[TUHS] origin of C header files

Doug McIlroy doug at cs.dartmouth.edu
Sun Dec 31 12:32:08 AEST 2017


Warning: off-topic info

> I was told once that McIlroy and Morris invented macro instructions
> for assembly language.   And certainly they wrote the definitive
> paper on macros, with, as I recall, something like 10 decisions you
> needed to make about a macro processor and you could generate 1024
> different macro systems that way.  I wonder if that ever got
> published 

The suggestion that I invented macros can also be found on-line, but
it's not true. I learned of macros in 1957 or before. GE had added
a basic macro capability to an assembler; I don't know whether they
invented the idea or borrowed it. In 1959 George Mealy suggested
that Bell Labs install a similar facility in SAP (SHARE assembly
program). Doug Eastwood wrote the definition part and I handled
expansions.

Vic Vyssotsky later asked whether a macro could define a macro--a
neat idea that was obviously useful. When we went to demonstrate
it, we were chagrinned that it didn't work: definition happening
during expansion resulted in colliding calls to a low-level
string-handling subroutine that was not re-entrant. Once that
was fixed, Steve Johnson (as a high-school intern!) observed
that it allowed the macro table to serve as an addressable
memory, for which the store and load operations were MOP
(macro define) and MAC (macro call).

Probably before Steve's bright insight, Eastwood had folded
the separate macro table into the opcode table, and I had
implemented conditional assembly, iteration over a list, and
automatic generation of symbols. These features yielded
a clean Turing-complete language-extension mechanism. I
believe we were the first to achieve this power via macros.
However, with GPM, Christopher Strachey showed you don't need
conditionals; the ability to generate new macro names is
enough. It's conceivable, but unlikely, that this trick
could be done with earlier macro mechanisms.

As for publication, our macroprocessor inspired my CACM
paper, "Macro nstruction extension of compiler languages",
but the note that Steve remembers circulated only in the
Labs. A nontrivial example of our original macros in
action--a Turing machine simulator that ran completely within
the assembler--was reproduced in Peter Wegner's programming
book, so confusingly described that I am glad not to have
been acknowledged as the original author.

Doug


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