Delete comment from: Ken Shirriff's blog
@AGCBubba: No, the computer could indeed be programmed both directly in assembler and in this other, interpreted, high(er)-level language. Assembly code would run directly on the AGC's CPU, whereas for the interpreter language, the AGC's ROM contained a program implementing a virtual machine which could be invoked and would then execute code written in that interpreted language.
Both kinds of code can be intermixed in the assembly listings and the assembler took care of transforming each kind of statement to the correct machine code.
Some more (and perhaps better) explanations can be found here:
https://www.ibiblio.org/apollo/assembly_language_manual.html#The_Interpreter_vs._the_CPU
https://www.ibiblio.org/apollo/assembly_language_manual.html#MarkSchulman
As for the bits about "operating system" and "do many things at once": The software for the AGC did actually support preemptive multitasking of various subprograms, so the OS moniker for the code controlling that doesn't seem that inappropriate.
Don Eyle's page has some description on how this worked:
https://www.doneyles.com/LM/Tales.html
Jul 17, 2021, 7:04:45 AM
Posted to A computer built from NOR gates: inside the Apollo Guidance Computer