Inception

When I was 14, I built an Altair 680b computer from a kit. This was in the earliest days of personal computing. Earlier than that, even. There were instructions. There was a technical manual. There was a CPU manual. But it came with no software, no operating system, no applications. There was not even a hard disk. For the first six months I had the machine, there was no way to save a program; everything was lost the instant I turned the computer off.

Later, I bought a 16K memory card (for $400 IIRC). It came with a text editor, an assembler program, and a debugger program. But, they were on paper tape, and I did not have a paper tape reader. I spent a lot of time looking at the paper tapes I had, longing for a tape reader or an assembler program I could use.

Eventually, I purchased a cassette tape interface, so I could store programs. As it happened, the tape interface came with a 4K Basic interpreter, on cassette tape. So, I finally had a programming language I could use, even if it did take 10 minutes to boot up. For a while, I forgot about an assembler. I wrote programs in Basic.

Years later, I decided to build a Z-80 based S-100 computer. I bought a CPU card, a 64K RAM card, a disk controller and two eight-inch floppy disk drives. But I never got the pieces to work together, and never wrote any code on that machine. By that time I was in college, writing code in C on a VAX-11/750. I was in heaven. I put the Z-80 parts in a box in my parents basement and forgot about them.

Recently, forty years later, I found the box of Z-80 parts in my own attic. I thought about putting them back together. And I realized: I had no operating system, no interpreter or compiler, no software. What would I do with my Z-80?

So, as I began assembling the parts to reconstitute the computer, I decided to write my own Z-80 assembler. From scratch. From first principles. Not copying anyone else’s design or code. 

This is the story of writing a Z-80 Assembler.