The BLISS language doesn't have any I/O capability built into the
language (as do BASIC, Fortran, COBOL, PL/I).
Sorry for the strange side trip ... you didn't parse my words careful (which I know can sometimes be a challenge). What I said was that CMU had a rich set of BLISS system
services where I/O was one set of those services.
I did not say it was part of the language
. But I/O
was very much part of way we programmed and we moved code from the PDP10 and the 11's reasonably freely that was not intended to be machine specific, particularly since the PDP11 compiler was a cross compiler that ran on the 10.