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“Hello World” Under ITS On PDP-10

From	Lawrence D’Oliveiro <ldo@nz.invalid>
Newsgroups	alt.folklore.computers
Subject	“Hello World” Under ITS On PDP-10
Date	2026-01-19 04:14 +0000
Organization	A noiseless patient Spider
Message-ID	<10kkb2u$rfa$1@dont-email.me> (permalink)

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Out of the handful of groundbreaking operating system developments on
various hardware platforms in the 1960s and 1970s, DEC’s PDP-10
computer has to be unique in being the host for, not one, but two of
them.

Among pre-RISC hardware architectures, the ’10 is fondly remembered as
being an elegant design, in that writing assembly language for it was
not the complete pain in the bum that it was on too many other
machines.

One of those OSes was ITS, which stood for “Incompatible Timesharing
System”, created at MIT. In the same sort of way that the “Unix” name
was a play on “MULTICS”, so ITS was a play on the name of one of the
earliest timesharing developments, “CTSS” (the “Compatible
Time-Sharing System”).

CTSS was “compatible” in that it had to coexist with the
vendor-provided batch-oriented OS on the IBM machines where it was
developed; the submission of regular production batch jobs could not
be disrupted by the running of this new, experimental OS. Whereas ITS
could be considered “incompatible” in that it didn’t need to coexist
with anything else, I guess.

ITS was a system created by hackers, for hackers. As an example, when
you logged in, instead of launching a command shell, you were running
a debugger as your primary user interface, instead. Using the system
involves issuing an interesting mix of debugging-specific commands and
commands that look more like regular command-shell ones.

Here <https://www.youtube.com/watch?v=kVmFqmiiM-0> is a presentation
by Lars Brinkhoff, showing what it’s like to write a simple “Hello
World” program in assembly language, build it and run it.

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“Hello World” Under ITS On PDP-10 Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-19 04:14 +0000
  Re: “Hello World” Under ITS On PDP-10 jayjwa <jayjwa@atr2.ath.cx.invalid> - 2026-01-19 12:12 -0500
    Re: “Hello World” Under ITS On PDP-10 Anthk <bozo@dev.null> - 2026-04-02 14:03 +0000
      Re: “Hello World” Under ITS On PDP-10 Rich Alderson <news@alderson.users.panix.com> - 2026-04-02 14:59 -0400

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