Misplaced Pages

#371628

123-407: Kernel space is strictly reserved for running a privileged operating system kernel , kernel extensions, and most device drivers . In contrast, user space is the memory area where application software and some drivers execute, typically one address space per process. The term user space (or userland ) refers to all code that runs outside the operating system's kernel. User space usually refers to

246-430: A computer program , such as arithmetic , logic, controlling, and input/output (I/O) operations. This role contrasts with that of external components, such as main memory and I/O circuitry, and specialized coprocessors such as graphics processing units (GPUs). The form, design , and implementation of CPUs have changed over time, but their fundamental operation remains almost unchanged. Principal components of

369-491: A language-based protection system , the kernel will only allow code to execute that has been produced by a trusted language compiler . The language may then be designed such that it is impossible for the programmer to instruct it to do something that will violate a security requirement. Advantages of this approach include: Disadvantages include: Examples of systems with language-based protection include JX and Microsoft 's Singularity . Edsger Dijkstra proved that from

492-446: A user mode separate from kernel mode involves operating system protection rings . Protection rings, in turn, are implemented using CPU modes . Typically, kernel space programs run in kernel mode , also called supervisor mode ; normal applications in user space run in user mode. Some operating systems are single address space operating systems —they have a single address space for all user-mode code. (The kernel-mode code may be in

615-462: A CPU include the arithmetic–logic unit (ALU) that performs arithmetic and logic operations , processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching (from memory) , decoding and execution (of instructions) by directing the coordinated operations of the ALU, registers, and other components. Modern CPUs devote

738-486: A CPU may also contain memory , peripheral interfaces, and other components of a computer; such integrated devices are variously called microcontrollers or systems on a chip (SoC). Early computers such as the ENIAC had to be physically rewired to perform different tasks, which caused these machines to be called "fixed-program computers". The "central processing unit" term has been in use since as early as 1955. Since

861-407: A bug in a device driver might crash the entire system – and the fact that large kernels can become very difficult to maintain; Thompson also stated that "It is also easier for [a monolithic kernel] to turn into a mess in a hurry as it is modified." Monolithic kernels, which have traditionally been used by Unix-like operating systems, contain all the operating system core functions and

984-516: A building block for privilege separation . A separate user mode can also be used to build efficient virtual machines – see Popek and Goldberg virtualization requirements . With enough privileges, processes can request the kernel to map part of another process's memory space to its own, as is the case for debuggers . Programs can also request shared memory regions with other processes, although other techniques are also available to allow inter-process communication . The most common way of implementing

1107-402: A cache had only one level of cache; unlike later level 1 caches, it was not split into L1d (for data) and L1i (for instructions). Almost all current CPUs with caches have a split L1 cache. They also have L2 caches and, for larger processors, L3 caches as well. The L2 cache is usually not split and acts as a common repository for the already split L1 cache. Every core of a multi-core processor has

1230-425: A checked copy of the request is passed through the system call. Hence, not far to travel at all. The monolithic Linux kernel can be made extremely small not only because of its ability to dynamically load modules but also because of its ease of customization. In fact, there are some versions that are small enough to fit together with a large number of utilities and other programs on a single floppy disk and still provide

1353-400: A code from the control unit indicating which operation to perform. Depending on the instruction being executed, the operands may come from internal CPU registers , external memory, or constants generated by the ALU itself. When all input signals have settled and propagated through the ALU circuitry, the result of the performed operation appears at the ALU's outputs. The result consists of both

SECTION 10

#1732775865372

1476-455: A computer system is responsible for executing programs. The kernel takes responsibility for deciding at any time which of the many running programs should be allocated to the processor or processors. Random-access memory (RAM) is used to store both program instructions and data. Typically, both need to be present in memory in order for a program to execute. Often multiple programs will want access to memory, frequently demanding more memory than

1599-461: A data word, which may be stored in a register or memory, and status information that is typically stored in a special, internal CPU register reserved for this purpose. Modern CPUs typically contain more than one ALU to improve performance. The address generation unit (AGU), sometimes also called the address computation unit (ACU), is an execution unit inside the CPU that calculates addresses used by

1722-458: A dedicated L2 cache and is usually not shared between the cores. The L3 cache, and higher-level caches, are shared between the cores and are not split. An L4 cache is currently uncommon, and is generally on dynamic random-access memory (DRAM), rather than on static random-access memory (SRAM), on a separate die or chip. That was also the case historically with L1, while bigger chips have allowed integration of it and generally all cache levels, with

1845-461: A device present on the system, or any form of communication with other processes requires the use of system calls. A system call is a mechanism that is used by the application program to request a service from the operating system. They use a machine-code instruction that causes the processor to change mode. An example would be from supervisor mode to protected mode. This is where the operating system performs actions like accessing hardware devices or

1968-468: A driver is abstraction; the function of the driver is to translate the OS-mandated abstract function calls (programming calls) into device-specific calls. In theory, a device should work correctly with a suitable driver. Device drivers are used for e.g. video cards, sound cards, printers, scanners, modems, and Network cards. At the hardware level, common abstractions of device drivers include: And at

