Misplaced Pages

#374625

128-468: Mach-O , short for Mach object file format, is a file format for executables , object code , shared libraries , dynamically loaded code, and core dumps . It was developed to replace the a.out format. Mach-O is used by some systems based on the Mach kernel . NeXTSTEP , macOS , and iOS are examples of systems that use this format for native executables, libraries and object code. Each Mach-O file

256-454: A system call or trap . The program uses a library to place data in a well known location in memory and then causes a fault , a type of error. When a system is first started, its kernel is set up to be the "handler" of all faults; thus, when a program causes a fault, the kernel takes over, examines the information passed to it, then carries out the instructions. Under Mach, the IPC system

384-491: 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

512-516: A POE Server, resulting in a kernel that was actually larger than the UNIX it was based on. The idea, however, was to move the UNIX layer out of the kernel into user-space, where it could be more easily worked on and even replaced outright. Unfortunately performance proved to be a major problem, and a number of architectural changes were made in order to solve this problem. Unwieldy UNIX licensing issues also plagued researchers, so this early effort to provide

640-486: A binary digit set one. The last eight binary digits is the section type value. The Mach-O loader records the symbol pointer sections and symbol stub sections. They are sequentially used by the indirect symbol table to load in method calls. Mach kernel Mach ( / m ɑː k / ) is a kernel developed at Carnegie Mellon University by Richard Rashid and Avie Tevanian to support operating system research, primarily distributed and parallel computing . Mach

768-407: 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

896-425: 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

1024-461: 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

1152-468: 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

1280-402: A feature of the basic system, allowing servers to be raised into the kernel space using built-in mechanisms. Mach 4 attempted to address these problems with a more radical set of upgrades. In particular, it was found that program code was typically not writable, so potential hits due to copy-on-write were rare. Thus it made sense to not map the memory between programs for IPC, but instead migrate

1408-428: A few strategies were developed to improve performance. Like its predecessor, Accent, Mach used a single shared-memory mechanism for physically passing the message from one program to another. Physically copying the message would be too slow, so Mach relies on the machine's memory management unit (MMU) to quickly map the data from one program to another. Only if the data is written to would it have to be physically copied,

SECTION 10

#1732781140375

1536-515: 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

1664-528: 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

1792-477: A new solution to the port concept, using the copy-on-write (COW) mechanism provided by the virtual memory system. Instead of copying data between programs, all that was required was to instruct the MMU to provide access to that same memory. This system would implement the interprocess communications (IPC) system with dramatically higher performance. This concept was picked up at Carnegie-Mellon, who adapted Aleph for

1920-425: A non-licensed UNIX-like system environment continued to find use, well into the further development of Mach. The resulting Mach 3 was released in 1990, and generated intense interest. A small team had built Mach and ported it to a number of platforms, including complex multiprocessor systems which were causing serious problems for older-style kernels. This generated considerable interest in the commercial market, where

2048-493: A number of companies were considering changing hardware platforms. If the existing system could be ported to run on Mach, it seemed it would then be easy to change the platform underneath. Mach received a major boost in visibility when the Open Software Foundation (OSF) announced they would be hosting future versions of OSF/1 on Mach 2.5, and were investigating Mach 3 as well. Mach 2.5 was also selected for

2176-423: 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

2304-684: 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

2432-592: 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

2560-419: A process called " copy-on-write ". Messages were also checked for validity by the kernel, to avoid bad data crashing one of the many programs making up the system. Ports were deliberately modeled on the UNIX file system concepts. This permits the user to find ports using existing file system navigation concepts, as well as assigning rights and permissions as they would on the file system. Development under such

2688-407: 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

SECTION 20

#1732781140375

2816-448: A segment is initially placed in the virtual address space, it is given the CPU access permissions specified by the initial virtual memory protections value. The permissions on a region of the virtual address space may be changed by application or library code with calls to routines such as mprotect() ; the maximum virtual memory protections limit what permissions may be granted for access to

