This is the RSTS/E port of PDP-11 ALGOL, originally by Barry James Folsom and modified by Gregory D. Hosler. It consists of the compiler proper and several utilities, all written in ALGOL, and a runtime system that executes the pseudo-machine code generated by the compiler.
This ALGOL is a dialect somewhat like Burroughs Extended ALGOL, as implemented on the B-5500 and B-6000 series mainframes. The similarity extends to the generated code, in that it looks a bit like "a 16-bit version of the B-5500 mainframe instruction set".
The original version was for RT-11; the sources for that runtime version appear to have been lost. The version found here is a RSTS/E port of the original, by Terry Grieb and Paul Koning of the RSTS/E development team. What precisely that involved I no longer remember; most likely it's mostly in the I/O interface routines and similar OS connections. It is likely that the main pseudo-code execution code is pretty much unchanged.
Code found here is the ALGOL source code, assembly language source code of the runtime machinery, and the binaries for the compiler (ready to be executed by the runtime code) and the runtime system.
The first version was published in 1973, and Gregory Hosler's modifications in 1974. There is no indication a copyright claim was made at that time, which would imply the code is in the public domain. The changes by Terry Grieb and Paul Koning were not released as far as I know; for lack of better information I have attached an MIT license to this repository.
Reassembling the runtime system requires files common.mac and err.stb, both of which can be found on a RSTS kit. They are not included here fore copyright reasons.