Comparison of operating system kernels

A kernel is the core component of every computer operating system. While kernels are highly technical in nature, and may be hidden from the user under many layers of software and applications, they do have distinguishing or characteristic features, such as computer architecture, design goals, as well as the more practical features that they provide. A direct comparison of operating system kernels can highlight these design choices, and provide insight into different niches and the evolving technology of kernels.

Comparison criterion

The following tables compare general and technical information for a number of widely used and currently available operating system kernels. Please see the individual products' articles for further information.

Even though there is a large number and variety of available Linux distributions, all of these kernels are grouped under a single entry in these tables, due to differences being of the patch level. See comparison of Linux distributions for a detailed comparison. Linux distributions that have highly modified kernels, for example Real-time computing kernels, should be listed separately. There are also a wide variety of minor BSD operating systems, many of which can be found at Comparison of BSD operating systems.

The tables specifically do not include subjective viewpoints on the merits of each kernel or operating system. For this kind of information, please see operating system advocacy.

Feature overview

The major contemporary general-purpose kernels are shown in comparison. Only an overview of the technical features is detailed.

Kernel name	Used in	Creator	Executable format (also see section below)	Type	Integrated firewall	SMP support	Multiple architecture support (also see section below).	Multitasking	Virtualization	Security	Profiling/Debugging	Soft real-time support	Hard real-time support	Can keep RTC in UT[1]
Agnix [3]	?	?	?	?	?	?	?	Yes	?	?	?	?	?	?
Amiga Exec	AmigaOS	Commodore International	HUNK	Exokernel (atypical)[2]	No	No	No	Yes	No	No	Yes	No	Yes	?
Amiga Exec SG (2nd Generation)	AmigaOS 4	Hyperion Entertainment	ELF/HUNK	Exokernel (atypical)	No	No	No	Yes	No	No	Yes	No	No	?
DragonFly BSD kernel [4]	DragonFly BSD	Matt Dillon	ELF, others - platform dependent	hybrid	IPFilter, Ipfirewall, PF	Yes	Yes	Yes	chroot, jail,vkernel	?	DDB[disambiguation needed ], KGDB	?	?	?
FreeBSD kernel	FreeBSD, Debian GNU/kFreeBSD, Gentoo/FreeBSD	The FreeBSD Project	ELF, others - platform dependent	monolithic	IPFilter, Ipfirewall, PF	Yes	Yes	Yes	chroot, jail	POSIX ACL[3], TrustedBSD MAC, OpenBSM	KDB, DDB[disambiguation needed ], KGDB, DTrace, hwpmc	Yes	?	?
GNU Hurd [5]	Bee GNU/Hurd, Debian GNU/Hurd	GNU Project/Free Software Foundation	ELF	microkernel	?	No	No	Yes	?	?	?	?	?	?
GNU Mach [6]	?	GNU Project/Free Software Foundation	?	microkernel	?	No	No	Yes	?	?	?	?	?	?
