... programming a kit micro-computer. In machine code. 'Twas a kit for electronics hobbyists featuring a Z80, 2k RAM, 20 button keypad, 6 LED digits and an expansion socket. Programming required copious pencil and paper, and an op-code listing.
It was really after I lost interest that I found out that the Z80 op-codes were designed by thinkers in octal, not hexadecimal.
Wade.