2091-458: A fully functional operating system (one of the most popular of which is muLinux ). This ability to miniaturize its kernel has also led to a rapid growth in the use of Linux in embedded systems . Central processing unit A central processing unit ( CPU ), also called a central processor , main processor , or just processor , is the most important processor in a given computer . Its electronic circuitry executes instructions of

2214-564: A global clock signal. Two notable examples of this are the ARM compliant AMULET and the MIPS R3000 compatible MiniMIPS. Rather than totally removing the clock signal, some CPU designs allow certain portions of the device to be asynchronous, such as using asynchronous ALUs in conjunction with superscalar pipelining to achieve some arithmetic performance gains. While it is not altogether clear whether totally asynchronous designs can perform at

2337-460: A hundred or more gates, was to build them using a metal–oxide–semiconductor (MOS) semiconductor manufacturing process (either PMOS logic , NMOS logic , or CMOS logic). However, some companies continued to build processors out of bipolar transistor–transistor logic (TTL) chips because bipolar junction transistors were faster than MOS chips up until the 1970s (a few companies such as Datapoint continued to build processors out of TTL chips until

2460-515: A kernel where I/O devices are handled uniformly with other processes, as parallel co-operating processes, was first proposed and implemented by Brinch Hansen (although similar ideas were suggested in 1967 ). In Hansen's description of this, the "common" processes are called internal processes , while the I/O devices are called external processes . Similar to physical memory, allowing applications direct access to controller ports and registers can cause

2583-528: A logical point of view, atomic lock and unlock operations operating on binary semaphores are sufficient primitives to express any functionality of process cooperation. However this approach is generally held to be lacking in terms of safety and efficiency, whereas a message passing approach is more flexible. A number of other approaches (either lower- or higher-level) are available as well, with many modern kernels providing support for systems such as shared memory and remote procedure calls . The idea of

SECTION 20

#1732775865372

2706-522: A lot of semiconductor area to caches and instruction-level parallelism to increase performance and to CPU modes to support operating systems and virtualization . Most modern CPUs are implemented on integrated circuit (IC) microprocessors , with one or more CPUs on a single IC chip. Microprocessor chips with multiple CPUs are called multi-core processors . The individual physical CPUs, called processor cores , can also be multithreaded to support CPU-level multithreading. An IC that contains

2829-411: A memory management unit, translating logical addresses into physical RAM addresses, providing memory protection and paging abilities, useful for virtual memory . Simpler processors, especially microcontrollers , usually don't include an MMU. A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from

2952-459: A number that identifies the address of the next instruction to be fetched. After an instruction is fetched, the PC is incremented by the length of the instruction so that it will contain the address of the next instruction in the sequence. Often, the instruction to be fetched must be retrieved from relatively slow memory, causing the CPU to stall while waiting for the instruction to be returned. This issue

3075-423: A particular (virtual) address may be different memory from what another process accesses at the same address. This allows every program to behave as if it is the only one (apart from the kernel) running and thus prevents applications from crashing each other. On many systems, a program's virtual address may refer to data which is not currently in memory. The layer of indirection provided by virtual addressing allows

3198-684: A point where it is as fast as or faster than the one that was specifically designed for the hardware, although more relevant in a general sense. Modern monolithic kernels, such as the Linux kernel , the FreeBSD kernel, the AIX kernel, the HP-UX kernel, and the Solaris kernel, all of which fall into the category of Unix-like operating systems, support loadable kernel modules , allowing modules to be loaded into

3321-592: A policy is a particular "mode of operation". Example: Because the mechanism and policy are separated, the policy can be easily changed to e.g. require the use of a security token . In minimal microkernel just some very basic policies are included, and its mechanisms allows what is running on top of the kernel (the remaining part of the operating system and the other applications) to decide which policies to adopt (as memory management, high level process scheduling, file system management, etc.). A monolithic kernel instead tends to include many policies, therefore restricting

3444-407: A process must be able to access the services provided by the kernel. This is implemented differently by each kernel, but most provide a C library or an API , which in turn invokes the related kernel functions. The method of invoking the kernel function varies from kernel to kernel. If memory isolation is in use, it is impossible for a user process to call the kernel directly, because that would be

3567-424: A single address space with the CPU executing in supervisor mode , mainly for speed. Microkernels run most but not all of their services in user space, like user processes do, mainly for resilience and modularity . MINIX 3 is a notable example of microkernel design. The Linux kernel is both monolithic and modular, since it can insert and remove loadable kernel modules at runtime. This central component of

3690-554: A time. Some CPU architectures include multiple AGUs so more than one address-calculation operation can be executed simultaneously, which brings further performance improvements due to the superscalar nature of advanced CPU designs. For example, Intel incorporates multiple AGUs into its Sandy Bridge and Haswell microarchitectures , which increase bandwidth of the CPU memory subsystem by allowing multiple memory-access instructions to be executed in parallel. Many microprocessors (in smartphones and desktop, laptop, server computers) have

3813-446: A useful computer requires thousands or tens of thousands of switching devices. The overall speed of a system is dependent on the speed of the switches. Vacuum-tube computers such as EDVAC tended to average eight hours between failures, whereas relay computers—such as the slower but earlier Harvard Mark I —failed very rarely. In the end, tube-based CPUs became dominant because the significant speed advantages afforded generally outweighed