2944-473: A segment load command has a relocation list offset that specifies the offsets in the section that must be adjusted based on the application's base address. The relocations are unnecessary if the application can be placed at its defined RAM address locations such as a base address of zero. 0x00000019 (Command type 64-bit) A segment name cannot be larger than 16 text characters in bytes. The unused characters are 0x00 in value. The segment command contains

3072-446: 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 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

3200-404: A separate area of memory, which is protected from access by application software or other less critical parts of 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

3328-451: A series of second generation microkernels, which further reduced the complexity of the system and placed almost all functionality in the user space. For instance, the L4 kernel (version 2) includes only seven system calls and uses 12k of memory, whereas Mach 3 includes about 140 functions and uses about 330k of memory. IPC calls under L4 on a 486DX-50 take only 5μs, faster than a UNIX syscall on

3456-520: A set of CPU subtype values, indicating a particular model of that CPU type for which the code is intended. Newer models of a CPU type may support instructions, or other features, not supported by older CPU models, so that code compiled or written for a newer model might contain instructions that are illegal instructions on an older model, causing that code to trap or otherwise fail to operate correctly when run on an older model. Code intended for an older model will run on newer models without problems. If

3584-414: A system would be easier. Not only would the code being worked on exist in a traditional program that could be built using existing tools, it could also be started, debugged and killed off using the same tools. With a monokernel a bug in new code would take down the entire machine and require a reboot, whereas under Mach this would require only that the program be restarted. Additionally the user could tailor

3712-417: A traditional syscall, but not by much. The rest, the majority of the actual problem, was due to the kernel performing tasks such as checking the message for port access rights. While it would seem this is an important security concern, in fact, it only makes sense in a UNIX-like system. For instance, a single-user operating system running a cell phone or robot might not need any of these features, and this

3840-449: 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

3968-456: A user-space Mach system it would take just under 500μs. When Mach was first being seriously used in the 2.x versions, performance was slower than traditional monolithic operating systems, perhaps as much as 25%. This cost was not considered particularly worrying, however, because the system was also offering multi-processor support and easy portability. Many felt this was an expected and acceptable cost to pay. When Mach 3 attempted to move most of

Mach-O - Misplaced Pages Continue

4096-478: 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

4224-466: Is 0xfeedface while the magic number for 64-bit architectures is 0xfeedfacf . The reserved value is only present in 64-bit Mach-O files. It is reserved for future use or extension of the 64-bit header. The CPU type indicates the instruction set architecture for the code. If the file is for the 64-bit version of the instruction set architecture, the CPU type value has the 0x01000000 bit set. The CPU type values are as follows: Each CPU type has

4352-447: 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

4480-432: Is a major issue for Mach 3 systems. However, the concept of a multi-server operating system is still promising, though it still requires some research. The developers have to be careful to isolate code into modules that do not call from server to server. For instance, the majority of the networking code would be placed in a single server, thereby minimizing IPC for normal networking tasks. Most developers instead stuck with

4608-423: 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 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)

4736-423: 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

4864-420: 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 –

4992-499: 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

5120-425: Is common in modern systems, Mach was the first system to define tasks and threads in this way. The kernel's job was reduced from essentially being the operating system to running the "utilities" and providing them access to the hardware. The existence of ports and the use of IPC is perhaps the most fundamental difference between Mach and traditional kernels. Under UNIX, calling the kernel consists of an operation named

5248-452: Is exactly the sort of system where Mach's pick-and-choose operating system would be most valuable. Likewise Mach caused problems when memory had been moved by the operating system, another task that only really makes sense if the system has more than one address space. DOS and the early Mac OS have a single large address space shared by all programs, so under these systems the mapping did not provide any benefits. These realizations led to

Mach-O - Misplaced Pages Continue

5376-472: Is followed by a sequence of Mach-O images. Each entry refers to a Mach-O image. The CPU type and subtype for an entry must be the same as the CPU type and subtype for the Mach-O image to which the entry refers. The file offset and size are the offset in the file of the beginning of the Mach-O image, and the size of the Mach-O image, to which the entry refers. The section alignment is the logarithm, base 2, of

