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OpenBSD: bsd.rd, a ramdisk kernel, source at http://cvsweb.openbsd.org/src/distrib/miniroot
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FreeBSD: bsdinstall, a Bourne shell script, source at https://github.com/freebsd/freebsd/tree/master/usr.sbin/bsdinstall
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NetBSD: sysinst, a C program, source at http://cvsweb.netbsd.org/bsdweb.cgi/src/usr.sbin/sysinst