On Sat, 16 Nov 2019, Peter Jeremy wrote:

> More than just the instruction set - IBM published a formal description
> of the S/360 (in APL in the IBM Systems Journal issue that announced the
> S/360).  The S/360 was (I believe) the first case where a company
> announced a computer architecture (rather than an implementation) and
> implementations were expected to precisely comply with the architecture
> (no more finding undocumented instructions and side-effects and writing
> code that depended on them).  This meant that clone makers could build a
> clone that accurately emulated a S/360.

Ah, I'd forgotten about the APL documentation; thanks!  Talk about giving
away the keys to the kingdom: Amdahl, Fujitsu, Hitachi...