Smal8 Linker

One of my objectives in choosing the SMAL assembler as the starting point for my relocating assembler is that it can also behave as a linker. The way this works is that the assembler resolves what symbols it can, keeping track of segment names and whatnot, and emits in the ".o" file a kind of stylized assembler code. Fully resolved instructions are emitted as constants, relocatable values are typically output as <symbol> or <symbol>+<constant>, and (in smal8) MRI instructions are represented as the MRI operation followed by one of the above. In addition, there are instructions to set the location counter (".") to constant or relocatable values.

Effectively then, the first assembly resolves all the local references, and emits a ".o" file suitable for another pass through the assembler, this time with the ".o" files for the other modules being linked. The result will also be a ".o" module; similar to the first, but hopefully without relocatable values. Unresolved references will give error messages, as you'd expect from a linker.

When the final ".o" file is produced, it will contain only constants and assignments of constants to the location counter. I wrote a simple Perl script (o2bin.pl), which reads this and converts it into the standard PDP-8 BIN loader format, if desired.

The other bit of magic that is needed in a linker, is the bit that searches the library for the relevant ".o" files, and includes them in the binary being linked. The solution for that is another Perl script (mkDIR.pl), which scans the library files, and builds a "RESOLVER" macro. This macro knows which symbols are in which files, and uses the conditional assembly features of smal8 to INCLUDE the relevant ".o" if one of it's symbols is an unresolved forward reference. The mkDIR.pl script creates this macro (and invocation) in clib/DIR, where it can be included by the linker shell script.

The linker shell script, then, just sets an origin, "INCLUDE"s any files explicitly referenced on the command line, and "INCLUDE"s clib/DIR.

You can go back to the "C" page, or on to the library page.




Last updated on 04/30/12 02:07

vrs