MIDAS
From Computer History Wiki
An assembler with macros written at MIT. First for the TX-0, then ported to the PDP-1, and then to the AI Lab PDP-6. The primary assembler in ITS.
Due to the popularity of some ITS applications such as Maclisp and MacHack VI, MIDAS was ported over to other PDP-10 operating systems in order to assemble those applications.