IIUC, that's what sci-fi LED panels of really old computers were. They showed all the internal statuses of the CPU as well as CPU-RAM bus. And the toggle switches allowed individual bit overrides.

The operator sets a CPU RESET switch to RESET, then powers on the machine, and start toggling RAM address and data switches, like HHLL HLLH HHHL LLLL. The operator then press and release the STEP push switch. The address 0b 1100 1001 is now set to 0b 1110 0000. This is repeated until the desired program or a bootloader is all complete. The operator finally sets CPU RESET to Normal, and CLOCK dial to RUN.

The CPU exits reset state, initializes program counter with reset vector, e.g. 0b1000, and start executing instruction at PC++. 1000, 1001, 1010, so on. Then oh no, the EXCEPTION indicator comes on, the LED shows 0b 1110 0000. That's divide r0 by 0, etc.

They didn't actually spend every half a day toggling those switches. They loaded their equivalents of bare minimum BIOS recovery code, then the rest wad loaded from magnetic or mechanical tapes. Only when computers were booted up blank slate or crashed and in need of debugging, the users resorted to that interface.

If they had the CPU-RAM main bus split into ROM and RAM address ranges in such ways that setting address to reset vector will yield the first byte of a BIOS program lithographically etched into the ROM chip, then simply powering on the machine will do the same thing as loading the BIOS manually.

There were also things like magnetic core memories. They didn't require lithography to fabricate, and there were both ROM and RAM kinds of those.

For the microcode ROMs they can just be “hardwired” with a zillion simpler gates. This has the added benefit of supporting way higher clock speed. For my planned program ROM you would either have to input manually like the first computers, or use other things like punch cards or your computer would be again “hardwired” to load programs from some other media

Actual application code was hardwired, entered manually with switches and lights, or with punch cards. Later, when ICs were sufficiently advanced, mask-programmed ROMs/PLAs.

When I was building embedded controllers with 6502 processors in the 1970s and 1980s we used UV erasable EPROMS and a programmer (my own design) with a ZIF socket built on an expansion board in an Apple ][. The prototype board also had a ZIF socket but the production boards would have ordinary DIL sockets, our production volume was too low to warrant ordering actual ROMS so we used the UV erasable ones and put a metallised sticker over the window.

All programming was done on the Apple in 6502 assembler. It took 45 minutes to assemble an 8kB rom image. This meant that you took extreme care to think about what the code was doing as assembling a new image was often the most time consuming part of the Edit-Assemble-Test loop.

He says that's for microcode ROMs though? As opposed to a user program written in machine code that you would use the CPU to execute. I don't believe ancient CPUs had microcode. Everything was implemented in hardware.

I'm going off memory (of a book, not that I was alive in the 40s, ha) so grain of salt etc but I believe the very earliest (edit: electronic, digital) computers were literally rewired every time they need to be re-programmed.

Plugboards! Think telephone exchange but used as a ROM.

Here is a blog post describing how to do it with a PDP8. Note: he used an assembler but in those days you had do translate to machine code by hand.

https://raymii.org/s/articles/Toggling_in_a_simple_program_o...