That reminds me of the APL\360 implementation running on the IBM 5100 and
5110 using a rudimental System/360 emulator written in PALM machine code
stored in the APL Executable ROS. The APL interpreter is stored in the APL
language ROS and was accessed like an I/O device, i.e. the emulator
fetched the 360 instructions with instructions like GETB and so on.
Also, I *think* that the PALM assembler used by IBM was written in APL, at
least that's the impression I got when reading the documents from H. J.
Myers "Instructions for the use of the Assembler Generator", and IBM
report no. ZZ20-6431 "A Fast Assembly Technique Using APL"