Page 136 - DCAP507_SYSTEM_SOFTWARE

System Software

Notes Presume program B has table known as NAMES; it can be accessed by program A as follows:
There are three types of external symbols, as depicted:
1. Segment Definition/Program Name (SD)-name on START
2. Load Definition (LD)—stated on ENTRY. There must be a label in similar program with
similar name.
3. External Reference (ER)—stated on EXTERNAL. There must be a matching ENTRY, or START
in another program with the similar name.
The TXT records enclose blocks of data and the relative address at which the data is to be
positioned. Once the loader has decided where to load the program, it simply adds the Program
Load Address (IPLA) to the relative address and moves the data into the consequential location.
The data on the TXT record may be instructions, non-relocated data, or initial values of address
constants.
The RLD records enclose the following information:
1. The location and length of each address constant that requires to be changed for relocation
or linking.
2. The external symbol by which the address constant should be altered (added or subtracted).
3. The operation to be done (add or subtracted).

This RLD information notifies the loader to add the absolute load address of A to the contents of
relative location 52 and then add the absolute load address of NAMES to the contents of relative
location 56.
The END record states the end of the object file. If the assembler END record has a symbol in the
operand field, it states a start of execution point for the whole program (all subroutines). This
address is recorded on the END record. There is a final record needed to state the end of a
collection of object files. The mainframe loaders typically use either a loader terminate (LOT) or
End of File (EOF) record.

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