Inferno kernel	Inferno	Bell Labs / Vita Nuova Holdings	?	virtual machine	?	Yes	Yes	Yes	?	?	?	?	?	?
L4	L4	Jochen Liedtke	?	microkernel	?	Yes	Yes	Yes	Yes	Afterburner/L4 [7], Marzipan [8]	Capabilities [9]	Fiasco L4 Kernel [10]	?	?
Linux kernel [11]	Linux	Linus Torvalds	ELF, others	monolithic	netfilter/ iptables	Yes	Yes	Yes	cgroups, chroot, Lguest, Xen, KVM, kvm-lite, LXC	POSIX ACL, POSIX Capabilities, keyctl, LSM(SELinux, SMACK, TOMOYO Linux, AppArmor)	OProfile, kprobe, SystemTap, JProbe, ftrace, KDB, KGDB, kernel marker, perf tools	CONFIG_PREEMPT	(CONFIG_PREEMPT_RT Patch)	Yes
Mach [12]	NextStep and OpenStep	Carnegie Mellon University	Mach-O	microkernel	?	?	?	?	?	?	?	?	?	?
Minix 3 [13]	Minix 3	Andrew S. Tanenbaum	a.out	microkernel	No	No	No	Yes	No	Unix permissions	Call profiling, statistical profiling, Minix Debugger (mdb)	No	No	?
MkLinux [14]	?	OSF Research Institute and Apple Computer	?	microkernel	?	?	?	?	?	?	?	?	?	?
NetBSD kernel	NetBSD, Debian GNU/NetBSD,	The NetBSD Project	ELF, others - platform dependent	monolithic	IPFilter PF	Yes	Yes	Yes	Xen, chroot	kauth	DDB[disambiguation needed ], KGDB, tprof	POSIX real-time scheduling extensions	Kernel preemption[citation needed]	?
NetWare kernel	Novell NetWare	Novell	NLM	hybrid	Yes	Yes	No	Yes	?	?	?	?	?	?
OpenBSD kernel	OpenBSD	OpenBSD developers	ELF, others - platform dependent	monolithic	PF	Yes	Yes	Yes	?	?	?	?	?	?
Plan 9 kernel	Plan 9 from Bell Labs	Bell Labs	?	hybrid	ipmux	Yes	Yes	Yes	?	?	?	?	?	?
ReactOS kernel	ReactOS	ReactOS Foundation	PE, others?	hybrid	?	?	?	Yes	?	?	?	?	?	?
Rockbox kernel	Rockbox	The Rockbox Project	?	?	?	?	Yes	?	?	?	?	?	?	?
SunOS kernel	SunOS	Sun Microsystems	a.out	monolithic	?	Yes	Yes	Yes	?	?	?	?	?	?
Solaris kernel	Solaris, OpenSolaris, Nexenta OS	Sun Microsystems	ELF (32-bit only until Solaris 7 in 1998)	monolithic	IPFilter	Yes	Yes	Yes	Zones, chroot	ACL, RBAC, Auditing, Privileges, Zones, Trusted Extensions	DTrace, CPU profiling, microstate accounting, perf counters, kernel lock stats, kstats, mdb, kmdb	Real-time scheduler	?	?
Trix	?	Massachusetts Institute of Technology	?	monolithic	?	?	?	?	?	?	?	?	?	?
Windows NT kernel [15]	Windows NT, 2000, XP, 2003, Vista, Windows 7	Microsoft	PE, others?	hybrid	Yes	Yes	Yes	Yes	?	?	?	?	?	No
XNU (Darwin kernel) [16]	Mac OS X, OpenDarwin, GNU/Darwin	Apple Computer	Mach-O	hybrid	Ipfirewall, PF (starting in Lion)	Yes	Yes	Yes	?	?	DTrace	?	?	?
SPARTAN kernel [17]	HelenOS	Jakub Jermar	ELF	microkernel	?	Yes	Yes	Yes	?	?	?	?	?	?
Kernel name	Used in	Creator	Executable format (also see section below)	Type	Integrated firewall	SMP support	Multiple architecture support (also see section below).	Multitasking	Virtualization	Security	Profiling/Debugging	Soft real-time support	Hard real-time support	Can keep RTC in UT

Transport protocol support

Kernel Name	TCP	UDP	SCTP	DCCP
FreeBSD kernel	Yes	Yes	Yes	No
Linux kernel	Yes	Yes	Yes	Yes
Solaris kernel	Yes	Yes	Yes	?
Windows NT kernel	Yes	Yes	No	?
XNU	Yes	Yes	No	No

