MicroArchaeology Mac OS

Additional diagram of macOS architecture (2017 update)

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The architecture of macOS describes the layers of the operating system that is the culmination of Apple Inc.'s decade-long research and development process to replace the classic Mac OS.

After the failures of their previous attempts -- Pink, which started as an Apple project but evolved into a joint venture with IBM called Taligent, and Copland, which started in 1994 and was cancelled two years later -- Apple began development of Mac OS X with the acquisition of NeXT's NeXTSTEP in 1997.

Note that Mac OS X was renamed to OS X in 2012 and then again to macOS in 2016.

Development[edit]

NeXTSTEP[edit]

NeXTSTEP used a hybrid kernel that combined the Mach 2.5 kernel developed at Carnegie Mellon University with subsystems from 4.3BSD. NeXTSTEP also introduced a new windowing system based on Display PostScript that intended to achieve better WYSIWYG systems by using the same language to draw content on monitors that drew content on printers. NeXT also included object-oriented programming tools based on the Objective-C language that they had acquired from Stepstone and a collection of Frameworks (or Kits) that were intended to speed software development. NeXTSTEP originally ran on Motorola's 68k processors, but was later ported to Intel's x86, Hewlett-Packard's PA-RISC and Sun Microsystems' SPARC processors. Later on, the developer tools and frameworks were released, as OpenStep, as a development platform that would run on other operating systems.

Rhapsody[edit]

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On February 4, 1997, Apple acquired NeXT and began development of the Rhapsody operating system. Rhapsody built on NeXTSTEP, porting the core system to the PowerPC architecture and adding a redesigned user interface based on the Platinum user interface from Mac OS 8. An emulation layer called Blue Box allowed Mac OS applications to run within an actual instance of the Mac OS and an integrated Java platform.^[1] The Objective-C developer tools and Frameworks were referred to as the Yellow Box and also made available separately for Microsoft Windows. The Rhapsody project eventually bore the fruit of all Apple's efforts to develop a new generation Mac OS, which finally shipped in the form of Mac OS X Server.

Mac OS X[edit]

A diagram of the Mac OS X architecture

At the 1998 Worldwide Developers Conference (WWDC), Apple announced a move that was intended as a response to complaints from Macintosh software developers who were not happy with the two options (Yellow Box and Blue Box) available in Rhapsody. Mac OS X would add another developer API to the existing ones in Rhapsody. Key APIs from the Macintosh Toolbox would be implemented in Mac OS X to run directly on the BSD layers of the operating system instead of in the emulated Macintosh layer. This modified interface, called Carbon, would eliminate approximately 2000 troublesome API calls (of about 8000 total) and replace them with calls compatible with a modern OS.^[2]

At the same conference, Apple announced that the Mach side of the kernel had been updated with sources from the OSFMK 7.3 (Open Source Foundation Mach Kernel) ^[3] and the BSD side of the kernel had been updated with sources from the FreeBSD, NetBSD and OpenBSD projects.^[2] They also announced a new driver model called I/O Kit, intended to replace the Driver Kit used in NeXTSTEP citing Driver Kit's lack of power management and hot-swap capabilities and its lack of automatic configuration capability.^[4]

At the 1999 WWDC, Apple revealed Quartz, a new Portable Document Format (PDF) based windowing system for the operating system that was not encumbered with licensing fees to Adobe like the Display PostScript windowing system of NeXTSTEP. Apple also announced that the Yellow Box layer had been renamed Cocoa and began to move away from their commitment to providing the Yellow Box on Windows. At this WWDC, Apple also showed Mac OS X booting off of a HFS Plus formatted drive for the first time.

The first public release of Mac OS X released to consumers was a Public Beta released on September 13, 2000.

References[edit]

^Apple PR (1997-11-19). 'Apple Extends Rhapsody Developer Release with Mac OS Compatibility Environment, Code-Named 'Blue Box''. apple.com. Archived from the original on 1998-12-02. Retrieved 2006-10-13.
^ ^a^bScott Anguish (May 1998). 'WWDC 98 Summary'. stepwise.com. Retrieved 2006-10-13.^{[dead link]}
^Apple WWDC Videos (2017-02-19), Apple WWDC 2000 Session 106 - Mac OS X: Kernel, retrieved 2018-07-06
^Scott Anguish (1998-05-14). 'Rhapsody Core OS: Intro to the I/O Driver Architecture'. stepwise.com. Retrieved 2006-10-13.^{[dead link]}

External links[edit]

Retrieved from 'https://en.wikipedia.org/w/index.php?title=Architecture_of_macOS&oldid=997169817'

OS X provides many benefits to the Macintoshuser and developer communities. These benefits include improvedreliability and performance, enhanced networking features, an object-basedsystem programming interface, and increased support for industrystandards.

