. After the sixth byte, you will see another TYPE=08 starting
another ISAM directory entry record. SYS6 is a partioned data set. The TYPE=08 records
are the directory entries for its members.
The ISAM directory data area is used by the SYSTEM loader to locate where a particular
member can be found in the file. The data area includes positioning information
indicating the exact byte position in the PDS which is the first record of the member.
The six-byte data field is further divided into sub fields. The first byte (in this case,
21) is the ISAM entry number. This entry number is provided to the system loader when a
library command is parsed by the command interpreter. The entry number is the PDS member
that will execute your request. The system loader searches the PDS directory for a
matching directory record. The next two-byte sub-field is the transfer address of the
member. The transfer address is contained in the directory so that more than one transfer
address can be applied to a member. Therefore, a member can have multiple entry points.
The last three-byte field is the triad pointer which points to the first byte of the
member. The triad pointer is composed of the Next Record Number (NRN) and Relative Byte
Offset for the member's first record byte. The system then positions to the pointer and
loads the member. Thus you have six bytes of data as specified by the LENGTH byte. Since
the process uses an index (the directory) to locate the member's starting byte then
proceeds to sequentially read the member, the access method is termed "Indexed Sequential
Access Method" (ISAM).
A TYPE-08 record can also have a 9-byte data area. In the PRO-PaDS utility available from
MISOSYS, the ISAM directory entry record includes a three-byte subfield which contains
the TRUE length of the member. The position of a member's logical end-of-file (EOF) can
thus be calculated by adding its length to its position and adjusting for sector
boundary alignment.
While you are looking at the first sector of SYS6, proceed to the first byte following
the last ISAM directory record. You will observe the sequence:
0A 01 00 04 01 00 01 02 00 26 ...
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