1971, TOPS-10 BASIC (BASIC-10) and BBCX.

The latter was a pseudo-assembler, actually written in BASIC (by my teacher), as a part of the then experimental curriculum for the first computer science national certificate (CSE) to be taught in UK schools. I think the name reflected the idea that the course was going to be backed by a series of BBC "Programmes for Schools" radio broadcasts, but I don't recall ever hearing any of them.

It simulated a processor that had 1 8-bit accumulator register, 1 flags register, a program counter and 32767 memory addresses. It had a fairly limited set of instructions:

• GET n -- load the accumulator with the value stored in memory location n (or a constant).
• PUT n -- store the value from the accumulator to memory location n (or a constant).
• ADD n -- Add the value from m.l. n (or a constant) to the accumulator.
• TST n -- Set the flags to reflect the result as if n (constant or memory) had been subtracted from the accumulator.
• JMP n -- Set the program counter to n.
• CALL n -- call the subroutine located at address n.
• ... etc.

It was a PIA to program, but very instructional. It certainly achieved it's aim of giving you a very good understanding of what was going on at the machine level. My first "big project" was to implement integer square root using the Newton method. It took me 3 weeks, though most of that was spent waiting for the twice weekly turnaround of coding sheets.