The early OS's were pretty simple. The best way to learn about these things is to study the other early ones. Some were little more than a bunch of routines sho

The early OS's were pretty simple. The best way to learn about these things is to study the other early ones. Some were little more than a bunch of routines shoved in ROM with a BASIC on top. Others were a little more sophisticated. All of them were hacked and abused in unspeakable ways. They really only had to do a few things: boot from the disk, map files somehow to the disk, read/write/append to said files on the disk, and load and execute a program. Look for how each of the different systems tried to accomplish those tasks. CP/M is the classic to look at. But consider things such as the Apple ROM, and the Apple DOS. The C64 ROM and how rather than having a DOS per se, it relied on a very "smart" disk drive. Atari has CIO, "Central I/O", which is an extensible device system. There's also FLEX, which is akin to CP/M in that it was a portable OS for 6800 based machines. TRS-DOS for the TRS-80 is a fascinating study, it has some really interesting features you find nowhere else. Don't forget to look at the old Forth implementations, "language, editor, assembler and OS in 8K of memory". It's very (very) simple, but many folks in the day used it. It needs little more than block disk I/O. But, at the time, thats easier said than done if you take in to account things like sector interleave and other exotic disk layouts. The computers were so slow that you didn't put sectors back to back on the floppy disk. This is because while trying to read two consecutive sectors, by the time the system got done handling the read of the first sector, the second sector (assuming it was adjacent) would have passed the head already and you'd have to wait until it all came around again, which slowed things down dramatically. So, you'd alternate sectors (1 every 2 or 3 sectors), "interleaving" the sectors along the track. Obviously not something you'd need to worry about with modern solid state storage, but back in the day mapping a simple "load sector 123" in to the appropriate track and sector on the disk drive was an entire little subsystem of the OS. The DOS part is the hardest part. Organizing files on the disk, keep track of the used sectors, the whole kit. There's a lot to do. The rest are little more than monitor routines. Read a serial port, write a serial port, handle an interrupt. All of the old OS's pretty much have source code published somewhere today that you can audit. They're all in assembly (Z80, 8080, 6502, 6800). But they're a good study. Doing a survey of them, you see how they try to all solve the same problems, but how they approach them in different ways.