In-kernel Security

Kernel Name	File access control	Disable memory execution support	Kernel ASLR	Mandatory access control	Capability-based security	In-kernel key management	Audit API	Sandbox	SYN flood protection	UDP flood protection	Ping flood protection	Smurf attack protection	Network Behavior Analysis
Linux Kernel	Traditional Unix permissions, POSIX ACL	Yes	Yes	LSM(SELinux, SMACK, TOMOYO Linux, AppArmor)	POSIX Capabilities	keyctl	fanotify	SELinux Sandbox, seccomp	SYN cookies	No?	No?	No?	No
FreeBSD Kernel	Traditional Unix permissions, POSIX ACL	Yes	?	TrustedBSD MAC	?	?	OpenBSM	Capsicum	?	?	?	?	?
Solaris Kernel	Traditional Unix permissions, POSIX ACL, ZFS ACL	Default	?	Solaris Trusted Extensions	?	?	?	?	?	?	?	?	?
Windows NT kernel	Access control list	DEP	Yes	Mandatory Integrity Control	?	?	?	?	?	?	?	?	?
XNU	Traditional Unix permissions, access control list	Yes	Yes	Yes	?	?	OpenBSM	Apple XNU Sandbox	?	?	?	?	?

In-kernel Virtualization

Kernel Name	Container (no resource management, no security)	Container (no resource management)	Container (resource management)	Paravirtualization	Full virtualization	User-space execution
Linux Kernel	chroot	LXC		Virtio, Hyper-V (guest only), Xen (guest only), VMI (guest only), kvm-lite, lguest	KVM	UML
FreeBSD	chroot	jail	No	?	?	?
Solaris kernel	chroot	Solaris Containers / Zones		No	No	No
Windows NT kernel	?	?	?	Hyper-V	Hyper-V	?
XNU	chroot	?	?	?	?	?

In-kernel server support

Kernel Name	HTTP	FTP	NFS	CIFS	Name server	transport-layer load balancer	application-layer load balancer	9P	SSL proxy
Linux kernel	(TUX web server patch)	(TUX web server patch)	knfsd	No	?	IP Virtual Server	(KTCPVS)	(patch available)	No
Solaris kernel	?	?	Yes	Yes[4]	?	Yes[5]	Yes[6]	?	KSSL
Windows NT kernel	HTTP.sys	?	?	Yes	?	Yes	Yes	No	?
XNU	No	No	Yes	No	No	No	No	No	No

Binary format support

A comparison of OS support for different binary formats (executables):

Kernel Name	a.out	ECOFF	ELF	FDPIC ELF binaries (mmu less)	flat binaries (superH)	HUNK	Mach-O	Misc (wrapper based, like interpreters)	PE	SOM (PA-RISC, HP-UX)	NLM	PEF
Amiga Exec	No	No	Yes[7]	No	No	Yes	No	No	No	No	?	?
HP-UX	No	No	Yes	No	No	No	No	No	No	Yes	No	No
Linux kernel	Yes	Yes	Yes	Yes	Yes	No	No	Yes	Some[8]	Yes	?	No
MINIX 3	Yes	No	No	No	No	No	No	No	No	No	No	No
NetBSD kernel	Yes	Yes	Yes	?	?	?	Yes	?	Yes	?	?	?
ReactOS kernel	?	?	?	?	?	?	?	?	Yes	?	?	?
Solaris kernel	Yes[9]	No	Yes[10]	No	No	No	No	No	Some[11]	No	?	?
Windows NT kernel	No	No	No	No	No	No	No	No	Yes	No	No	No
XNU	No	No	No	No	No	No	Yes	No	No	No	No	No

File system support

Physical file systems:

Kernel	Acorn ADFS	Amiga FFS	BeFS	BFS	cramfs	EFS	ext2	ext3	ext4	FAT	FreeVxFS	HFS	HFS+	HPFS	ISO 9660	JFFS	JFFS2	JFS	Minix fs	NSS	NTFS	OCFS	QNX4 FS	System V FS	UDF	UFS	XFS	ZFS	ReiserFS	Reiser4	Btrfs	HAMMER	Tux3	exFAT
FreeBSD kernel	No	No	No	No	No	No	Yes	Yes	Yes	Yes	No	No	No	No	Yes	No	No	No	No	No	limited write	No	No	No	Yes	Yes	read only	Yes	read only	No	No	No	No	?
Linux kernel	Yes [12]	Yes	read only	Yes	Yes	read only	Yes	Yes	Yes	Yes	read only	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	limited write [13]	Yes	Yes [12] [14]	write support?	Yes	Yes [12]	Yes	both FUSE and native	Yes	Yes	Yes	No	Yes	(experimental patch is available[15])
MINIX 3	No	No	No	No	No	No	Yes	No	No	Tools available, cannot mount	No	No	No	No	Yes	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
NetWare kernel	No	No	No	No	No	No	No	No	No	Yes	No	No	No	No	Yes	No	No	No	No	Yes	No	No	No	No	Yes	No	No	No	No	No	No	No	No	?
ReactOS kernel	No	No	No	No	No	No	No	No	No	Yes[16]	No	No	No	No	Yes[16]	No	No	No	No	No	No	No	No	No	Yes[16]	No	No	No	No	No	No	No	No	No
Solaris kernel	?	?	?	?	?	?	Yes	?	?	Yes	?	?	?	?	Yes	?	?	?	?	No	No	?	?	?	Yes	Yes	?	Yes	No	No	No	No	No	?
Windows NT kernel	?	?	?	?	?	?	Yes [17]	Yes [18]	?	Yes	?	Yes [18]	Yes [18]	No	Yes	?	?	?	?	No	Yes	?	?	?	Yes	?	?	No	No	No	No	No	No	Yes
XNU	No	No	No	No	No	No	Yes [19]	Yes [19]	No	Yes	No	Yes	Yes	No	Yes	No	No	No	No	No	limited write	No	No	No	Yes	Yes	No	read only	No	No	No	No	No	Yes

Networked filesystem support

Kernel Name	NFS	AFS	CIFS	Coda	9P	Ceph
FreeBSD kernel	Yes	Yes	Yes	No	No	No
Linux kernel	Yes	Yes	Yes	Yes	Yes	Yes
Solaris kernel	Yes	Yes	Yes	No	No	No
Windows NT kernel	Yes	Yes	Yes	No	No	No
XNU	Yes	Yes	Yes	No	No	No

Supported CPU architectures

kernel	DEC		ARM Holdings		HP	Intel				MIPS Technologies		AIM		IBM		Renesas Electronics					Sun Microsystems			Motorola		Analog Devices	Xilinx	Renesas Electronics		Tensilica		Axis Communications	Fujitsu	Atmel	Panasonic	Hyperstone	Altera			WDC	Sunplus Technology	Tilera	Texas Instruments
	VAX	Alpha	ARM		PA-RISC	x86		i960	IA-64	MIPS		PowerPC		System/ 390	z/Architecture	H8300	M16C	M32R	SuperH		SPARC			m68k		Blackfin (nommu)	MicroBlaze	78K	V850	Xtensa		ETRAX CRIS	FR-V	AVR32	MN10300	E1 (nommu)	Nios (nommu)	Nios II		WDC 65C816	S+core	Tilera	C6X
			mmu	nommu		x86	x86-64			mmu	nommu	32-bit	64-bit						mmu	nommu	32-bit	64-bit	nommu	mmu	nommu					mmu	nommu							mmu	nommu
DragonFly BSD kernel	?	No	No	No	No	Yes	Yes	?	No	No	No	No	No	No	No	?	?	?	No	No	No	No	No	No	No	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
FreeBSD kernel	?	Yes	Yes	?	No	Yes	Yes	?	Yes	No	No	Yes	No	No	No	?	?	?	No	No	No	Yes	?	No	No	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
Linux kernel	No	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	?	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	No (dropped in 2.6.x)	Yes	Yes	Yes	Yes	Yes	Yes	No	No	Yes	Yes	No	Yes	Yes	No
MINIX 3	?	No	In progress	?	No	Yes	In progress	No	No	In progress	?	No	No	No	No	No	?	No	No	No	No	No	No	No	No	No	No	?	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
NetBSD kernel	Yes	Yes	Yes	No[20]	Yes	Yes	Yes	?	Yes	Yes	No[20]	Yes	No	No	No	?	?	?	Yes	No[20]	Yes	Yes	No[20]	Yes	No[20]	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
NetWare kernel	No	No	No	No	No	Yes	No	?	No	No	No	No	No	No	No	?	?	?	No	No	No	No	No	No	No	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
OpenBSD kernel	Yes	Yes	Yes	?	Yes	Yes	Yes	?	No	Yes	?	Yes	No	No	No	?	?	?	Yes	?	Yes	Yes	?	Yes	Yes	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
Rockbox kernel	?	?	Yes	?	?	?	?	?	?	?	?	?	?	?	No	?	?	?	Yes	?	?	?	?	Yes	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
Solaris kernel	No	No	No	No	No	Yes	Yes	?	No	No	No	No	No	No	No	?	?	?	No	No	Yes	Yes	?	No	No	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
Windows NT kernel	No	NT 5.0 RC1 and below only	Yes	No	No	Yes	Yes	No	Yes	NT 4.0 and below only	No	NT 3.51 and NT 4.0 only	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
Windows CE kernel	No	No	Yes	?	No	Yes	?	No	No	Yes	?	No	No	No	No	No	?	No	Yes	?	No	No	No	No	No	No	No	?	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
XNU	No	No	Yes	?	No	Yes	Yes	?	No	No	No	Yes	Yes	No	No	?	?	?	No	No	No	No	No	No	No	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
SPARTAN kernel	?	No	Yes	?	No	Yes	Yes	?	Yes	Yes	?	Yes	In Progress	No	No	?	?	?	No	No	No	Yes	?	No	No	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?	?
FreeRTOS kernel	?	?	?	Yes	?	?	?	?	?	?	?	Yes	?	?	No	?	?	?	?	?	?	?	?	?	?	Yes	Yes	Yes	Yes	?	?	?	?	Yes	?	?	?	?	Yes	?	?	?	?

