As a electronics engineer - I believe it would be entirelyy possible to start a computer or microprocessor from nothing, no boot rom or nothing. Would need RAM
How?
Sequence would be
Power on switch - turn ON.
Pause/halt computer to ON - ( ie computer is now not operating instructions but when pause/halt switch is reverted it will start to ....)
Now use hard ware to directly write data to a nominated memory address.
This hardware could be permanently included in computer circuitry or temporarily connected.
This hardware would have entry switches ( could be simple switches( on switch for each binary bit ) OR BCD encoded ( rotary? switch ) to enter the memory address and data.
The person entering the program would need to be familiar of the memory map of the computer / processor.
Ie it may need certain (interrupt/reset/restart) vectors , start addresses set correctly
Person would enter address and data and then press a switch that would cause the data to be written to the address nominated by switches.
Would be laborious to enter the needed few hundred bytes but would be possible.
Could be other included switches to make this memory write protected etc ...
A summary of the above would be "...use abank of physical switches to directly input binary data to memory " while the computer is halted.
Back in the "old days" the 80's ... we would get a static RAM chip (6116) put a battery on top of the chip along with a bit of circuitry to make "battery backed" could use the physical switch method to load a program into such a setup and then plug this RAM into computer, would need to be a right location in memory map for it to work...
From Google - describes such a circuit / mod from Elektor magazine ...
"An Elektor article from June 1985 describes a plugin circuit for using two battery-backed 6116 RAM chips as a replacement for a 2732 EPROM. This circuit uses a 2-bit binary decoder to select the correct 6116 RAM chip, ensuring proper operation and data retention via a battery backup, a key feature of the low-power "LA" version of the 6116 chip. ..."
How? Sequence would be Power on switch - turn ON. Pause/halt computer to ON - ( ie computer is now not operating instructions but when pause/halt switch is reverted it will start to ....)
Now use hard ware to directly write data to a nominated memory address. This hardware could be permanently included in computer circuitry or temporarily connected.
This hardware would have entry switches ( could be simple switches( on switch for each binary bit ) OR BCD encoded ( rotary? switch ) to enter the memory address and data.
The person entering the program would need to be familiar of the memory map of the computer / processor. Ie it may need certain (interrupt/reset/restart) vectors , start addresses set correctly
Person would enter address and data and then press a switch that would cause the data to be written to the address nominated by switches.
Would be laborious to enter the needed few hundred bytes but would be possible.
Could be other included switches to make this memory write protected etc ...
Sorry if the above is a bit confusing .....