Yes, at least in the early hobbyist phase. When you bought a machine (or parts of a machine, the line is a little blurry with S-100 bus), you usually also got t

Yes, at least in the early hobbyist phase. When you bought a machine (or parts of a machine, the line is a little blurry with S-100 bus), you usually also got the ASM source code for the BIOS. If you wanted to add a printer or other peripheral usually you had to make changes to the BIOS and recompile. It was actually pretty easy, the source code had a lot of comments and was easy to modify. Later on, vendors such as CompuPro sold complete "turnkey" S-100 bus systems that were intended for less technical users.