Supported kernel execution environment

Kernel name	Linux	Darwin	Windows NT kernel	FreeBSD	NetBSD	Solaris	OSF/1	Amiga Unix	SunOS	BSDI	IBCS2 systems	IRIX	Ultrix	NDIS	SVR4
FreeBSD	Yes[21]	No	No	Yes	No	No	No	No	No	No	No	No	No	Yes[21]	Yes[21]
Linux	Yes	No	No	No	No	No	No	No	No	No	No	No	No	No	No
NetBSD	Yes[22]	No? [18]?	No? [19]?	Yes[22]	Yes	Yes[22]	Yes[22]	Yes[22]	Yes[22]	Yes[22]	Yes[22]	Yes[22]	Yes[22]	Yes [20]	Yes
OpenBSD	Yes	No	No	Yes	Yes	Yes	No	No	Yes	?	?	?	?	No	Yes
Windows NT kernel	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No	No
ReactOS kernel	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No	No
XNU	No	Yes	No	No	No	No	No	No	No	No	No	No	No	No	No
Kernel name	Linux	Darwin	Windows NT kernel	FreeBSD	NetBSD	Solaris	OSF/1	Amiga Unix	SunOS	BSDI	IBCS2 systems	IRIX	Ultrix	NDIS	SVR4

Supported cipher algorithms

This may be usable on some situations like file system encrypting.

Kernel name	DES	AES	Blowfish	Triple DES	Serpent	Twofish	CAST-128	DES-X	IDEA	RC2	RC5	SEED	Skipjack	TEA	XTEA	CAST-256	RC4	Camellia	Anubis	KHAZAD	Salsa20	FCrypt
Linux	Yes	Yes	Yes	Yes	Yes	Yes	Yes	?	?	?	?	Yes	?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Windows	Yes	Yes	No	Yes	No	No	No	No	No	Yes	No	No	No	No	No	No	Yes	No	No	No	No	No
Kernel name	DES	AES	Blowfish	Triple DES	Serpent	Twofish	CAST-128	DES-X	IDEA	RC2	RC5	SEED	Skipjack	TEA	XTEA	CAST-256	RC4	Camellia	Anubis	KHAZAD	Salsa20	FCrypt

Supported compression algorithms

This may be usable on some situations like compression file system.

