These machines have a peculiar (to me) memory format, it's not really obvious, so I worked it out and wrote it up here http://wps.com/projects/LGP-21/Documentation/arithmetic.html simh adds to the confusion, somewhat, because it displays memory by default in "normal" mode. e -m address does the right thing for instructions, but it's missing a -option to display numerical data; for example a "2" stored in LGP memory shows as "4" in simh because of the spacer bit. Memory is 31 bits wide, the accumulator is 32 bits...