Page 135 - DCAP507_SYSTEM_SOFTWARE

Unit 9: Design of an Absolute Loader

Alternatively, consider adjusting the above example: Notes
DAWG DATA 5
CAT ADDR A(DAWG)
END

CAT must enclose the absolute address of DAWG. The assembler recognizes only that DAWG is
96 bytes from the starting of the program, so the loader must add to this the load address of the
program in locating the absolute address to be included in CAT.
Let us elucidate the scope of the address constant problem. An address constant may be
1. absolute;
2. simple relocatable; or

3. complex relocatable.
The direct linking loader processes programs produced by the assembler, FORTRAN compiler,
or some other compiler.

Did u know? Neither the original source program nor the assembler symbol table is
obtainable to the loader.

!
Caution The object file must enclose all information required for relocation and linking.
There are four segments to the object file (and four corresponding formats):

1. External Symbol Dictionary (ESD)
2. Instructions and data records, called "text" of program (TXT)
3. Relocation and Linkage Directory (RLD)

4. End (END)
The ESD records enclose the information essential to build the external symbol dictionary or
symbol table. External symbols are symbols that can be referred away from the subroutine
level. Only the assembler utilizes the normal labels in the source program, and information
regarding them is not incorporated in the object file.

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