User space and kernel space - Misplaced Pages Continue

3936-449: A user-level application may only be permitted to perform some of these operations (e.g., it may only be allowed to read the file). A common implementation of this is for the kernel to provide an object to the application (typically so called a "file handle") which the application may then invoke operations on, the validity of which the kernel checks at the time the operation is requested. Such a system may be extended to cover all objects that

4059-439: A very small number of ICs; usually just one. The overall smaller CPU size, as a result of being implemented on a single die, means faster switching time because of physical factors like decreased gate parasitic capacitance . This has allowed synchronous microprocessors to have clock rates ranging from tens of megahertz to several gigahertz. Additionally, the ability to construct exceedingly small transistors on an IC has increased

4182-478: A violation of the processor's access control rules. A few possibilities are: An important consideration in the design of a kernel is the support it provides for protection from faults ( fault tolerance ) and from malicious behaviours ( security ). These two aspects are usually not clearly distinguished, and the adoption of this distinction in the kernel design leads to the rejection of a hierarchical structure for protection . The mechanisms or policies provided by

4305-447: Is a computer program encapsulating, monitoring and controlling a hardware device (via its Hardware/Software Interface (HSI) ) on behalf of the OS. It provides the operating system with an API, procedures and information about how to control and communicate with a certain piece of hardware. Device drivers are an important and vital dependency for all OS and their applications. The design goal of

4428-423: Is an example of inter-process communication (IPC). The above listed tasks and features can be provided in many ways that differ from each other in design and implementation. The principle of separation of mechanism and policy is the substantial difference between the philosophy of micro and monolithic kernels. Here a mechanism is the support that allows the implementation of many different policies, while

4551-420: Is an exokernel. In a monolithic kernel, all OS services run along with the main kernel thread, thus also residing in the same memory area. This approach provides rich and powerful hardware access. UNIX developer Ken Thompson stated that "it is in [his] opinion easier to implement a monolithic kernel". The main disadvantages of monolithic kernels are the dependencies between system components –

4674-499: Is common in conventional commercial systems; in fact, every module needing protection is therefore preferably included into the kernel. This link between monolithic design and "privileged mode" can be reconducted to the key issue of mechanism-policy separation; in fact the "privileged mode" architectural approach melds together the protection mechanism with the security policies, while the major alternative architectural approach, capability-based addressing , clearly distinguishes between

4797-400: Is defined by the CPU's instruction set architecture (ISA). Often, one group of bits (that is, a "field") within the instruction, called the opcode, indicates which operation is to be performed, while the remaining fields usually provide supplemental information required for the operation, such as the operands. Those operands may be specified as a constant value (called an immediate value), or as

4920-494: Is generally referred to as the " classic RISC pipeline ", which is quite common among the simple CPUs used in many electronic devices (often called microcontrollers). It largely ignores the important role of CPU cache, and therefore the access stage of the pipeline. Some instructions manipulate the program counter rather than producing result data directly; such instructions are generally called "jumps" and facilitate program behavior like loops , conditional program execution (through

5043-483: Is greater or whether they are equal; one of these flags could then be used by a later jump instruction to determine program flow. Fetch involves retrieving an instruction (which is represented by a number or sequence of numbers) from program memory. The instruction's location (address) in program memory is determined by the program counter (PC; called the "instruction pointer" in Intel x86 microprocessors ), which stores

User space and kernel space - Misplaced Pages Continue

5166-400: Is largely addressed in modern processors by caches and pipeline architectures (see below). The instruction that the CPU fetches from memory determines what the CPU will do. In the decode step, performed by binary decoder circuitry known as the instruction decoder , the instruction is converted into signals that control other parts of the CPU. The way in which the instruction is interpreted

5289-530: Is most often credited with the design of the stored-program computer because of his design of EDVAC, and the design became known as the von Neumann architecture , others before him, such as Konrad Zuse , had suggested and implemented similar ideas. The so-called Harvard architecture of the Harvard Mark I , which was completed before EDVAC, also used a stored-program design using punched paper tape rather than electronic memory. The key difference between

5412-505: Is that any security policy can be implemented in an application regardless of kernel support. According to Mars Research Group developers, a lack of isolation is one of the main factors undermining kernel security. They propose their driver isolation framework for protection, primarily in the Linux kernel. Typical computer systems today use hardware-enforced rules about what programs are allowed to access what data. The processor monitors

5535-737: Is the IBM PowerPC -based Xenon used in the Xbox 360 ; this reduces the power requirements of the Xbox 360. Another method of addressing some of the problems with a global clock signal is the removal of the clock signal altogether. While removing the global clock signal makes the design process considerably more complex in many ways, asynchronous (or clockless) designs carry marked advantages in power consumption and heat dissipation in comparison with similar synchronous designs. While somewhat uncommon, entire asynchronous CPUs have been built without using

5658-452: Is the portion of the operating system code that is always resident in memory and facilitates interactions between hardware and software components. A full kernel controls all hardware resources (e.g. I/O, memory, cryptography) via device drivers , arbitrates conflicts between processes concerning such resources, and optimizes the utilization of common resources e.g. CPU & cache usage, file systems, and network sockets. On most systems,

