We had devices like those in university, left from previous generations where they'd manually program Z80s ROM by hand through switches and see the output on LEDs.
Then programing(not the correct term) CPLDs via obscure programming languages and some MS-DOS tool via the parallel port.
Well, at the recent VCF (vintage computer festival) Midwest they showed a robot using the Altair's front panel to toggle in a game:
Of course you can also try to reprogram the BIOS ROM on the motherboard, but you're likely to need a full OS to first reverse engineer it enough to know where to stick your debugger code.
For example, you could build yourself a makeshift hex keyboard and load a driver for that keyboard onto an ATmega. Good! from that point on, you can enter bytes faster.
Then, you hook a few seg7 together to have some kind of display. Even better!
... and so on, until you can drive that FDD.
ATmega8 happens to satisfy those requirements. It's also not too hard to program microcontrollers that would support human level inputs.
In practice, the bootstrap device will be like a Windows7 laptop we've got squirreled away somewhere. It's probably cheaper to buy up a ton of used / old laptops that are sufficient for bootstrapping rather than trying to build methodologies that bootstrap from scratch.
The next step up is to use a hex keypad to enter 16 bits at a time. Some single board computers from the 1970's provided this. It was marginally practical - I used to enter programs of a couple of hundred bytes into a KIM-1 this way, it only took a few minutes.
Those computers did provide a simple monitor in ROM to support this, but that code was very small - Steve Wozniak's Wozmon monitor for the Apple I was 256 bytes. You could probably design some circuitry not much more complicated than in this article to replace its row of switches with a keypad.
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 .....
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. ..."
cameron_b•3mo ago
I've seen a lot of hand-waving explanations of how boot-strapping happens (and granted it is very implementation-specific) but this is delightfully complete without veering off into the rough.