Architettura Windows NT

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L'architettura della famiglia di sistemi operativi Windows NT è strutturata in due livelli (user mode e kernel mode), entrambi i quali includono vari moduli.

Windows 2000, Windows XP, Windows Server 2003, e Windows Vista sono dei sistemi operativi facenti parte della famiglia Windows NT prodotta da Microsoft. Tutti sono sistemi operativi a Multitasking con prelazione, progettati per lavorare su macchine Intel x86 sia a singolo sia a multiprocessore (SMP).

A partire da Windows XP, Microsoft ha cominciato a sviluppare il supporto nei suoi sistemi operativi, prima il suo sistema operativo era basato su un modello a 32-bit. L'architettura della linea di sistemi operativi NT e' altamente modulare e si struttura in due strati principali: user mode e kernel mode. I programmi ed i sottosistemi in user mode sono limitati in termini di risorse di sistema alle quali possono accedere, mentre cio' che gira in kernel mode gode di un accesso alla memoria centrale ed ai device esterni senza restrizioni. Il kernel dei sistemi operativi di questa linea e' conosciuto come kernel ibrido - tuttavia non e' essenzialmente un kernel monolitico in qualche modo strutturato come un microkernel. L'architettura comprende un kernel ibrido, un Livello di astrazione dell'Hardware (HAL), driver che vivono in kernel mode.

L'user mode nella linea Windows NT e' realizzato tramite un sottosistema capace di passare le richieste I/O agli appropriati driver software in kernel mode usando un manager di I/O. L'user mode di WIndows 2000 e' fatto di due sottosistemi: Il sottosistema d'Ambiente, che puo' eseguire applicazioni scritte per diversi sistemi operativi, ed il sottosistema integrato, che svolge le funzioni specifiche del sistema operativo. Il kernel mode in Windows 2000 ha accesso completo alle risorse hardware di sistema, inoltre ha la funzione di interdire ai servizi in user mode l'accesso ad aree critiche del sistema operativo alle quali non dovrebbero avere accesso.

L'Executive dialoga con tutti i sottosistemi dell'user mode: I/O, gestione degli oggetti, gestione dei processi e della sicurezza. Il kernel si pone tra l'HAL e l'Executive per la sincronizzazione multiprocessore, i thread ed interrupt scheduling e dispatching, trap handling ed exception dispatching. Il kernel ha inoltre la funzione di inizializzare i driver dei dispositivi all'avvio. I driver possono girare in Kernel mode in tre livelli: alto (highest level drivers), medio (intermediate drivers) e basso (low level drivers). Il Modello di driver Windows (Windows Driver Model, WDM) è di livello medio e fu ideato per essere compatibile sia a livello binario che di codice sorgente tra Windows 98 e Windows 2000. I driver di basso livello possono essere o driver supportati in retrocompatibilità che controllano un dispositivo in modo diretto, o un bus hardware Plug and Play.

[modifica] Modalità utente

La modalità utente è costituira sal sottosistema che trasmette le richieste I/O i driver appropriati della modalità kernel tramite l'I/O manager (che risiede nella kernel mode). Due sottosistemi costituiscono lo strato di per la modalità utente di Windows 2000: ilSottosistema d'ambient e il Sottosistema integral.

Il sottosistema d'ambiente è progettato per eseguire applicazioni scritte per sistemi operativi diversi. Nessuno dei sottosistemi d'ambiente può accedere direttamente all'hardware, e deve richiedere l'accesso alle risorse attraverso il Manager della memoria virtuale che esegue in kernel mode. Inoltre le applicazioni girano a livello di priorità basso rispetto ai processi in modalità kernel. Attualmente ci sono tre principali sottosistemi d'ambiente: il sottosistema Win32, un sottosistema OS/2 e un sottosistema per POSIX.

