In 1985, I was developing a simple serial network for barcode readers, to run on the original IBM PC. It needed to run as a TSR (Terminate and Stay Resident) as

In 1985, I was developing a simple serial network for barcode readers, to run on the original IBM PC. It needed to run as a TSR (Terminate and Stay Resident) assembler routine and use the 8250 UART chip, and the 8259 Programmable Interupt Controller. At that point the only documentation we had access to was the IBM Technical Reference Manual, which gave the schematics and BIOS listing, but little else. I was very familiar with GW-BASIC and had written some BASIC code to use the RS232 serial interface from it, so decided to reverse engineer GW-BASIC itself, to work out how it was addressing those chips. A month or two later, we came across Peter Norton's "Programmers Guide to the IBM PC", which could have saved us weeks. Exciting times.