In creating OS X, Apple has completely re-engineered theMac OS core operating system. Forming the foundation of OS Xis the kernel. Figure 3-1 illustrates the OS X architecture.

The kernel provides many enhancements for OS X. Theseinclude preemption, memory protection,enhanced performance, improved networking facilities, support for bothMacintosh (Extended and Standard) and non-Macintosh (UFS, ISO 9660,and so on) file systems, object-oriented APIs, and more. Two ofthese features, preemption and memory protection, lead to a morerobust environment.

In Mac OS 9, applications cooperate to share processor time.Similarly, all applications share the memory of the computer amongthem. Mac OS 9 is a cooperative multitasking environment.The responsiveness of all processes is compromised if even a single applicationdoesn’t cooperate. On the other hand, real-time applications suchas multimedia need to be assured of predictable, time-critical,behavior.

In contrast, OS X is a preemptive multitasking environment.In OS X, the kernel provides enforcement of cooperation, schedulingprocesses to share time (preemption). This supports real-time behaviorin applications that require it.

In OS X, processes do not normally share memory. Instead,the kernel assigns each process its own addressspace, controlling access to these address spaces. Thiscontrol ensures that no application can inadvertently access ormodify another application’s memory (protection). Size is notan issue; with the virtual memory system included in OS X, eachapplication has access to its own 4 GB address space.

Viewed together, all applications are said to run in userspace, but this does not imply that they share memory. User spaceis simply a term for the combined address spaces of all user-levelapplications. The kernel itself has its own address space, calledkernel space. In OS X, no application can directly modify thememory of the system software (the kernel).

Although user processes do not share memory by default asin Mac OS 9, communication (and even memory sharing) between applicationsis still possible. For example, the kernel offers a rich set ofprimitives to permit some sharing of information among processes. Theseprimitives include shared libraries, frameworks, and POSIX sharedmemory. Mach messaging provides another approach, handing memoryfrom one process to another. Unlike Mac OS 9, however, memory sharingcannot occur without explicit action by the programmer.

Darwin

The OS X kernel is an Open Source project.The kernel, along with other core parts of OS X are collectivelyreferred to as Darwin. Darwin is a complete operatingsystem based on many of the same technologies that underlie OS X. However, Darwin does not include Apple’s proprietary graphicsor applications layers, such as Quartz, QuickTime, Cocoa, Carbon,or OpenGL.

Figure 3-2 shows the relationship between Darwin and OS X. Both build upon the same kernel, but OS X adds Core Services,Application Services and QuickTime, as well as the Classic, Carbon, Cocoa,and Java (JDK) application environments. Both Darwin and OS X include the BSD command-line application environment; however,in OS X, use of environment is not required, and thus it ishidden from the user unless they choose to access it.

Darwin technology is based on BSD, Mach3.0, and Apple technologies. Best of all, Darwin technology is OpenSource technology, which means that developers have full accessto the source code. In effect, OS X third-party developers canbe part of the Darwin core system software development team. Developerscan also see how Apple is doing things in the core operating systemand adopt (or adapt) code to use within their own products. Referto the Apple Public Source License (APSL) fordetails.

Because the same software forms the core of both OS Xand Darwin, developers can create low-level software that runs onboth OS X and Darwin with few, if any, changes. The only differenceis likely to be in the way the software interacts with the applicationenvironment.

Darwin is based on proven technology from many sources. Alarge portion of this technology is derived from FreeBSD, a versionof 4.4BSD that offers advanced networking, performance, security,and compatibility features. Other parts of the system software,such as Mach, are based on technology previously used in Apple’sMkLinux project, in OS X Server, and in technology acquiredfrom NeXT. Much of the code is platform-independent. All of thecore operating-system code is available in source form.

The core technologies have been chosen for several reasons.Mach provides a clean set of abstractions for dealing with memorymanagement, interprocess (and interprocessor) communication (IPC),and other low-level operating-system functions. In today’s rapidly changinghardware environment, this provides a useful layer of insulationbetween the operating system and the underlying hardware.

BSD is a carefully engineered, mature operating system withmany capabilities. In fact, most of today’s commercial UNIX andUNIX-like operating systems contain a great deal of BSD code. BSDalso provides a set of industry-standard APIs.

New technologies, such as the I/O Kit and Network Kernel Extensions(NKEs), have been designed and engineered by Apple to take advantageof advanced capabilities, such as those provided by an object-orientedprogramming model. OS X combines these new technologies withtime-tested industry standards to create an operating system thatis stable, reliable, flexible, and extensible.

