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.    Oddly, for 
some reason, all the DEC OSes use EMT instructions for their system 
calls.   This came in handy when JHU ported BasicPlus from RSTS to UNIX. 
     That executable could run fine on UNIX because we caught the few EMT 
traps that mattered to us and simulated them.   The only thing we had to 
do other than that was to add a "nostack()" system call that got rid of 
the normal UNIX-maintained stack starting at the address space (RSTS 
executables like many DEC OSs used a stack that started around 1000).

Many of the UNIX signals come straight from PDP-11 traps:  SIGFPE, 
SIGIOT, SIGSEGV, SIGBUS, SIGILL, SIGEMT. and those traps invoked those 
signals.

FPE - floating point exception
ILL - illegal exception (either unknown opcode or CERTAIN of the 
privileged instructions, others were ignored)
BUS - fatal unibus timeout trap.   Usually an attempt to access a 
memory/unibus address that doesn't respond, or to do word accesses on 
odd boundaries.
SEGV - accessing memory not mapped to you
IOT - the IOT instruction
BPT - the BPT instruction
TRAP, EMT - these instructions