5781-452: Is to simulate capabilities using commonly supported hierarchical domains. In this approach, each protected object must reside in an address space that the application does not have access to; the kernel also maintains a list of capabilities in such memory. When an application needs to access an object protected by a capability, it performs a system call and the kernel then checks whether the application's capability grants it permission to perform

5904-441: Is typically implemented using CPU modes . Many kernels provide implementation of "capabilities", i.e., objects that are provided to user code which allow limited access to an underlying object managed by the kernel. A common example is file handling: a file is a representation of information stored on a permanent storage device. The kernel may be able to perform many different operations, including read, write, delete or execute, but

6027-488: The IBM z13 has a 96 KiB L1 instruction cache. Most CPUs are synchronous circuits , which means they employ a clock signal to pace their sequential operations. The clock signal is produced by an external oscillator circuit that generates a consistent number of pulses each second in the form of a periodic square wave . The frequency of the clock pulses determines the rate at which a CPU executes instructions and, consequently,

6150-546: The Manchester Mark 1 ran its first program during the night of 16–17 June 1949. Early CPUs were custom designs used as part of a larger and sometimes distinctive computer. However, this method of designing custom CPUs for a particular application has largely given way to the development of multi-purpose processors produced in large quantities. This standardization began in the era of discrete transistor mainframes and minicomputers , and has rapidly accelerated with

6273-401: The compiler . Approaches that delegate enforcement of security policy to the compiler and/or the application level are often called language-based security . The lack of many critical security mechanisms in current mainstream operating systems impedes the implementation of adequate security policies at the application abstraction level . In fact, a common misconception in computer security

SECTION 50

#1732775865372

6396-474: The main memory . A cache is a smaller, faster memory, closer to a processor core , which stores copies of the data from frequently used main memory locations . Most CPUs have different independent caches, including instruction and data caches , where the data cache is usually organized as a hierarchy of more cache levels (L1, L2, L3, L4, etc.). All modern (fast) CPUs (with few specialized exceptions ) have multiple levels of CPU caches. The first CPUs that used

6519-408: The memory management unit . Generally the operating system provides a library that sits between the operating system and normal user programs. Usually it is a C library such as Glibc or Windows API. The library handles the low-level details of passing information to the kernel and switching to supervisor mode. System calls include close, open, read, wait and write. To actually perform useful work,

6642-453: The AGU, various address-generation calculations can be offloaded from the rest of the CPU, and can often be executed quickly in a single CPU cycle. Capabilities of an AGU depend on a particular CPU and its architecture . Thus, some AGUs implement and expose more address-calculation operations, while some also include more advanced specialized instructions that can operate on multiple operands at

6765-431: The ALU's output word size), an arithmetic overflow flag will be set, influencing the next operation. Hardwired into a CPU's circuitry is a set of basic operations it can perform, called an instruction set . Such operations may involve, for example, adding or subtracting two numbers, comparing two numbers, or jumping to a different part of a program. Each instruction is represented by a unique combination of bits , known as

6888-468: The CPU can fetch the data from actual memory locations. Those address-generation calculations involve different integer arithmetic operations , such as addition, subtraction, modulo operations , or bit shifts . Often, calculating a memory address involves more than one general-purpose machine instruction, which do not necessarily decode and execute quickly. By incorporating an AGU into a CPU design, together with introducing specialized instructions that use

7011-479: The CPU to access main memory . By having address calculations handled by separate circuitry that operates in parallel with the rest of the CPU, the number of CPU cycles required for executing various machine instructions can be reduced, bringing performance improvements. While performing various operations, CPUs need to calculate memory addresses required for fetching data from the memory; for example, in-memory positions of array elements must be calculated before

7134-422: The CPU to malfunction. Another major issue, as clock rates increase dramatically, is the amount of heat that is dissipated by the CPU . The constantly changing clock causes many components to switch regardless of whether they are being used at that time. In general, a component that is switching uses more energy than an element in a static state. Therefore, as clock rate increases, so does energy consumption, causing

7257-467: The CPU to require more heat dissipation in the form of CPU cooling solutions. One method of dealing with the switching of unneeded components is called clock gating , which involves turning off the clock signal to unneeded components (effectively disabling them). However, this is often regarded as difficult to implement and therefore does not see common usage outside of very low-power designs. One notable recent CPU design that uses extensive clock gating

7380-431: The advent and eventual success of the ubiquitous personal computer , the term CPU is now applied almost exclusively to microprocessors. Several CPUs (denoted cores ) can be combined in a single processing chip. Previous generations of CPUs were implemented as discrete components and numerous small integrated circuits (ICs) on one or more circuit boards. Microprocessors, on the other hand, are CPUs manufactured on

7503-428: The advent of the transistor . Transistorized CPUs during the 1950s and 1960s no longer had to be built out of bulky, unreliable, and fragile switching elements, like vacuum tubes and relays . With this improvement, more complex and reliable CPUs were built onto one or several printed circuit boards containing discrete (individual) components. In 1964, IBM introduced its IBM System/360 computer architecture that

