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Microsoft Just Released MS-DOS Source Code!
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My DOS computer from the 90s is sluggish with some of the more complex hand-written assembly things. C and UNIX were for powerful multi-user uses.
Roller Coaster Tycoon was written in assembly for optimization too.
The Amiga's OS was written in 68k asm and BCPL (C's grandfather) for the kernel, and C was used for the utilities and GUI. But this was considered quite ahead of its time for a single user micro.