5504-462: Is made up of one Mach-O header, followed by a series of load commands, followed by one or more segments, each of which contains between 0 and 255 sections. Mach-O uses the REL relocation format to handle references to symbols. When looking up symbols Mach-O uses a two-level namespace that encodes each symbol into an 'object/symbol name' pair that is then linearly searched for, first by the object and then

5632-455: Is often considered one of the earliest examples of a microkernel . However, not all versions of Mach are microkernels. Mach's derivatives are the basis of the operating system kernel in GNU Hurd and of Apple 's XNU kernel used in macOS , iOS , iPadOS , tvOS , and watchOS . The project at Carnegie Mellon ran from 1985 to 1994, ending with Mach 3.0, which is a true microkernel . Mach

5760-505: 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

5888-409: Is that it is not clear how to find ports. Under UNIX this problem was solved over time as programmers agreed on a number of "well known" locations in the file system to serve various duties. While this same approach worked for Mach's ports as well, under Mach the operating system was assumed to be much more fluid, with ports appearing and disappearing all the time. Without some mechanism to find ports and

6016-452: 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

6144-441: 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

6272-663: 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 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

6400-479: The NeXTSTEP system and a number of commercial multiprocessor vendors. Mach 3 led to a number of efforts to port other operating systems parts for the microkernel, including IBM 's Workplace OS and several efforts by Apple to build a cross-platform version of the classic Mac OS . Support for running DOS applications in a Mach 3.0 environment was demonstrated by researchers, following on from earlier work running

6528-517: The PERQ workstation and implemented it using copy-on-write. The port was successful, but the resulting Accent kernel was of limited practical use because it did not run existing software. Moreover, Accent was as tightly tied to PERQ as Aleph was to the Eclipse. The major change between these experimental kernels and Mach was the decision to make a version of the existing 4.2BSD kernel re-implemented on

SECTION 50

#1732781140375

6656-401: 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

6784-408: 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,

6912-743: The 1990s. The primary remaining widespread uses of the Mach kernel are Apple's macOS and its sibling iOS, which run atop a heavily modified hybrid Open Software Foundation Mach Kernel (OSFMK 7.3) called " XNU " also used in OSF/1. In XNU, the file systems, networking stacks, and process and memory management functions are implemented in the kernel; and file system, networking, and some process and memory management functions are invoked from user mode via ordinary system calls rather than message passing; XNU's Mach messages are used for communication between user-mode processes, and for some requests from user-mode code to

7040-437: The 32-bit version of the segment load command, while 0x00000019 is used to specify the 64-bit version of the segment load command., The segment load command varies if the Mach-O header is 32-bit, or 64-bit. This is because 64-bit processor architecture uses 64-bit addresses while 32-bit architectures use 32-bit addresses. All virtual RAM addresses are added to a base address to keep applications spaced apart. Each section in

7168-486: The Accent message-passing concepts. Such a kernel would be binary compatible with existing BSD software, making the system immediately available for everyday use while still being a useful experimental platform. Additionally, the new kernel would be designed from the start to support multiple processor architectures, even allowing heterogeneous clusters to be constructed. In order to bring the system up as quickly as possible,

7296-668: The BSD developers at CSRG , and appears in modern BSD-derived Unix systems such as FreeBSD . Mach is the logical successor to Carnegie Mellon's Accent kernel . Mach's lead developer Richard Rashid has been employed at Microsoft since 1991; he founded the Microsoft Research division. Co-founding Mach developer Avie Tevanian, was formerly head of software at NeXT , then Chief Software Technology Officer at Apple Inc. until March 2006. The developers rode bicycles to lunch through rainy Pittsburgh's mud puddles, and Tevanian joked

7424-524: The BSD system were re-written to call into Mach, and a change to 4.3BSD was also made during this process. By 1986 the system was complete to the point of being able to run on its own on the DEC VAX . Although doing little of any practical value, the goal of making a microkernel was realized. This was soon followed by versions on the IBM RT PC and for Sun Microsystems 68030 -based workstations, proving

7552-401: The CPU type is ARM then the subtypes are as follows: If the CPU type is x86 then the subtypes are as follows: After the subtype value is the file type value. After the file type value is the number of load commands and the total number of bytes the load commands are after the Mach-O header, then a 32-bit flag with the following possible settings. Multiple binary digits can be set to one in