Il sottosistema d'ambiente Win32 può eseguire applicazioni Windows a 32-bit. Contiene la console come supportato dalla finestra di testo, lo spegnimento e una gestione di hard-error per tutti lgi altri sottosistemi d'ambiente. Supporta anche la Virtual DOS Machine (VDMs), che permette alle applicazioni MS-DOS e Windows 3.x a 16-bit di essere eseguite su Windows. C'è una specifica MS-DOS VDM che gira nel proprio spazio di indirizzamento a che emula un Intel 80486 che esegue MS-DOS 5. I programmi Win16, comunque, girano in una VDM Win16. Ogni programma, di default, gira nello stesso processo, usando lo stesso spazio di indirizzamento, e la VDM Win16 concede ad ogni programma il suo proprio thread da eseguire. Tuttavia, Windows 2000 permette agli utenti di eseguire un programma Win16 in una separata Win16 VDM, che consente al programma di essere soggetto al multitasking poiché Windows 2000 will pre-empt the whole VDM process, which only contains one running application. The OS/2 environment subsystem supports 16-bit character-based OS/2 applications and emulates OS/2 1.x, but not 2.x or later OS/2 applications. The POSIX environment subsystem supports applications that are strictly written to either the POSIX.1 standard or the related ISO/IEC standards.

The integral subsystem looks after operating system specific functions on behalf of the environment subsystem. It consists of a security subsystem, a workstation service and a server service. The security subsystem deals with security tokens, grants or denies access to user accounts based on resource permissions, handles logon requests and initiates logon authentication, and determines which system resources need to be audited by Windows 2000. It also looks after Active Directory. The workstation service is an API to the network redirector, which provides the computer access to the network. The server service is an API that allows the computer to provide network services.

[modifica] Kernel mode

Windows 2000 kernel mode has full access to the hardware and system resources of the computer and runs code in a protected memory area. It controls access to scheduling, thread prioritisation, memory management and the interaction with hardware. The kernel mode stops user mode services and applications from accessing critical areas of the operating system that they should not have access to as user mode processes ask the kernel mode to perform such operations on its behalf.

Kernel mode consists of executive services, which is itself made up on many modules that do specific tasks, kernel drivers, a kernel and a Hardware Abstraction Layer, or HAL.

[modifica] Executive

The Executive interfaces with all the user mode subsystems. It deals with I/O, object management, security and process management. It contains various components, including the I/O Manager, the Security Reference Monitor, the Object Manager, the IPC Manager, the Virtual Memory Manager (VMM), a PnP Manager and Power Manager, as well as a Window Manager which works in conjunction with the Windows Graphics Device Interface (GDI). Each of these components exports a kernel-only support routine allows other components to communicate with one another. Grouped together, the components can be called executive services. No executive component has access to the internal routines of any other executive component.

The object manager is a special executive subsystem that all other executive subsystems must pass through to gain access to Windows 2000 resources — essentially making it a resource management infrastructure service. The object manager is used to reduce the duplication of object resource management functionality in other executive subsystems, which could potentially lead to bugs and make development of Windows 2000 harder^[1]. To the object manager, each resource is an object, whether that resource is a physical resource (such as a file system or peripheral) or a logical resource (such as a file). Each object has a structure or object type that the object manager must know about. When another executive subsystem requests the creation of an object, they send that request to the object manager which creates an empty object structure which the requesting executive subsystem then fills in^[2]. Object types define the object procedures and any data specific to the object. In this way, the object manager allows Windows 2000 to be an object oriented operating system, as object types can be thought of as classes that define objects.

Each instance of an object that is created stores its name, parameters that are passed to the object creation function, security attributes and a pointer to its object type. The object also contains an object close procedure and a reference count to tell the object manager how many other objects in the system reference that object and thereby determines whether the object can be destroyed when a close request is sent to it^[3]. Every object exists in a hierarchical object namespace.

