V7M/src/cmd/as/README

To assemble files intended for overlay use, a special
assembler is needed.  The changes are in as11.s and as27.s.
The code is controlled by the symbol 'overlay', look at the
makefile.  The files ov.s and nov.s define the symbol
appropriately for the overlay or non-overlay version of the
assembler.  The overlay assembler is installed as /bin/ovas
and /lib/ovas2.  Note that this could probably be better done
with an option to the assembler, but I was in a hurry and the
assembler is pretty ugly to muck around with.

The basic problem is that all resolution of global text addresses
must be delayed until the file is processed with the C overlay
loader (covld) so that if the file lives in an overlay, all references
to the global text symbols can be changed to references to the thunk
that corresponds to the procedure.  The normal assembler resolves all
references to text symbols that are defined in the current assembly.
The overlay assembler forces all references to global text symbols
to be treated as references to undefined external symbols, to be
resolved by covld.