SECTION 60

#1732775865372

7626-490: The appropriate drivers. As device management is a very OS -specific topic, these drivers are handled differently by each kind of kernel design, but in every case, the kernel has to provide the I/O to allow drivers to physically access their devices through some port or memory location. Important decisions have to be made when designing the device management system, as in some designs accesses may involve context switches , making

7749-564: The complexity and number of transistors in a single CPU many fold. This widely observed trend is described by Moore's law , which had proven to be a fairly accurate predictor of the growth of CPU (and other IC) complexity until 2016. While the complexity, size, construction and general form of CPUs have changed enormously since 1950, the basic design and function has not changed much at all. Almost all common CPUs today can be very accurately described as von Neumann stored-program machines. As Moore's law no longer holds, concerns have arisen about

7872-423: The complexity scale, a machine language program is a collection of machine language instructions that the CPU executes. The actual mathematical operation for each instruction is performed by a combinational logic circuit within the CPU's processor known as the arithmetic–logic unit or ALU. In general, a CPU executes an instruction by fetching it from memory, using its ALU to perform an operation, and then storing

7995-439: The computer has available. The kernel is responsible for deciding which memory each process can use, and determining what to do when not enough memory is available. I/O devices include, but are not limited to, peripherals such as keyboards, mice, disk drives, printers, USB devices, network adapters, and display devices . The kernel provides convenient methods for applications to use these devices which are typically abstracted by

8118-486: The control unit as part of the von Neumann architecture . In modern computer designs, the control unit is typically an internal part of the CPU with its overall role and operation unchanged since its introduction. The arithmetic logic unit (ALU) is a digital circuit within the processor that performs integer arithmetic and bitwise logic operations. The inputs to the ALU are the data words to be operated on (called operands ), status information from previous operations, and

8241-457: The controller to malfunction, or system to crash. With this, depending on the complexity of the device, some devices can get surprisingly complex to program, and use several different controllers. Because of this, providing a more abstract interface to manage the device is important. This interface is normally done by a device driver or hardware abstraction layer. Frequently, applications will require access to these devices. The kernel must maintain

8364-478: The data requested by the program. The program can then be resumed from the point where it was stopped. This scheme is generally known as demand paging . Virtual addressing also allows creation of virtual partitions of memory in two disjointed areas, one being reserved for the kernel ( kernel space ) and the other for the applications ( user space ). The applications are not permitted by the processor to address kernel memory, thus preventing an application from damaging

8487-453: The desired operation. The action is then completed, typically in response to a clock pulse. Very often the results are written to an internal CPU register for quick access by subsequent instructions. In other cases results may be written to slower, but less expensive and higher capacity main memory . For example, if an instruction that performs addition is to be executed, registers containing operands (numbers to be summed) are activated, as are

8610-518: The device drivers. A monolithic kernel is one single program that contains all of the code necessary to perform every kernel-related task. Every part which is to be accessed by most programs which cannot be put in a library is in the kernel space: Device drivers, scheduler, memory handling, file systems, and network stacks. Many system calls are provided to applications, to allow them to access all those services. A monolithic kernel, while initially loaded with subsystems that may not be needed, can be tuned to

8733-429: The drawbacks of globally synchronous CPUs. For example, a clock signal is subject to the delays of any other electrical signal. Higher clock rates in increasingly complex CPUs make it more difficult to keep the clock signal in phase (synchronized) throughout the entire unit. This has led many modern CPUs to require multiple identical clock signals to be provided to avoid delaying a single signal significantly enough to cause

8856-453: The early 1980s). In the 1960s, MOS ICs were slower and initially considered useful only in applications that required low power. Following the development of silicon-gate MOS technology by Federico Faggin at Fairchild Semiconductor in 1968, MOS ICs largely replaced bipolar TTL as the standard chip technology in the early 1970s. As the microelectronic technology advanced, an increasing number of transistors were placed on ICs, decreasing

8979-474: The entire operating system. Even in systems where the kernel is included in application address spaces , memory protection is used to prevent unauthorized applications from modifying the kernel. The kernel's interface is a low-level abstraction layer . When a process requests a service from the kernel, it must invoke a system call , usually through a wrapper function . There are different kernel architecture designs. Monolithic kernels run entirely in

9102-578: The era of specialized supercomputers like those made by Cray Inc and Fujitsu Ltd . During this period, a method of manufacturing many interconnected transistors in a compact space was developed. The integrated circuit (IC) allowed a large number of transistors to be manufactured on a single semiconductor -based die , or "chip". At first, only very basic non-specialized digital circuits such as NOR gates were miniaturized into ICs. CPUs based on these "building block" ICs are generally referred to as "small-scale integration" (SSI) devices. SSI ICs, such as

9225-432: The execution and stops a program that violates a rule, such as a user process that tries to write to kernel memory. In systems that lack support for capabilities, processes are isolated from each other by using separate address spaces. Calls from user processes into the kernel are regulated by requiring them to use one of the above-described system call methods. An alternative approach is to use language-based protection. In

9348-503: The execution of an instruction, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter . If a jump instruction was executed, the program counter will be modified to contain the address of the instruction that was jumped to and program execution continues normally. In more complex CPUs, multiple instructions can be fetched, decoded and executed simultaneously. This section describes what

