PDP-11 Architecture - PDP-11 Lore

PDP-11 Lore

A (false) folk myth is that the instruction set architecture of the PDP-11 influenced the idiomatic use of the B programming language. The PDP-11's increment and decrement addressing modes correspond to the −−i and i++ constructs in C. If i and j were both register variables, an expression such as *(−−i) = *(j++) could be compiled to a single machine instruction. Dennis Ritchie unambiguously contradicts this folk myth, noting that the PDP-11 did not yet exist at the time of B's creation. He notes however that these addressing modes may have been suggested by the auto-increment cells of the PDP-7, though the implementation of B did not utilize this hardware feature. The C programming language did however take advantage of several low level PDP-11 dependent programming features, resulting in the propagation of these features into new processors.