This is a collection of disassembled and commented source of parts of MS-DOS 1.0, which can be assembled back into (almost) identical binaries (using NASM).
The DOS 1.0 boot sector. See Reverse-Engineering DOS 1.0 – Part 1: The Boot Sector for more info.
The DOS 1.0 I/O library. See Reverse-Engineering DOS 1.0 – Part 2: IBMBIO.COM for more info.
The DOS 1.0 Kernel. There is no disassembly for it yet.
The original code is Microsoft's. The disassembly was done by Michael Steil.