Page 137 - DCAP507_SYSTEM_SOFTWARE

Unit 9: Design of an Absolute Loader

The simple programs PGA and PGB demonstrate a broad range of relocation and linking Notes
situations. The display shows the ESD, TXT, and RLD records formed by the assembler for PGA
and PGB, respectively. Lastly, we show, the contents of main storage after the programs have
been assigned space, relocated, linked, and loaded.
You should scrutinize these figures cautiously and validate the accuracy and reasons for each
value. A few particular points in these examples should be observed. Both PGA and PGB enclose
an address constant of the form A(A2-A1-3).

You should observe that both instances of this address constant (location 152, 200) have the
similar value. Since both A2 and A1 are symbols internal to PGA, the assembler processing PGA,
can work out the whole expression and find out the value of 7. We observe that the TXT record
for location 48-51 includes the 7 and there are no connected RLD records for address constant.
Alternatively, these symbols are external PGB; thus, the assembler, while processing PGB has no
means of assessing the address constant. This is demonstrated also. The TXT record for relative
location 32-35 comprises a -3, the only part of the address constant that can be gauged by the
assembler. The last two RLD records inform the loader to add the load address of A2 to location
32-35 and then subtract the load address of A1. When processed by the loader, this address
constant in PGB will certainly have the similar value as the one in PGA. As the direct linking
loader may come across external references in an object file which can not be assessed until a
later object file is processed, this type of loader needs two passes. Their functions are very alike
to those of the two passes of an assembler. The main function of pass 1 of a direct linking loader
is to assign and assign each program location in core and create a symbol table filling in the
values of the external symbols. The main function of pass 2 is to load the definite program text
and carry out the relocation modification of any address constants requiring to be altered. The
first pass assigns storage locations to all segments and accumulates the values of an external
symbols in a symbol table.

Did u know? The external symbol occurs as local definitions on the ESD records of another
assembled program.

!
Caution For every external reference symbol there must be an analogous internal symbol
in some other program.

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