Are you guys talking about “A formal description of System/360” by Falkoff, Iverson and Sussenguth? It uses an APL like notation but not exactly a S/360 emulator in APL! Much more concise than the S/360 POP.

http://www.cs.trinity.edu/~jhowland/class.files.cs2321.html/falkoff.pdf

>> On Nov 18, 2019, at 8:43 AM, Clem Cole <clemc at ccc.com> wrote:
> 
> 
> 
>> On Sun, Nov 17, 2019 at 12:14 AM Dave Horsfall <dave at horsfall.org> wrote:
>> 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...
> 
> The cat was already out and poking around with the publishing of:  IBM 360 Principles of Operation, DOC A22-6821-0. The APL version of spec just gave it more area to roam.
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