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The fundamental services and primitives ofthe OS X kernel are based on Mach3.0. Apple has modified and extended Mach to better meet OS X functional and performance goals.

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Mach 3.0 was originally conceived as a simple, extensible,communications microkernel. It iscapable of running as a stand–alone kernel, with other traditionaloperating-system services such as I/O, file systems, and networkingstacks running as user-mode servers.

However, in OS X, Mach is linked with other kernel componentsinto a single kernel address space. This is primarily for performance;it is much faster to make a direct call between linked componentsthan it is to send messages or do remote procedure calls (RPC) betweenseparate tasks. This modular structure results in a more robustand extensible system than a monolithic kernel would allow, withoutthe performance penalty of a pure microkernel.

Thus in OS X, Mach is not primarily a communication hubbetween clients and servers. Instead, its value consists of itsabstractions, its extensibility, and its flexibility. In particular,Mach provides

object-basedAPIs with communication channels (for example, ports) as object references
highly parallel execution, including preemptively scheduled threads and support for SMP
a flexible scheduling framework, with support for real-time usage
a complete set of IPC primitives, including messaging, RPC, synchronization, and notification
support for large virtual address spaces, shared memoryregions, and memory objects backed by persistent store
proven extensibility and portability, for example across instructionset architectures and in distributed environments
security and resource management as a fundamental principleof design; all resources are virtualized

Mach Kernel Abstractions

Mach provides a small set of abstractions that have been designedto be both simple and powerful. These are the main kernel abstractions:

Tasks. Theunits of resource ownership; each task consists of a virtual addressspace, a portrightnamespace, and one or more threads.(Similar to a process.)
Threads. The units of CPU execution withina task.
Addressspace. In conjunction with memory managers, Mach implementsthe notion of a sparse virtual address space and shared memory.
Memoryobjects. The internal units of memory management. Memoryobjects include named entries and regions; they are representationsof potentially persistent data that may be mapped into address spaces.
Ports.Secure, simplex communication channels, accessible only via sendand receive capabilities (known as port rights).
IPC.Message queues, remote procedure calls, notifications, semaphores,and lock sets.
Time.Clocks, timers, and waiting.

At the trap level, the interface to most Mach abstractionsconsists of messages sent to and from kernel ports representingthose objects. The trap-level interfaces (such as mach_msg_overwrite_trap)and message formats are themselves abstracted in normal usage bythe Mach Interface Generator(MIG).MIG is used to compile procedural interfaces to the message-basedAPIs, based on descriptions of those APIs.

Tasks and Threads

OS X processes and POSIXthreads (pthreads)are implemented on top of Mach tasks and threads, respectively.A thread is a point of control flow in a task. A task exists to provideresources for the threads it contains. This split is made to providefor parallelism and resource sharing.

A thread

is a pointof control flow in a task.
has access to all of the elements of the containing task.
executes (potentially) in parallel with other threads, eventhreads within the same task.
has minimal state information for low overhead.

A task

is a collectionof system resources. These resources, with the exception of theaddress space, are referenced by ports. These resources may be sharedwith other tasks if rights to the ports are so distributed.
provides a large, potentially sparse address space, referencedby virtual address. Portions of this space may be shared throughinheritance or external memory management.
contains some number of threads.

Note that a task has no life of its own—only threads executeinstructions. When it is said that “task Y does X,” what isreally meant is that “a thread contained within task Y does X.”

A task is a fairly expensive entity. It exists to be a collectionof resources. All of the threads in a task share everything. Twotasks share nothing without an explicit action (although the actionis often simple) and some resources (such as port receive rights) cannotbe shared between two tasks at all.

A thread is a fairly lightweight entity. It is fairly cheapto create and has low overhead to operate. This is true becausea thread has little state information (mostly its register state). Itsowning task bears the burdenof resource management. On a multiprocessor computer, it is possiblefor multiple threads in a task to execute in parallel. Even whenparallelism is not the goal, multiple threads have an advantagein that each threadcan use a synchronous programming style, instead of attempting asynchronousprogramming with a single thread attempting to provide multipleservices.

A threadis the basic computational entity. A thread belongs to one and onlyone task that defines its virtual address space. To affect the structureof the address space or to reference any resource other than theaddress space, the thread must execute a special trap instructionthat causes the kernel to perform operations on behalf of the threador to send a message to some agent on behalf of the thread. In general,these traps manipulate resources associated with the task containingthe thread. Requests can be made of the kernel to manipulate theseentities: to create them, delete them, and affect their state.

Mach provides a flexible framework for thread–schedulingpolicies. Early versions of OS X support both time-sharing and fixed-priority policies.A time-sharing thread’s priority is raised and lowered to balanceits resource consumption against other time-sharing threads.

Fixed-priority threads execute for a certain quantum of time, and then areput at the end of the queue of threads of equal priority. Settinga fixed priority thread’s quantum level to infinity allows thethread to run until it blocks, or until it is preempted by a threadof higher priority. High priority real-time threads are usuallyfixed priority.

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OS X also provides time constraint scheduling for real-timeperformance. This scheduling allows you to specify that your threadmust get a certain time quantum within

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