9471-401: The faster the clock, the more instructions the CPU will execute each second. To ensure proper operation of the CPU, the clock period is longer than the maximum time needed for all signals to propagate (move) through the CPU. In setting the clock period to a value well above the worst-case propagation delay , it is possible to design the entire CPU and the way it moves data around the "edges" of

9594-418: The firmware does not support protection mechanisms, it is possible to simulate protection at a higher level, for example by simulating capabilities by manipulating page tables , but there are performance implications. Lack of hardware support may not be an issue, however, for systems that choose to use language-based protection. An important kernel design decision is the choice of the abstraction levels where

9717-559: The individual transistors used by the PDP-8 and PDP-10 to SSI ICs, and their extremely popular PDP-11 line was originally built with SSI ICs, but was eventually implemented with LSI components once these became practical. Lee Boysel published influential articles, including a 1967 "manifesto", which described how to build the equivalent of a 32-bit mainframe computer from a relatively small number of large-scale integration circuits (LSI). The only way to build LSI chips, which are chips with

9840-399: The kernel at runtime, permitting easy extension of the kernel's capabilities as required, while helping to minimize the amount of code running in kernel space. Most work in the monolithic kernel is done via system calls. These are interfaces, usually kept in a tabular structure, that access some subsystem within the kernel such as disk operations. Essentially calls are made within programs and

9963-425: The kernel can be classified according to several criteria, including: static (enforced at compile time ) or dynamic (enforced at run time ); pre-emptive or post-detection; according to the protection principles they satisfy (e.g., Denning ); whether they are hardware supported or language based; whether they are more an open mechanism or a binding policy; and many more. Support for hierarchical protection domains

10086-462: The kernel is one of the first programs loaded on startup (after the bootloader ). It handles the rest of startup as well as memory, peripherals , and input/output (I/O) requests from software , translating them into data-processing instructions for the central processing unit . The critical code of the kernel is usually loaded into a separate area of memory, which is protected from access by application software or other less critical parts of

10209-402: The kernel itself or the kernel can also rely on other processes it is running. Although the kernel must provide IPC in order to provide access to the facilities provided by each other, kernels must also provide running programs with a method to make requests to access these facilities. The kernel is also responsible for context switching between processes or threads. The kernel has full access to

10332-431: The kernel manages, and indeed to objects provided by other user applications. An efficient and simple way to provide hardware support of capabilities is to delegate to the memory management unit (MMU) the responsibility of checking access-rights for every memory access, a mechanism called capability-based addressing . Most commercial computer architectures lack such MMU support for capabilities. An alternative approach

10455-405: The kernel so that applications do not need to know their implementation details. Key aspects necessary in resource management are defining the execution domain ( address space ) and the protection mechanism used to mediate access to the resources within a domain. Kernels also provide methods for synchronization and inter-process communication (IPC). These implementations may be located within

10578-475: The kernel will be rewritten if the available hardware changes), configured by the user (typical on older PCs and on systems that are not designed for personal use) or detected by the operating system at run time (normally called plug and play ). In plug-and-play systems, a device manager first performs a scan on different peripheral buses , such as Peripheral Component Interconnect (PCI) or Universal Serial Bus (USB), to detect installed devices, then searches for

10701-439: The limits of integrated circuit transistor technology. Extreme miniaturization of electronic gates is causing the effects of phenomena like electromigration and subthreshold leakage to become much more significant. These newer concerns are among the many factors causing researchers to investigate new methods of computing such as the quantum computer , as well as to expand the use of parallelism and other methods that extend

10824-489: The list of these devices by querying the system for them in some way. This can be done through the BIOS, or through one of the various system buses (such as PCI/PCIE, or USB). Using an example of a video driver, when an application requests an operation on a device, such as displaying a character, the kernel needs to send this request to the current active video driver. The video driver, in turn, needs to carry out this request. This

10947-408: The location of a value that may be a processor register or a memory address, as determined by some addressing mode . In some CPU designs, the instruction decoder is implemented as a hardwired, unchangeable binary decoder circuit. In others, a microprogram is used to translate instructions into sets of CPU configuration signals that are applied sequentially over multiple clock pulses. In some cases

11070-406: The machine language opcode . While processing an instruction, the CPU decodes the opcode (via a binary decoder ) into control signals, which orchestrate the behavior of the CPU. A complete machine language instruction consists of an opcode and, in many cases, additional bits that specify arguments for the operation (for example, the numbers to be summed in the case of an addition operation). Going up

11193-421: The memory that stores the microprogram is rewritable, making it possible to change the way in which the CPU decodes instructions. After the fetch and decode steps, the execute step is performed. Depending on the CPU architecture, this may consist of a single action or a sequence of actions. During each action, control signals electrically enable or disable various parts of the CPU so they can perform all or part of

11316-710: The number of individual ICs needed for a complete CPU. MSI and LSI ICs increased transistor counts to hundreds, and then thousands. By 1968, the number of ICs required to build a complete CPU had been reduced to 24 ICs of eight different types, with each IC containing roughly 1000 MOSFETs. In stark contrast with its SSI and MSI predecessors, the first LSI implementation of the PDP-11 contained a CPU composed of only four LSI integrated circuits. Since microprocessors were first introduced they have almost completely overtaken all other central processing unit implementation methods. The first commercially available microprocessor, made in 1971,

