[TUHS] speaking of early C compilers

[John Cowan]

Ronald Natalie scripsit:

> Amusingly, if you read the DEC processor handbook, it says the TRAP
> instruction is designed to invoke system calls.    UNIX did this
> of course.   Amuslingly, the DEC operating systems, including RSTS,
> used EMT to invoke system calls.   The book says this was put there
> to allow emulating other OSs.    Well this made it relatively easy.

Unless I have utterly forgotten, EMT was to be used by DEC and TRAP
by users.  Since Bell Labs was a user, they used TRAP (I asked ken --
I think it was ken, definitely not Ken -- about this).  Of course,
DEC didn't anticipate that any user would write an operating system!

On RSTS/E as opposed to RSTS, both EMT and TRAP went to the user
executive.  To reach the kernel, you did EMT 377 followed by another
EMT, but EMT 377; EMT 377 was vectored back to the user as an EMT 377.
That's what allowed RSTS/E to host other OSes that used EMT.

-- 
John Cowan          http://www.ccil.org/~cowan        cowan at ccil.org
Nobody expects the RESTifarian Inquisition!  Our chief weapon is
surprise ... surprise and tedium  ... tedium and surprise ....
Our two weapons are tedium and surprise ... and ruthless disregard
for unpleasant facts....  Our three weapons are tedium, surprise, and
ruthless disregard ... and an almost fanatical devotion to Roy Fielding....