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Microkernel
In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC).
If the hardware provides multiple rings or CPU modes, the microkernel may be the only software executing at the most privileged level, which is generally referred to as supervisor or kernel mode. Traditional operating system functions, such as device drivers, protocol stacks and file systems, are typically removed from the microkernel itself and are instead run in user space.
In terms of the source code size, microkernels are often smaller than monolithic kernels. The MINIX 3 microkernel, for example, has only approximately 12,000 lines of code.
Microkernels trace their roots back to Danish computer pioneer Per Brinch Hansen and his tenure in Danish computer company Regnecentralen where he led software development efforts for the RC 4000 computer. In 1967, Regnecentralen was installing a RC 4000 prototype in the Zakłady Azotowe Puławy fertilizer plant in Poland. The computer used a small real-time operating system tailored for the needs of the plant. Brinch Hansen and his team became concerned with the lack of generality and reusability of the RC 4000 system. They feared that each installation would require a different operating system so they started to investigate novel and more general ways of creating software for the RC 4000. In 1969, their effort resulted in the completion of the RC 4000 Multiprogramming System. Its nucleus provided inter-process communication based on message-passing for up to 23 unprivileged processes, out of which 8 at a time were protected from one another. It further implemented scheduling of time slices of programs executed in parallel, initiation and control of program execution at the request of other running programs, and initiation of data transfers to or from peripherals. Besides these elementary mechanisms, it had no built-in strategy for program execution and resource allocation. This strategy was to be implemented by a hierarchy of running programs in which parent processes had complete control over child processes and acted as their operating systems.
Following Brinch Hansen's work, microkernels have been developed since the 1970s. The term microkernel itself first appeared no later than 1981. Microkernels were meant as a response to changes in the computer world, and to several challenges adapting existing "mono-kernels" to these new systems. New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time. This code was normally located in the monolithic kernel, and thus required considerable work and careful code management to work on. Microkernels were developed with the idea that all of these services would be implemented as user-space programs, like any other, allowing them to be worked on monolithically and started and stopped like any other program. This would not only allow these services to be more easily worked on, but also separated the kernel code to allow it to be finely tuned without worrying about unintended side effects. Moreover, it would allow entirely new operating systems to be "built up" on a common core, aiding OS research.
Microkernels were a very hot topic in the 1980s when the first usable local area networks were being introduced.[citation needed]. The AmigaOS Exec kernel was an early example, introduced in 1986 and used in a PC with relative commercial success. The lack of memory protection, considered in other respects a flaw, allowed this kernel to have very high message-passing performance because it did not need to copy data while exchanging messages between user-space programs.
The same mechanisms that allowed the kernel to be distributed into user space also allowed the system to be distributed across network links. The first microkernels, notably Mach created by Richard Rashid, proved to have disappointing performance, but the inherent advantages appeared so great that it was a major line of research into the late 1990s. However, during this time the speed of computers grew greatly in relation to networking systems, and the disadvantages in performance came to overwhelm the advantages in development terms.[citation needed]
Many attempts were made to adapt the existing systems to have better performance, but the overhead was always considerable and most of these efforts required the user-space programs to be moved back into the kernel. By 2000, most large-scale Mach kernel efforts had ended, although Apple's macOS, released in 2001, still uses a hybrid kernel called XNU, which combines a heavily modified (hybrid) OSF/1's Mach kernel (OSF MK 7.3 kernel) with code from BSD UNIX, and this kernel is also used in iOS, tvOS, and watchOS. Windows NT, starting with NT 3.1 and continuing with Windows 11, uses a hybrid kernel design. As of 2012[update], the Mach-based GNU Hurd is also functional and included in testing versions of Arch Linux and Debian.
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Microkernel
In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC).
If the hardware provides multiple rings or CPU modes, the microkernel may be the only software executing at the most privileged level, which is generally referred to as supervisor or kernel mode. Traditional operating system functions, such as device drivers, protocol stacks and file systems, are typically removed from the microkernel itself and are instead run in user space.
In terms of the source code size, microkernels are often smaller than monolithic kernels. The MINIX 3 microkernel, for example, has only approximately 12,000 lines of code.
Microkernels trace their roots back to Danish computer pioneer Per Brinch Hansen and his tenure in Danish computer company Regnecentralen where he led software development efforts for the RC 4000 computer. In 1967, Regnecentralen was installing a RC 4000 prototype in the Zakłady Azotowe Puławy fertilizer plant in Poland. The computer used a small real-time operating system tailored for the needs of the plant. Brinch Hansen and his team became concerned with the lack of generality and reusability of the RC 4000 system. They feared that each installation would require a different operating system so they started to investigate novel and more general ways of creating software for the RC 4000. In 1969, their effort resulted in the completion of the RC 4000 Multiprogramming System. Its nucleus provided inter-process communication based on message-passing for up to 23 unprivileged processes, out of which 8 at a time were protected from one another. It further implemented scheduling of time slices of programs executed in parallel, initiation and control of program execution at the request of other running programs, and initiation of data transfers to or from peripherals. Besides these elementary mechanisms, it had no built-in strategy for program execution and resource allocation. This strategy was to be implemented by a hierarchy of running programs in which parent processes had complete control over child processes and acted as their operating systems.
Following Brinch Hansen's work, microkernels have been developed since the 1970s. The term microkernel itself first appeared no later than 1981. Microkernels were meant as a response to changes in the computer world, and to several challenges adapting existing "mono-kernels" to these new systems. New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time. This code was normally located in the monolithic kernel, and thus required considerable work and careful code management to work on. Microkernels were developed with the idea that all of these services would be implemented as user-space programs, like any other, allowing them to be worked on monolithically and started and stopped like any other program. This would not only allow these services to be more easily worked on, but also separated the kernel code to allow it to be finely tuned without worrying about unintended side effects. Moreover, it would allow entirely new operating systems to be "built up" on a common core, aiding OS research.
Microkernels were a very hot topic in the 1980s when the first usable local area networks were being introduced.[citation needed]. The AmigaOS Exec kernel was an early example, introduced in 1986 and used in a PC with relative commercial success. The lack of memory protection, considered in other respects a flaw, allowed this kernel to have very high message-passing performance because it did not need to copy data while exchanging messages between user-space programs.
The same mechanisms that allowed the kernel to be distributed into user space also allowed the system to be distributed across network links. The first microkernels, notably Mach created by Richard Rashid, proved to have disappointing performance, but the inherent advantages appeared so great that it was a major line of research into the late 1990s. However, during this time the speed of computers grew greatly in relation to networking systems, and the disadvantages in performance came to overwhelm the advantages in development terms.[citation needed]
Many attempts were made to adapt the existing systems to have better performance, but the overhead was always considerable and most of these efforts required the user-space programs to be moved back into the kernel. By 2000, most large-scale Mach kernel efforts had ended, although Apple's macOS, released in 2001, still uses a hybrid kernel called XNU, which combines a heavily modified (hybrid) OSF/1's Mach kernel (OSF MK 7.3 kernel) with code from BSD UNIX, and this kernel is also used in iOS, tvOS, and watchOS. Windows NT, starting with NT 3.1 and continuing with Windows 11, uses a hybrid kernel design. As of 2012[update], the Mach-based GNU Hurd is also functional and included in testing versions of Arch Linux and Debian.
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