Microsoft open-sourced 86-DOS 1.00 kernel sources, PC-DOS 1.00 snapshots, and CHKDSK utilities via its developer notes, predating MS-DOS branding (Microsoft Blog, 2026; Ars Technica, 2026). Tim Paterson developed the code at Seattle Computer Products for Intel 8086 kits before Microsoft licensed and acquired it for the IBM 5150. The release documents direct lineage from QDOS adaptations of CP/M structures to the 1981 PC-DOS 1.00 kernel. This artifact aligns with prior Microsoft disclosures of MS-DOS 1.25 through 2.0 sources and Windows NT 3.1, confirming a pattern of legacy OS archival that began in 2012. Coverage understates the code's value for tracing interrupt handling and FAT filesystem precursors absent from secondary histories. Primary diffs between 86-DOS and PC-DOS snapshots reveal incremental IBM hardware accommodations not detailed in the Ars summary.