A list of operating systems which have source code available
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CP/M 1.2 68000, written in C language

IBM DMS Release 2 Version 12 (used for IBM 1130) 1965, language is assembly
IBM OS/360-MVT 21.8F (1972), language is assembly

Unix v5, runs on PDP-11, language is C (1974)
Unix v6, runs on PDP-11, language is C (1975)
Unix v7, runs on PDP-11, language is C (1979)

RT-11FB V05.03 runs on PDP-11/53 (1985), language is MACRO-11

4.3BSD (Quasijarus) runs on MicroVAX 3000, language is C (1986)

Minix (1987)

2.11BSD backport of 4.3BSD to PDP-11 (1992), language is C

Linux version 1.0 (1994), language is C

MenuetOS (2000) 32-bit version is GPL, written in assembly language IA-32

Plan 9 4th edition (2002)

Kolibri OS (2004), language is x86 assembly

Haiku (2008), language is C++

various modern Linux distros including Debian, Fedora and Slackware

various BSD distros including NetBSD, FreeBSD and OpenBSD

OpenSXCE - An open-source enterprise-class highly scalable OS for Developers and System Administrators
OpenIndiana - forked from OpenSolaris