emularity VS MS-DOS

This 4.0 code contains references to 4.00, though: https://github.com/microsoft/MS-DOS/blob/main/v4.0/src/BOOT/...

I recently stumbled across the MS-DOS 1.25 and 2.0 source code [1].

[1] https://github.com/microsoft/MS-DOS

“Despite this reduction in scope for MS-DOS 2.0, it did carry many bits of XENIX. The system adopted I/O redirection via less-than and greater-than symbols, piping, a hierarchical directory tree, file handles […]”

The source code for MSDOS 2 is available and the file descriptor stuff appears to be in https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/... and XENIX2.ASM. It stands in contrast to the File Control Block API which MSDOS 1 (née 86-DOS) modeled after CP/M’s API.

Asynchronous I/O figures in prominently in Windows NT. I was really surprised to see[0]:

Each driver in the chain defines two entry points; the strategy routine and the interrupt routine. The 2.0 DOS does not really make use of two entry points (it simply calls strategy, then immediately calls interrupt). This dual entry point scheme is designed to facilitate future multi-tasking versions of MS-DOS. In multi-tasking environments I/O must be asynchronous, to accomplish this the strategy routine will be called to queue (internally) a request and return quickly. It is then the responsibility of the interrupt routine to perform the actual I/O at interrupt time by picking requests off the internal queue (set up by the strategy routine), and process them. When a request is complete, it is flagged as "done" by the interrupt routine. The DOS periodically scans the list of requests looking for ones flagged as done, and "wakes up" the process waiting for the completion of the request.

I didn't realize that kind of forwarding-looking perspective was going into the design of MS-DOS.

[0] https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/...

>Any others I'm missing?

I would suggest MS-DOS: https://github.com/microsoft/MS-DOS