Comment by doug-moen

> You surely aren't advocating that hardware predating PDP-11 for a decade are more powerful.

That's correct. The PDP-11 used for the first Unix system had 24KBytes of memory, and no virtual memory. The kernel and the current running process had to both fit in 24KB. This PDP-11 minicomputer was vastly underpowered compared to ten year old mainframes (but was also far less expensive). The ability of Unix to run on such underpowered (and cheap) machines was a factor in its early popularity.

BCPL was first implemented on an IBM 7094 running CTSS at Project Mac at MIT. This was one of the most powerful mainframes of its era. It had 12× the memory of the Unix group’s PDP-11, plus memory protection to separate kernel memory from user memory. One of the historical papers about C noted that a BCPL compiler could not be made to run on the PDP-11 because it needed too much memory. It needed to keep the entire parse tree of a function in memory while generating code for that function. C was designed so that machine code could be generated one statement at a time while parsing a function.