Imagesource: https://youtu.be/DM4rZZBqXVM

Once you have completed the ISA and the basic architecture of your own CPU as well as input and output devices, you quickly face the next challenge. How do you boot your machine, as long as input and output devices can't be addressed yet, because no software can be pulled through the CPU byte by byte for exactly this task. Chicken & egg. 🐥

The solution nowadays is called BIOS, and when this did not exist yet, one packed basic routines into a ROM, which was placed in the address space in such a way, that the entry vector of the CPU pointed to these as the very first.

But what about machines built, when ROM was extremely expensive?

Dave Plummer from Dave's Garage asked himself this very question, and took up the challenge with an IMSAI 8080. The solution: addresses as well as data bytes of the boot code are manually poked into the machine's RAM using switches. Another exciting video after which you might be a lot smarter.