Back in the late 70s, I did the bring-up of a Z80 system. I coded an EPROM (2708) with 1KB. Wrote a very small bootstrap monitor that did not require RAM to be operational. Drove a serial terminal (autobaud on CR entry). Display/Set memory and Go to address. Just enough to enter a memory test and a disk bootstrap, which was then added to the EPROM. So, there are examples of "zero RAM" programs. With no stack, use a single level of subroutine - LD DE,ret addr, LD HL,subroutine, JMP (HL), and at the end of the subroutine: LD HL,DE and JMP (HL) did it. The EPROM had to work on both 8080 and Z80 CPUs. The code had to be small, because it was manually entered into an EPROM programming device. Fairly common stuff back then... I enjoyed this kind of work. No debugger, no safety net. The code had to be simple, understandable, and work. However, these days I still don't use debuggers (much). This may be slowing me down; not sure about that.