Earliest 86-DOS and PC-DOS code released as open source

Had a 486 SX/25 (Ambra Hurdla) back then. Accidentally bought 4x4MB server SIMMs instead of what I needed and ended up with 20MB which was kind of absurd in 1993. Ran the whole games directory as a RAM disk just because I could. Spent way too long learning the segmented memory model by breaking config.sys rather than reading anything. Would've been useful to have the actual source back then.