We just open-sourced DOS 4 (and found binaries of Multitasking DOS 4) https://www.hanselman.com/blog/open-sourcing-dos-4
@shanselman thanks!🙂 it’s a fascinating read. “This module has been modified extensively for my personal use.” https://github.com/microsoft/MS-DOS/blob/2d04cacc5322951f187bb17e017c12920ac8ebe2/v4.0/src/CMD/ATTRIB/ATTRIBA.ASM#L5