APL on the 5100/5110/5120 was based on the IBM mainframe APL/SV, (follow-on to
APL\360), with several added features. The "360 emulator" was not a full
emulator of 360/370, but only emulated exactly those instructions needed by the
APL interpreter (that was written in 360/370 assembler language).
Similarly, the System/3 emulator that was developed for the 5100/5110/5120
BASIC implemented only those instructions that were needed by the S/3 BASIC
software. System/3 was a machine with limited amounts of real CORE memory, so
developers recognized early on that they needed to implement a kind of a
"virtual machine" for BASIC, and that was coded in S/3 assembler language. This
layer also provided the "virtual" memory for BASIC use, done via "software
paging" on a machine (S/3) that had no hardware for virtual memory, relocation,
etc.
See also:
https://www.glennsmuseum.com/items/s3m6/
Glenn Henry was instrumental in the development of the S/3 Model 6 BASIC, and
later led the effort to create the IBM System/38. He was eventually promoted
to become an IBM Fellow. The design of the virtual instruction set used for
S/3 BASIC was used to create the "scientific instructions" for the S/32, S/34
and S/36 that was used by BASIC and FORTRAN on those systems.
Mark S Waterbury