7680-598: The Mach kernel would also have to be reentrant, in practice this is not an issue because its response times are so fast it can simply wait and serve requests in turn. Mach also included a server that could forward messages not just between programs, but even over the network, which was an area of intense development in the late 1980s and early 1990s. Unfortunately, the use of IPC for almost all tasks turned out to have serious performance impact. Benchmarks on 1997 hardware showed that Mach 3.0-based UNIX single-server implementations were about 50% slower than native UNIX. Study of

7808-402: The Mach-O header and the size in bytes to where the load commands end. The size of load commands is used as a redundancy check. When the last load command is read and the number of bytes for the load commands do not match, or if we go outside the number of bytes for load commands before reaching the last load command, then the file may be corrupted. Each load command is a sequence of entries in

SECTION 60

#1732781140375

7936-405: The address to write the section in virtual address space plus the application's base address. The number of bytes to write to the address location (Address size). After the address information is the file offset the segment data is located in the Mach-O binary, and the number of bytes to read from the file. When the address size is larger than the number of bytes to read from the file, the rest of

8064-490: 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

8192-498: The basic system was sound, and went on to demonstrate computer clustering by copying the memory over an early Ethernet interface. Around this time a new generation of central processors (CPUs) were coming to market, offering a 32-bit address space and (initially optional) support for a memory management unit (MMU). The MMU handled the instructions needed to implement a virtual memory system by keeping track of which pages of memory were in use by various programs. This offered

8320-454: The byte alignment in the file required for the Mach-O image to which the entry refers; for example, a value of 14 means that the image must be aligned on a 2-byte boundary, i.e. a 16384-byte boundary. This is required by tools that modify the multi-architecture binary, in order for them to keep the image properly aligned. The load commands are read immediately after the Mach-O header. The Mach-O header tells us how many load commands exist after

8448-461: The bytes in RAM space are set 0x00 . There is a segment that is called __PAGEZERO, which has a file offset of zero and a size of zero in the file. It has a defined RAM address and size. Since it reads zero bytes from the file it fills the address location with zeros to where the binary is going to be placed in RAM. This segment is necessary to rid the section of any data from a prior application. When

8576-464: The caller has required access rights and that the message is valid. If it is, there is another context switch and memory mapping to complete the call into the user-space server. The process must then be repeated to return the results, adding up to a total of four context switches and memory mappings, plus two message verifications. This overhead rapidly compounds with more complex services, where there are often code paths passing through many servers. This

8704-493: The classic Mac OS and MultiFinder under Mach 2.5. Mach was originally intended to be a replacement for classical monolithic UNIX, and for this reason contained many UNIX-like ideas. For instance, Mach provided a permissions and security system similar to that used by UNIX's file system. Since the kernel was privileged (running in kernel-space ) over other OS servers and software, it was possible for malfunctioning or malicious programs to send it commands that would cause damage to

8832-457: 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

8960-478: 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

9088-518: 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

9216-399: The drivers out of the microkernel. The main difference with UNIX is that instead of utilities handling files, they can handle any "task". More operating system code was moved out of the kernel and into user space, resulting in a much smaller kernel and the rise of the term microkernel . Unlike traditional systems, under Mach a process, or "task", can consist of a number of threads. While this

9344-411: The entire task, or alternately, used files to pass data, which was resource expensive and time-consuming. Pipes were built on the underlying input/output system. This system is, in turn, based on a model where drivers are expected to periodically "block" while they wait for tasks to complete. For instance, a printer driver might send a line of text to a line printer and then have nothing to do until

9472-400: The exact nature of the performance problems turned up a number of interesting facts. One was that the IPC was not the problem: there was some overhead associated with the memory mapping needed to support it, but this added only a small amount of time to making a call. The rest, 80% of the time being spent, was due to additional tasks the kernel was running on the messages. Primary among these was