11439-501: The objects that they are not allowed to access. This approach has been implemented in JXOS , Unununium as well as Microsoft's Singularity research project. Operating system kernel The kernel is a computer program at the core of a computer 's operating system and generally has complete control over everything in the system. The kernel is also responsible for preventing and mitigating conflicts between different processes. It

11562-583: The ones used in the Apollo Guidance Computer , usually contained up to a few dozen transistors. To build an entire CPU out of SSI ICs required thousands of individual chips, but still consumed much less space and power than earlier discrete transistor designs. IBM's System/370 , follow-on to the System/360, used SSI ICs rather than Solid Logic Technology discrete-transistor modules. DEC's PDP-8 /I and KI10 PDP-10 also switched from

11685-528: The operating system to use other data stores, like a hard drive , to store what would otherwise have to remain in main memory ( RAM ). As a result, operating systems can allow programs to use more memory than the system has physically available. When a program needs data which is not currently in RAM, the CPU signals to the kernel that this has happened, and the kernel responds by writing the contents of an inactive memory block to disk (if necessary) and replacing it with

11808-548: The operating system. The kernel performs its tasks, such as running processes, managing hardware devices such as the hard disk , and handling interrupts, in this protected kernel space . In contrast, application programs such as browsers, word processors, or audio or video players use a separate area of memory, user space . This separation prevents user data and kernel data from interfering with each other and causing instability and slowness, as well as preventing malfunctioning applications from affecting other applications or crashing

11931-445: The operation very CPU-intensive and easily causing a significant performance overhead. In computing, a system call is how a process requests a service from an operating system's kernel that it does not normally have permission to run. System calls provide the interface between a process and the operating system. Most operations interacting with the system require permissions not available to a user-level process, e.g., I/O performed with

12054-409: The parts of the arithmetic logic unit (ALU) that perform addition. When the clock pulse occurs, the operands flow from the source registers into the ALU, and the sum appears at its output. On subsequent clock pulses, other components are enabled (and disabled) to move the output (the sum of the operation) to storage (e.g., a register or memory). If the resulting sum is too large (i.e., it is larger than

12177-544: The physical wiring of the computer. This overcame a severe limitation of ENIAC, which was the considerable time and effort required to reconfigure the computer to perform a new task. With von Neumann's design, the program that EDVAC ran could be changed simply by changing the contents of the memory. EDVAC was not the first stored-program computer; the Manchester Baby , which was a small-scale experimental stored-program computer, ran its first program on 21 June 1948 and

12300-501: The popularization of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of nanometers . Both the miniaturization and standardization of CPUs have increased the presence of digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in electronic devices ranging from automobiles to cellphones, and sometimes even in toys. While von Neumann

12423-473: The possible exception of the last level. Each extra level of cache tends to be bigger and is optimized differently. Other types of caches exist (that are not counted towards the "cache size" of the most important caches mentioned above), such as the translation lookaside buffer (TLB) that is part of the memory management unit (MMU) that most CPUs have. Caches are generally sized in powers of two: 2, 8, 16 etc. KiB or MiB (for larger non-L1) sizes, although

12546-451: The processor. It tells the computer's memory, arithmetic and logic unit and input and output devices how to respond to the instructions that have been sent to the processor. It directs the operation of the other units by providing timing and control signals. Most computer resources are managed by the CU. It directs the flow of data between the CPU and the other devices. John von Neumann included

12669-478: The reliability problems. Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs. Clock signal frequencies ranging from 100 kHz to 4 MHz were very common at this time, limited largely by the speed of the switching devices they were built with. The design complexity of CPUs increased as various technologies facilitated the building of smaller and more reliable electronic devices. The first such improvement came with

12792-435: The requested action, and if it is permitted performs the access for it (either directly, or by delegating the request to another user-level process). The performance cost of address space switching limits the practicality of this approach in systems with complex interactions between objects, but it is used in current operating systems for objects that are not accessed frequently or which are not expected to perform quickly. If

12915-488: The rest of the system to rely on them. Per Brinch Hansen presented arguments in favour of separation of mechanism and policy. The failure to properly fulfill this separation is one of the major causes of the lack of substantial innovation in existing operating systems, a problem common in computer architecture. The monolithic design is induced by the "kernel mode"/"user mode" architectural approach to protection (technically called hierarchical protection domains ), which

13038-409: The result to memory. Besides the instructions for integer mathematics and logic operations, various other machine instructions exist, such as those for loading data from memory and storing it back, branching operations, and mathematical operations on floating-point numbers performed by the CPU's floating-point unit (FPU). The control unit (CU) is a component of the CPU that directs the operation of

13161-484: The rising and falling clock signal. This has the advantage of simplifying the CPU significantly, both from a design perspective and a component-count perspective. However, it also carries the disadvantage that the entire CPU must wait on its slowest elements, even though some portions of it are much faster. This limitation has largely been compensated for by various methods of increasing CPU parallelism (see below). However, architectural improvements alone do not solve all of

