content: I learned to program on a PET, first in BASIC, then I started writing little assembly programs to speed up the slower bits of my programs. With a machine like a PET, every bit of hardware is directly addressable. To program it in this way you need a manual that gives you the direct memory addresses of every bit of hardware on board. there is no abstraction layer whatsoever. Want to read from a serial port? you pull a value from a specific location. want to place a character on the screen? you need to know the address of that position on the screen and then push the ascii value into it. With a computer this simple, it's well within a hobbyists capability to learn the machine completely. This got progressively more difficult, and it's probably not really worth the insane effort required to learn what makes something like an Amiga or a modern PC tick like this unless you're actually a genius working in hardware development. So it's not really about what the machine can do, but what you can learn about all the functions that still make up a modern computer, at the level of the metal, in a way that would be near impossible now due to the incredible complexity of a modern PC.