On Wed, Sep 1, 2021 at 12:07 PM Ron Natalie <ron@ronnatalie.com> wrote:
I disagree.  TRAP according to the processor handbook was intended to be used for what UNIX calls system calls.    EMT was the emulator trap used to simulate other operating systems on the same hardware.

The 1969 PDP-11 Handbook <http://gordonbell.azurewebsites.net/digital/pdp%2011%20handbook%201969.pdf> says on p. 41 (PDF page 49): "[Trap] instructions provide for calls to emulators, I/O monitors, debugging packages, and user-defined interpreters", but it does not define "emulators".  (The OED has several citations for this sense of "emulator", but before about 1985 the context seems to be hardware emulation only.)

Nevertheless, DOS/BATCH-11 (1970-71) already uses EMT as the system call instruction, and it is clear that TRAP was for user use.  At that point, the only other operating system that could be emulated would be the Paper Tape Software that I mentioned, which used IOT.

(BTW, RSTS/E's hypervisor would reflect any EMT instruction to the RTS (the actual or emulated supervisor running in any given process, such as Basic-Plus, RT-11, RSX-11, or Teco), with the exception of an EMT 377, EMT 377 sequence, which was a syscall to the hypervisor itself.)