PDP-11 Binary that can execute on Unix and RT-11

A binary executable image on PDP-11 Unix and RT-11 have headers that almost
don't overlap. With care, a binary image can be created that can execute on
either Unix or RT-11.

The Unix header is the first 16 bytes, and immediately followed by the entry

The RT-11 header is from &o40 to &o57 and specifies the entry point.

With care, the first 256 bytes of a binary image can be written to allow the
code to be loaded and executed on Unix or RT-11. With different entry points
the code can then set things up internally so that the mainline code calls
the appropriate system calls, TRAPs for Unix and EMTs for RT-11.

UnixRT11 shows how the Unix and RT-11 headers compare and how to arrange
them in a single binary to run on either Unix or RT-11.

UnixRT.mac is a demo program that does this.

This screenshot shows the same binary running on RT-11 and Unix.

This is based on Paramucho's adaptation of the Hello Word demo program for
the AsmPDP assembler.