9600-432: 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

9728-491: The expensive IPC needed to call it in. Other performance problems were related to Mach's support for multiprocessor systems. From the mid-1980s to the early 1990s, commodity CPUs grew in performance at a rate of about 60% a year, but the speed of memory access grew at only 7% a year. This meant that the cost of accessing memory grew tremendously over this period, and since Mach was based on mapping memory around between programs, any "cache miss" made IPC calls slow. IPC overhead

9856-418: 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

9984-487: The flags to identify any information or settings that apply to the binary. Now the load commands are read as we have reached the end of the Mach-O header. Multiple Mach-O files can be combined in a multi-architecture binary . This allows a single binary file to contain code to support multiple instruction set architectures, for example for different generations and types of Apple devices, including different processor architectures such as ARM64 and x86-64 . All fields in

10112-435: The following form: The load command type identifies what the parameters are in the load command. If a load command starts with 0x80000000 bit set that means the load command is necessary in order to be able to load or run the binary. This allows older Mach-O loaders to skip commands not understood by the loader that are not mandatory for loading the application. Mach-O binaries that use load command type 0x00000001 use

10240-439: The hardware, including memory and the ports, while conventional programs using the ports system implemented all behavior, from device drivers to user programs. This concept greatly reduced the size of the kernel, and permitted users to experiment with different drivers simply by loading them and connecting them together at runtime. This greatly eased the problems when developing new operating system code, which would otherwise require

10368-457: The kernel and from the kernel to user-mode servers. Further analysis demonstrated that the IPC performance problem was not as obvious as it seemed. Recall that a single-side of a syscall took 20μs under BSD and 114μs on Mach running on the same system. Of the 114, 11 were due to the context switch, identical to BSD. An additional 18 were used by the MMU to map the message between user-space and kernel space. This adds up to only 29μs, longer than

10496-399: 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

10624-425: 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

10752-402: 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

10880-431: 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

11008-405: 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

11136-475: 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

11264-519: 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 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

11392-411: The kernel. In order to test the end-to-end performance, MkLinux in co-located mode was compared with an L4 port running in user-space. L4 added about 5%–10% overhead, compared to Mach's 29%. The following is a list of operating system kernels derived from Mach and operating systems with kernels derived from Mach: Kernel (operating system) The critical code of the kernel is usually loaded into

11520-489: 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

11648-416: The machine to be restarted. The overall concept of a small kernel and external drivers became known as a microkernel. Aleph was implemented on Data General Eclipse minicomputers and was tightly bound to them. This machine was far from ideal, since it required memory to be copied between programs, which resulted in considerable performance overhead. It was also quite expensive. Nevertheless, Aleph proved that

11776-413: The operating system into user-space, the overhead became higher still: benchmarks between Mach and Ultrix on a MIPS R3000 showed a performance hit as great as 67% on some workloads. For example, getting the system time involves an IPC call to the user-space server maintaining system clock . The caller first traps into the kernel, causing a context switch and memory mapping. The kernel then checks that

11904-468: The operating system only, in order to permit user space programs to handle things such as controlling hardware. Under Mach, and like UNIX, the operating system again becomes primarily a collection of utilities. As with UNIX, Mach keeps the concept of a driver for handling the hardware. Therefore, all the drivers for the present hardware have to be included in the microkernel. Other architectures based on hardware abstraction layer or exokernels could move

12032-479: 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

12160-445: 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

12288-599: The original POE concept of a single large server providing the operating system functionality. In order to ease development, they allowed the operating system server to run either in user-space or kernel-space. This allowed them to develop in user-space and have all the advantages of the original Mach idea, and then move the debugged server into kernel-space in order to get better performance. Several operating systems have since been constructed using this method, known as co-location , among them Lites , MkLinux , OSF/1 , and NeXTSTEP/OPENSTEP/macOS. The Chorus microkernel made this

12416-407: The original Unix operating system is the idea of a pipe . A pipe is an abstraction allowing data to be moved as an unstructured stream of bytes between programs. Using pipes, users can link together multiple programs to complete tasks, feeding data through several consecutive small programs. This contrasts with typical operating systems of the era, which require a single large program that can handle

12544-404: The pipe again. In contrast to a file, where the entire file has to be read or written before the next program can use it, pipes made the movement of data across multiple programs occur in a piecemeal fashion without any programmer intervention. However, implementing pipes in memory buffers forced data to be copied from program to program, a time-consuming and resource intensive operation. This made