Architecture

The foundation layer of Darwin and OS X is composed ofseveral architectural components, as shown in Figure 3-3. Taken together, thesecomponents form the kernel environment.

Important: Note that OS X uses the term kernel somewhatdifferently than you might expect.

“A kernel, in traditional operating-system terminology,is a small nucleus of software that provides only the minimal facilitiesnecessary for implementing additional operating-system services.”— from The Design and Implementation of the 4.4 BSDOperating System, McKusick, Bostic, Karels, and Quarterman,1996.

Similarly, in traditional Mach-based operating systems, thekernel refers to the Mach microkernel and ignores additional low-levelcode without which Mach does very little.

In OS X, however, the kernel environment contains muchmore than the Mach kernel itself. The OS X kernel environmentincludes the Mach kernel, BSD, the I/O Kit, file systems, and networkingcomponents. These are often referred to collectively as the kernel.Each of these components is described briefly in the following sections.For further details, refer to the specific component chapters orto the reference material listed in the bibliography.

Because OS X contains three basic components (Mach, BSD,and the I/O Kit), there are also frequently as many as three APIsfor certain key operations. In general, the API chosen should matchthe part of the kernel where it is being used, which in turn isdictated by what your code is attempting to do. The remainder ofthis chapter describes Mach, BSD, and the I/O Kit and outlines thefunctionality that is provided by those components.

Mach

Mach manages processor resources such as CPU usage and memory,handles scheduling, provides memory protection, and provides a messaging-centeredinfrastructure to the rest of the operating-system layers. The Machcomponent provides

untyped interprocesscommunication (IPC)
remote procedure calls (RPC)
scheduler support for symmetric multiprocessing (SMP)
support for real-time services
virtual memory support
support for pagers
modular architecture

General information about Mach may be found in the chapter Mach Overview. Informationabout scheduling can be found in the chapter Mach Scheduling and Thread Interfaces.Information about the VM system can be found in Memory and Virtual Memory.

BSD

Above the Mach layer, the BSD layer provides “OS personality”APIs and services. The BSD layer is based on the BSD kernel, primarily FreeBSD.The BSD component provides

file systems
networking (except for the hardware device level)
UNIX security model
syscall support
the BSD process model, including process IDs and signals
FreeBSD kernel APIs
many of the POSIX APIs
kernel support for pthreads (POSIX threads)

The BSD component is described in more detail in the chapter BSD Overview.

Networking

OS X networking takes advantage of BSD’s advanced networkingcapabilities to provide support for modern features, such as NetworkAddress Translation (NAT) and firewalls.The networking component provides

4.4BSD TCP/IPstack and socket APIs
support for both IP and DDP (AppleTalk transport)
multihoming
routing
multicast support
server tuning
packet filtering
Mac OS Classic support (through filters)

More information about networking may be found in the chapter Network Architecture.

File Systems

OS X provides support for numerous types of file systems,including HFS, HFS+, UFS, NFS, ISO9660, and others. The default file-system type is HFS+;OS X boots (and “roots”) from HFS+, UFS, ISO, NFS, and UDF.Advanced features of OS X file systems include an enhanced VirtualFile System (VFS) design. VFS provides for alayered architecture (file systems are stackable).The file system component provides

UTF-8 (Unicode)support
increased performance over previous versions of Mac OS.

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More information may be found in the chapter File Systems Overview.

I/O Kit

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The I/O Kit provides a framework for simplified driver development,supporting many categories of devices.The I/O Kit features an object-orientedI/O architecture implemented in a restricted subset of C++. TheI/O Kit framework is both modular and extensible. The I/O Kit componentprovides

true plugand play
dynamic device management
dynamic (“on-demand”) loading of drivers
power management for desktop systems as well as portables
multiprocessor capabilities

The I/O Kit is described in greater detail in the chapter I/O Kit Overview.

Kernel Extensions

OS X provides a kernel extension mechanism as a meansof allowing dynamic loading of pieces of code into kernel space,without the need to recompile. These pieces of code are known genericallyas plug-ins or, in the OS X kernel environment,as kernel extensions or KEXTs.

Because KEXTs provide both modularity and dynamic loadability,they are a natural choice for any relatively self-contained servicethat requires access to interfaces that are not exported to userspace. Many of the components of the kernel environment supportthis extension mechanism, though they do so in different ways.

For example, some of the new networking features involve theuse of network kernel extensions (NKEs). Theseare discussed in the chapter Network Architecture.

The ability to dynamically add a new file-system implementationis based on VFS KEXTs. Device drivers and device families in theI/O Kit are implemented using KEXTs. KEXTs make development mucheasier for developers writing drivers or those writing code to supporta new volume format or networking protocol. KEXTs are discussedin more detail in the chapter Kernel Extension Overview.