MIT PDP-10 'Info' file converted to Hypertext 'html' format by Henry Baker
In programming the PDP-10 it is convenient to imagine that your
program occupies contiguous virtual memory locations from 0 to some
maximum address. All memory locations are equivalent for most
purposes (but some operating systems reserve some of your space for
their own purposes).
Sixteen memory locations (addresses 0 to 17 - note that addresses
will appear in octal) are distinguished by their use as general
purpose registers (also called accumulators or index registers).
Most PDP-10 instructions address one memory operand and one
accumulator (so-called "one and a half address" architecture). This
means that nearly all instruction affect some accumulator. These
registers are actually implemented in high speed solid state memory
rather than in slower core. For any purpose where it is convenient
to do so, a user may reference an accumulator as memory.