12672-735: The pipe concept unsuitable for tasks where quick turnaround or low latency was needed, such as in most device drivers . The operating system's kernel and most core functionality was instead written in a single large program. When new functionality, such as computer networking , was added to the operating system, the size and complexity of the kernel grew, too. Unix pipes offered a conceptual system that could be used to build arbitrarily complex solutions out of small cooperating programs. These smaller programs were easier to develop and maintain, and had well-defined interfaces that simplified programming and debugging. These qualities are even more valuable for device drivers, where small size and bug-free performance

12800-417: The port rights checking and message validity. In benchmarks on an 486 DX-50, a standard UNIX system call took an average of 21 μs to complete, while the equivalent operation with Mach IPC averaged 114μs. Only 18μs of this was hardware related; the rest was the Mach kernel running various routines on the message. Given a syscall that does nothing, a full round-trip under BSD would require about 40μs, whereas on

12928-415: The printer completes printing that line. In this case, the driver indicates that it was blocked, and the operating system allows some other program to run until the printer indicates it is ready for more data. In the pipes system the limited resource was memory, and when one program filled the memory assigned to the pipe, it would naturally block. Normally this would cause the consuming program to run, emptying

13056-477: The program code being used into the local space of the program. This led to the concept of "shuttles" and it seemed performance had improved, but the developers moved on with the system in a semi-usable state. Mach 4 also introduced built-in co-location primitives, making it a part of the kernel. By the mid-1990s, work on microkernel systems was largely stagnant, although the market had generally believed that all modern operating systems would be microkernel based by

13184-435: 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

13312-488: 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

13440-435: 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

13568-414: The same system, and over 20 times as fast as Mach. Of course this ignores the fact that L4 is not handling permissioning or security; but by leaving this to the user-space programs, they can select as much or as little overhead as they require. The potential performance gains of L4 are tempered by the fact that the user-space applications will often have to provide many of the functions formerly supported by

13696-410: 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

13824-405: The segment. Then after the CPU address protection settings is the number of sections that are within this segment that are read after the segments flag settings. The segment flag settings are as follows: The number of sections in the segment is a set of entries that are read as follows: The section's segment name must match the segments load command name. The sections entries locate to data in

13952-399: The segment. Each section locates to the relocation entries for adjusting addresses in the section if the application base address is added to anything other than zero. The section size applies to both the size of the section at its address location and size in the file at its offset location. The section Flag/Type value is read as follows: Any of the settings that apply to the section have

14080-479: The services they represented, much of this flexibility would be lost. Mach was initially hosted as additional code written directly into the existing 4.2BSD kernel, allowing the team to work on the system long before it was complete. Work started with the already functional Accent IPC/port system, and moved on to the other key portions of the OS: tasks, threads, and virtual memory. As portions were completed various parts of

14208-413: 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

14336-419: The symbol name. The basic structure—a list of variable-length "load commands" that reference pages of data elsewhere in the file—was also used in the executable file format for Accent . The Accent file format was in turn, based on an idea from Spice Lisp . All multi-byte values in all data structures are written in the byte order of the host for which the code was produced. The magic number for 32-bit code

14464-563: The system to be distributed over multiple processors, either by using shared memory directly as in most Mach messages, or by adding code to copy the message to another processor if needed. In a traditional kernel this is difficult to implement; the system has to be sure that different programs do not try to write to the same region of memory from different processors. However, using Mach ports makes this well defined and easy to implement, so Mach ports were made first-class citizens in that system. The IPC system initially had performance problems, so

14592-435: The system to include, or exclude, whatever features they required. Since the operating system was simply a collection of programs, they could add or remove parts by simply running or killing them as they would any other program. Finally, under Mach, all of these features were deliberately designed to be extremely platform neutral. To quote one text on Mach: There are a number of disadvantages, however. A relatively mundane one

