Assembler. And before that, nightmare territory of having to program directly at the per-bit level, by "literally" throwing the switches.
The earliest concepts of what one might think of as an assembler appeared around 1947 on the EDSAC.
https://www.davidsalomon.name/assem.advertis/asl.pdf#page=16
All the power we have on tap today of course means an assemblers role is somewhat diminished by brute force capability. But there's a lot to be said for being able to operate right down at the hardware level. How many attack vectors are symptoms of design decisions in higher abstraction layers? Too many.
Fully accept practicality of writing a full blown OS on todays myriad and modular hardware is a tough ask. Some people have tried to do it.
Biting the hand that feeds IT
