APEXC

The APE(X)C, or All Purpose Electronic (X) Computer series was designed by Andrew Donald Booth at Birkbeck College, London in the early 1950s.

[5] In 1947, along with his collaborator and future spouse Kathleen Britten, he spent a few months with von Neumann's team, which was the leading edge in computer research at the time.

The APEHC was a punched card machine while the APEXC, APERC and APENC were teletypers (keyboard and printer, plus paper tape reader and puncher).

In March 1951, the British Tabulating Machine Company (BTM) sent a team to Andrew Booth's workshop.

The machine code is made of 15 instructions only, namely addition, subtraction, multiplication, load (3 variants), store (2 variants), conditional branch, right arithmetic bit shift, right bit rotation, punched-card input, punched-card output, machine stop, and bank-switching (which is never used on the APEXC, since it only has 1024 words of storage, and addresses are 10-bit-long).

A so-called vector mode enables to repeat the same operation 32 times with 32 successive memory locations.

The processor is fairly efficient: most instructions take only 2 word cycles (1 for fetch, 1 for read operand and execute), with the exception of stores, shifts and multiplications.

Note there is no read-only memory (ROM), and therefore no bootstrap loader or default start-up program whatsoever.

It is believed that no executive or operating system was ever written for the APEXC, although there were subroutine libraries of sorts for common arithmetic, I/O and debug tasks.

When starting the machine, the address of the first instruction of the program to be executed must be entered in the control panel, then the run switch must be pressed.