For about five years before the IBM PC, lots of us used CP/M, before the development of the 8086 class of "16-bit" microprocessors, a floppy disk-based operating system. There's been a lot written about it, and there's source code available for it and for nearly all its utilities.

In fact, if you were to find all the source files, and a PL/M-51 compiler that runs on something you have available, e.g. a PC, you could take all the PL/M-80 sources and modify them to run on an 805x-core. Since there was a PL/M-51 compiler for CP/M, you should be able to run that on one of the numerous CP/M simulators that are available for the PC. You would, of course, have to translate the portions written in ASM80 and write an original BIOS, which, BTW, is not the same thing as PC's BIOS, as it is part of the OS and not a part of the platform hardware.

If you don't need a disk file system in your setup (the term "operating system" is often misapplied to what amounts really to just a monitor program) For that purpose, I'd recommend taking a hard look at UltraMON51, which I use from time to time. It has an assembler and a disassembler and is capable of tracing, etc. You'll have to read the documentation, of course.

In the years before the IBM PC, we even had CPM/86, which was also a relatively simple operating system.

Before that, we had OS-8 for the PDP 8, and other primitive DEC OS'. It was the falling price of 8080 chips, memory, and floppy disk drives, popularized on the IBM 370, among others, that made the "personal" computer a reality. We used CP/M to do word processing, payroll and accounting, general ledger, database operations, and wrote programs in COBOL, FORTRAN, LISP, BASIC, PASCAL, 'C', PL/1, PL/M, and, of course, 8080 assembler, under CP/M. There was a text editor, an assembler, a debugger (DDT), and, later, some pretty well-liked communication programs, e.g. XMODEM, that were developed and widely used under CP/M.

Go ahead! Read about CP/M. Read about the machines on which it was run before the "commercial" home-computer became popular (before 1977). You could build a complete system nowadays with just a CPU (an MCU containing a bootloader), one static meomory IC (64Kx8), and an IDE disk interface, attached to a CF module. That would allow you to run MUCH faster than the old 8080 at 2.5 MHz that I used back in the day. The entire thing, including the CF module and adapter could occupy a board about the size of a playing card. You could even substitute an LCD module for the serially interfaced (RS232) dumb-terminal that original CP/M used to expect, provided you can find a way to interface a keyboard.

RE