14720-416: The system would be implemented by starting with the existing BSD code, and gradually re-implementing it as inter-process communication -based (IPC-based) programs. Thus Mach would begin as a monolithic system similar to existing UNIX systems, and progress toward the microkernel concept over time. Mach started largely being an effort to produce a clearly defined, UNIX-based, highly portable Accent. The result

14848-431: 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

14976-480: The system's portability. By 1987 the list included the Encore Multimax and Sequent Balance machines, testing Mach's ability to run on multiprocessor systems. A public Release 1 was made that year, and Release 2 followed the next year. Throughout this time the promise of a "true" microkernel had not yet been delivered. These early Mach versions included the majority of 4.3BSD in the kernel, a system known as

15104-417: The system, and for this reason the kernel checked every message for validity. Additionally most of the operating system functionality was to be located in user-space programs, so this meant there needed to be some way for the kernel to grant these programs additional privileges, e.g. to directly access hardware. Some of Mach's more esoteric features were also based on this same IPC mechanism. For instance, Mach

15232-629: 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,

15360-442: The universal header are big-endian. The universal header is in the following form: The magic number in a multi-architecture binary is 0xcafebabe in big-endian byte order, so the first 4 bytes of the header will always be 0xca 0xfe 0xba 0xbe , in that order. The number of binaries is the number of entries that follow the header. The header is followed by a sequence of entries in the following form: The sequence of entries

15488-518: The word "muck" could form a backronym for their Multi-User (or Multiprocessor Universal) Communication Kernel. Italian CMU engineer Dario Giuse later asked project leader Rick Rashid about the project's current title and received "MUCK" as the answer, though not spelled out but just pronounced IPA: [mʌk] . According to the Italian alphabet , he wrote "Mach". Rashid liked Giuse's spelling "Mach" so much that it prevailed. A key concept in

15616-509: Was a short list of generic concepts: Mach developed on Accent's IPC concepts, but made the system much more UNIX-like in nature, making it possible to run UNIX programs with little or no modification. To do this, Mach introduced the port, representing each endpoint of a two-way IPC. Ports had a concept of permissions like files under UNIX, permitting a very UNIX-like model of protection to be applied to them. Additionally, Mach allowed any program to handle privileges that would normally be given to

15744-404: Was able to support multi-processor machines with ease. In a traditional kernel extensive work needs to be carried out to make it reentrant or interruptible , as programs running on different processors could call into the kernel at the same time. Under Mach, the bits of the operating system are isolated in servers, which are able to run, like any other program, on any processor. Although in theory

15872-516: Was developed as a replacement for the kernel in the BSD version of Unix , not requiring a new operating system to be designed around it. Mach and its derivatives exist within several commercial operating systems. These include all using the XNU operating system kernel which incorporates an earlier non-microkernel Mach as a major component. The Mach virtual memory management system was also adopted in 4.4BSD by

16000-533: Was extremely important. There was a strong desire to model the kernel on the same basis of small cooperating programs. One of the first systems to use a pipe-like system underpinning the operating system was the Aleph kernel developed at the University of Rochester . This introduced the concept of ports, which were essentially a shared memory implementation. In Aleph, the kernel was reduced to providing access to

16128-414: Was not possible because the kernel had no real idea what the operating system consisted of. Instead they had to use a single one-size-fits-all solution, which added to the performance problems. Mach 3 attempted to address this problem by providing a simple pager, relying on user-space pagers for better specialization. But this turned out to have little effect. In practice, any benefits it had were wiped out by

16256-412: Was not the only source of performance problems. Another centered on the problems of trying to handle memory properly when physical memory ran low and paging had to occur. In the traditional monolithic operating systems the authors had direct experience with which parts of the kernel called which others, allowing them to fine-tune their pager to avoid paging out code that was about to be used. Under Mach this

16384-640: Was used for this role instead. To call system functionality, a program would ask the kernel for access to a port, then use the IPC system to send messages to that port. Although sending a message requires a system call, just as a request for system functionality on other systems requires a system call, under Mach sending the message is pretty much all the kernel does; handling the actual request would be up to some other program. Thread and concurrency support benefited by message passing with IPC mechanisms since tasks now consist of multiple code threads which Mach could freeze and unfreeze during message handling. This permits

#374625