Kernel name	Deflate	zlib	LZO	LZJB	gzip
Linux	Yes	Yes	Yes	?	Yes
Solaris	?	?	?	Yes	Yes
Kernel name	Deflate	zlib	LZO	LZJB	gzip

Supported hash algorithms

Kernel name	CRC-32 (IEEE)	CRC32c	MD2	MD4	MD5	SHA-1	SHA-256	SHA-384	SHA-512	Michael MIC	RIPEMD-128	RIPEMD-160	RIPEMD-256	RIPEMD-320	Tiger	Whirlpool	HMAC	MDC2	GOST	LASH-160	LASH-256	LASH-384	LASH-512
Linux	Yes[23]	Yes[24]	Yes[25]	Yes	Yes	Yes	Yes	Yes[26]	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	?	?	?	?	?	?	?
Solaris	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	?	?	Yes	?	?	Yes	?	Yes	Yes	?	?	?	?	?
Windows NT	?	?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	No	No	No	No	No	No	Yes	No	No	No	No	No	No
Kernel name	CRC-32 (IEEE)	CRC32c	MD2	MD4	MD5	SHA-1	SHA-256	SHA-384	SHA-512	Michael MIC	RIPEMD-128	RIPEMD-160	RIPEMD-256	RIPEMD-320	Tiger	Whirlpool	HMAC	MDC2	GOST	LASH-160	LASH-256	LASH-384	LASH-512

Supported Bluetooth protocols

Kernel name	ACL	SCO	LMP	HCI	L2CAP	BNEP	RFComm.	SDP	TCP	AVTCP	AVDTP	OBEX	CMTP	HIDP	HCRP	CAPI	PPP
FreeBSD kernel	?	?	?	Yes	Yes	?	Yes	Yes	?	?	?	?	?	?	?	?	?
Linux	?	Yes	?	Yes	Yes	Yes	Yes	?	?	?	?	?	Yes	Yes	?	?	?
Kernel name	ACL	SCO	LMP	HCI	L2CAP	BNEP	RFComm.	SDP	TCP	AVTCP	AVDTP	OBEX	CMTP	HIDP	HCRP	CAPI	PPP

See also

• Comparison of open source operating systems
• Comparison of Linux distributions
• Comparison of BSD operating systems
• Comparison of Windows versions
• List of operating systems
• Comparison of file systems
• Comparison of operating systems
• Operating system advocacy
• XvsXP

Footnotes

1. ^ IBM PC Real Time Clock should run in UT
2. ^ The Amiga hardware lacked support for memory protection, so the strong isolation goals of the microkernel design could not be achieved.^{[citation needed]}
3. ^ http://www.freebsd.org/doc/en/books/handbook/fs-acl.html
4. ^ OpenSolaris Project Weaves CIFS Server Into the Solaris Kernel
5. ^ Integrated Load Balancer
6. ^ Integrated Load Balancer
7. ^ AmigaOS up to version 3.9 could use the ELF format for PowerPC executables and libraries through ppc.library, also known as PowerUP. AmigaOS 4, uses ELF as its native executable format.
8. ^ The Linux kernel can recognize PE binaries through binfmt_misc and run them using Wine
9. ^ to support SunOS 4.x binaries
10. ^ including a Linux compatibility option
11. ^ The Solaris kernel can PE using Wine
12. ^ ^{a b c} experimental and dangerous write support
13. ^ The only supported operation is overwriting existing files, without changing the file length so a loop file on a ntfs volume can be written; better write support can be achieved through ntfs-3g, although that is a FUSE filesystem and therefore not strictly a kernel feature
14. ^ write support currently broken
15. ^ Fat64 / exFat fs and Linux
16. ^ ^{a b c} Version Status - ReactOS
17. ^ additional driver needed - see http://www.fs-driver.org/
18. ^ ^{a b c} additional driver needed
19. ^ ^{a b} Third party module required. Mac OS X Kernel Module available here
20. ^ ^{a b c d e} NetBSD Projects - Support for MMU-less systems
21. ^ ^{a b c} Linux emulation in FreeBSD - 2 A look inside...
22. ^ ^{a b c d e f g h i j} NetBSD Binary Emulation
23. ^ [1]
24. ^ [2]
25. ^ MD2 Linux man page
26. ^ GNU SHA2 utilities