Fun fact: The code for the IMP (Internet Message Processor), a modified Honeywell 516, was programmed on a PDP-1. Particularly, this was the PDP-1d at BBN and the code was programmed by Bernie Cosell, Will Crowther – yes, this Will Crowther! –, and Dave Walden.
This is really a testament to the versatility of this early workstation/small mainframe, which was first presented in 1959. When the BBN PDP-1d was connected to the ARPANET, new software was loaded into the IMPs via the network itself. This may have been well the first occurrence of update by network downloads.
More information on how the IMP code became to be can be found here: https://walden-family.com/bbn/imp-code.pdf
One of my recent blog post brought me into contact with Marc Ramsey, who had programmed the ELF nodes, which linked up a bunch of mainframes (CDC 6600, 7600, Cray 1, and IBM 360/370). This was a PDP-11/40 running RSX-11M that emulated a variety of remote job entry stations, with a hard-wired interface to an IMP. The interface code was written in Fortran IV-Plus. Marc says, he doesn’t remember what ELF actually stood for, and that seems to have been lost to history.
Indeed, as early as 1978, the J.A. Payne report on Host to Host Access refers slightly embarrassed to the system just as “the PDP-11 operating system known as Elf”.