The Creation of the UNIX* Operating System: From B language to NB to C

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From B language to NB to C

The first version of UNIX was written in assembler language, but Thompson's intention was that it would be written in a high-level language.

Thompson first tried in 1971 to use Fortran on the PDP-7, but gave up after the first day. Then he wrote a very simple language he called B, which he got going on the PDP-7. It worked, but there were problems. First, because the implementation was interpreted, it was always going to be slow. Second, the basic notions of B, which was based on the word-oriented BCPL, just were not right for a byte-oriented machine like the new PDP-11.

Ritchie used the PDP-11 to add types to B, which for a while was called NB for "New B," and then he started to write a compiler for it. "So that the first phase of C was really these two phases in short succession of, first, some language changes from B, really, adding the type structure without too much change in the syntax; and doing the compiler," Ritchie said.

"The second phase was slower," he said of rewriting UNIX in C. Thompson started in the summer of 1972 but had two problems: figuring out how to run the basic co-routines, that is, how to switch control from one process to another; and the difficulty in getting the proper data structure, since the original version of C did not have structures.

"The combination of the things caused Ken to give up over the summer," Ritchie said. "Over the year, I added structures and probably made the compiler code somewhat better -- better code -- and so over the next summer, that was when we made the concerted effort and actually did redo the whole operating system in C."

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