13284-435: The running kernel. This fundamental partition of memory space has contributed much to the current designs of actual general-purpose kernels and is almost universal in such systems, although some research kernels (e.g., Singularity ) take other approaches. To perform useful functions, processes need access to the peripherals connected to the computer, which are controlled by the kernel through device drivers . A device driver

13407-444: The same address space, or it may be in a second address space). Other operating systems have a per-process address space, with a separate address space for each user-mode process. Another approach taken in experimental operating systems is to have a single address space for all software, and rely on a programming language's semantics to make sure that arbitrary memory cannot be accessed – applications cannot acquire any references to

13530-410: The security mechanisms and policies should be implemented. Kernel security mechanisms play a critical role in supporting security at higher levels. One approach is to use firmware and kernel support for fault tolerance (see above), and build the security policy for malicious behavior on top of that (adding features such as cryptography mechanisms where necessary), delegating some responsibility to

13653-540: The short switching time of a transistor in comparison to a tube or relay. The increased reliability and dramatically increased speed of the switching elements, which were almost exclusively transistors by this time; CPU clock rates in the tens of megahertz were easily obtained during this period. Additionally, while discrete transistor and IC CPUs were in heavy usage, new high-performance designs like single instruction, multiple data (SIMD) vector processors began to appear. These early experimental designs later gave rise to

13776-413: The software level, device driver abstractions include: For example, to show the user something on the screen, an application would make a request to the kernel, which would forward the request to its display driver, which is then responsible for actually plotting the character/pixel. A kernel must maintain a list of available devices. This list may be known in advance (e.g., on an embedded system where

13899-431: The system's memory and must allow processes to safely access this memory as they require it. Often the first step in doing this is virtual addressing , usually achieved by paging and/or segmentation . Virtual addressing allows the kernel to make a given physical address appear to be another address, the virtual address. Virtual address spaces may be different for different processes; the memory that one process accesses at

14022-439: The term "CPU" is generally defined as a device for software (computer program) execution, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer . The idea of a stored-program computer had been already present in the design of John Presper Eckert and John William Mauchly 's ENIAC , but was initially omitted so that it could be finished sooner. On June 30, 1945, before ENIAC

14145-629: The two, leading naturally to a microkernel design (see Separation of protection and security ). While monolithic kernels execute all of their code in the same address space ( kernel space ), microkernels try to run most of their services in user space, aiming to improve maintainability and modularity of the codebase. Most kernels do not fit exactly into one of these categories, but are rather found in between these two designs. These are called hybrid kernels . More exotic designs such as nanokernels and exokernels are available, but are seldom used for production systems. The Xen hypervisor, for example,

14268-422: The use of a conditional jump), and existence of functions . In some processors, some other instructions change the state of bits in a "flags" register . These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. For example, in such processors a "compare" instruction evaluates two values and sets or clears bits in the flags register to indicate which one

14391-431: The usefulness of the classical von Neumann model. The fundamental operation of most CPUs, regardless of the physical form they take, is to execute a sequence of stored instructions that is called a program. The instructions to be executed are kept in some kind of computer memory . Nearly all CPUs follow the fetch, decode and execute steps in their operation, which are collectively known as the instruction cycle . After

14514-440: The various programs and libraries that the operating system uses to interact with the kernel: software that performs input/output , manipulates file system objects, application software , etc. Each user space process normally runs in its own virtual memory space, and, unless explicitly allowed, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and

14637-616: The von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both. Most modern CPUs are primarily von Neumann in design, but CPUs with the Harvard architecture are seen as well, especially in embedded applications; for instance, the Atmel AVR microcontrollers are Harvard-architecture processors. Relays and vacuum tubes (thermionic tubes) were commonly used as switching elements;

14760-538: Was made, mathematician John von Neumann distributed a paper entitled First Draft of a Report on the EDVAC . It was the outline of a stored-program computer that would eventually be completed in August 1949. EDVAC was designed to perform a certain number of instructions (or operations) of various types. Significantly, the programs written for EDVAC were to be stored in high-speed computer memory rather than specified by

14883-647: Was so popular that it dominated the mainframe computer market for decades and left a legacy that is continued by similar modern computers like the IBM zSeries . In 1965, Digital Equipment Corporation (DEC) introduced another influential computer aimed at the scientific and research markets—the PDP-8 . Transistor-based computers had several distinct advantages over their predecessors. Aside from facilitating increased reliability and lower power consumption, transistors also allowed CPUs to operate at much higher speeds because of

15006-399: Was the Intel 4004 , and the first widely used microprocessor, made in 1974, was the Intel 8080 . Mainframe and minicomputer manufacturers of the time launched proprietary IC development programs to upgrade their older computer architectures , and eventually produced instruction set compatible microprocessors that were backward-compatible with their older hardware and software. Combined with

15129-429: Was used in a series of computers capable of running the same programs with different speeds and performances. This was significant at a time when most electronic computers were incompatible with one another, even those made by the same manufacturer. To facilitate this improvement, IBM used the concept of a microprogram (often called "microcode"), which still sees widespread use in modern CPUs. The System/360 architecture

#371628