On Sat, Apr 11, 2015 at 11:13 AM, Marvin Johnston <marvin at west.net> wrote: That was the MR16-D "constants memory", M7325. It stored 24 words of 16 bits each, so the overall capacity was 48 bytes. The module sold for $150 in 1972. Unlike the other PDP-n machines, the PDP-16 was not a computer with a standard instruction set, but rather was essentially a set of Register Transfer Modules (RTM) primarily intended for building hardwired systems using asynchronous control logic, though there were also 16-bit data path and memory modules, including larger ROM modules that could be used as a microcode control store. The asynchronous control flow worked in a similar manner to many early PDP-n machines, where at any given moment usually only one functional was active, and when it was done, as it became inactive it sent a pulse to the next functional unit. Branches in flow were done by having a unit decide which of several units to send the pulse to. In the PDP-16 RTMs there was also provision for subroutines. However, DEC did offer a preconfigured PDP-16/M computer, which was built from the PDP-16 RTM modules, but used them to implement a more conventional 16-bit processor which did have a standard instruction set (though not matching that of any other DEC machine). A normal PDP-16/M used the PDP-8/M chassis and power supply, though it could also be purchased as only the backplane and modules.