I cut my teeth on a TRS-80 Model 1 with 4K of RAM. I was like 13 or so and got one in the late 70s. I learned BASIC on it, but quickly ran out of CPU and memory with the interpreted BASIC and switched to Z-80 assembler. I didn't have an assembler however, so would write out all of the Z-80 instructions by hand on paper, then, when I thought I had debugged it enough on paper, I would convert it to the hex OpCodes. At that point I would either use a small BASIC program to POKE the hex into memory and run it, or after I got a hex editor, I would use that to type in the hex values. Of course, all of this was saved to cassette tapes! If there was a bug I would go back to the paper and figure out where I went wrong. I still remember some Z-80 OpCodes to this day from the countless hours of converting assembler to hex.
One really cool way I would use to debug programs, or at least know if they were running, was to keep an AM radio next to the computer. The TRS-80/Z-80 would give off enough RF so you could "hear" the program running on the radio. In fact, you could devise loops and delays in your assembler such that you could "play" music on the radio. :-)