A bit more on the machine, from the 1958 Programmers’ Reference Manual. It’s core-based, with 28 bit words but addressed at the 6 bit character level. An instruction is 56 bits (eight characters) and takes two cycles, or 30 microseconds, to fetch. Addressing is 3 characters (18 bits) and each instruction has two addresses. Instructions may operate on characters or on words, or on records.
The machine’s arithmetic can be binary or BCD, in excess 3 style.
There’s a drum store for random access, paper tape, and lots of mag tape units can be attached.
The arithmetic unit does everything twice, to be sure: operating on the true and on the complement of the inputs. And the mag tape also stores every bit twice (it’s 16 tracks wide.) And there’s parity everywhere, at the character level.
Eight lowercase letters (ahqrstvx) are available on the Monitor Printer, but not on paper tape. The Online Printer offers 51 printable characters, and prints a 00 as an overprinted = + and @. None of which are individually printable.
The instructions allow addresses to be indexed, which offers some 5 index registers, implemented in particular triplets of locations in the core.
There’s an arrangement for slow I/O instructions to continue to completion while other instructions continue: one write and one read can be in progress while execution proceeds.