Further executive subsystems are the following:

• I/O Manager: allows devices to communicate with user-mode subsystems. It translates user-mode read and write commands in read or write IRPs which it passes to device drivers. It accepts file system I/O requests and translates them into device specific calls, and can incorporate low-level device drivers that directly manipulate hardware to either read input or write output. It also includes a cache manager to improve disk performance by caching read requests and write to the disk in the background
• Security Reference Monitor (SRM): the primary authority for enforcing the security rules of the security integral subsystem^[4]. It determines whether an object or resource can be accessed, via the use of access control lists (ACLs), which are themselves made up of access control entries (ACEs). ACEs contain a security identifier (SID) and a list of operations that the ACE gives a select group of trustees — a user account, group account, or logon session^[5] — permission (allow, deny, or audit) to that resource.^[6][7]
• IPC Manager: short for Interprocess Communication Manager, this manages the communication between clients (the environment subsystem) and servers (components of the Executive). It can use two facilities: the Local Procedure Call (LPC) facility (clients and servers on the one computer) and the Remote Procedure Call (RPC) facility (where clients and servers are situated on different computers. Microsoft has had significant security issues with the RPC facility^[8].
• Virtual Memory Manager: manages virtual memory, allowing Windows 2000 to use the hard disk as a primary storage device (although strictly speaking it is secondary storage). It controls the paging of memory in and out of physical memory to disk storage.
• Process Manager: handles process and thread creation and termination
• PnP Manager: handles Plug and Play and supports device detection and installation at boot time. It also has the responsibility to stop and start devices on demand — sometimes this happens when a bus gains a new device and needs to have a device driver loaded to support that device. Both FireWire and USB are hot-swappable and require the services of the PnP Manager to load, stop and start devices. The PnP manager interfaces with the HAL, the rest of the executive (as necessary) and with device drivers.
• Power Manager: the power manager deals with power events and generates power IRPs. It coordinates these power events when several devices send a request to be turned off it determines the best way of doing this.
• The display system has been moved from user mode into the kernel mode as a device driver contained in the file Win32k.sys. There are two components in this device driver — the Window Manager and the GDI:
  • Window Manager: responsible for drawing windows and menus. It controls the way that output is painted to the screen and handles input events (such as from the keyboard and mouse), then passes messages to the applications that need to receive this input
  • GDI: the Graphics Device Interface is responsible for tasks such as drawing lines and curves, rendering fonts and handling palettes. Windows 2000 introduced native alpha blending into the GDI.

[modifica] Kernel & kernel-mode drivers

The kernel sits between the HAL and the Executive and provides multiprocessor synchronization, thread and interrupt scheduling and dispatching, and trap handling and exception dispatching. The kernel often interfaces with the process manager.^[9] The kernel is also responsible for initialising device drivers at bootup that are necessary to get the operating system up and running.

Windows 2000 uses kernel-mode device drivers to enable it to interact with hardware devices. Each of the drivers has well defined system routines and internal routines that it exports to the rest of the operating system. All devices are seen by user mode code as a file object in the I/O manager, though to the I/O manager itself the devices are seen as device objects, which it defines as either file, device or driver objects. Kernel mode drivers exist in three levels: highest level drivers, intermediate drivers and low level drivers. The highest level drivers, such as file system drivers for FAT and NTFS, rely on intermediate drivers. Intermediate drivers consist of function drivers — or main driver for a device — that are optionally sandwiched between lower and higher level filter drivers. The function driver then relies on a bus driver — or a driver that services a bus controller, adapter, or bridge — which can have an optional bus filter driver that sits between itself and the function driver. Intermediate drivers rely on the lowest level drivers to function. The Windows Driver Model (WDM) exists in the intermediate layer. The lowest level drivers are either legacy Windows NT device drivers that control a device directly or can be a PnP hardware bus. These lower level drivers directly control hardware and do not rely on any other drivers.

[modifica] Hardware abstraction layer

The Windows 2000 Hardware Abstraction Layer, or HAL, is a layer between the physical hardware of the computer and the rest of the operating system. It was designed to hide differences in hardware and therefore provide a consistent platform on which applications may run. The HAL includes hardware specific code that controls I/O interfaces, interrupt controllers and multiple processors.

Windows 2000 was designed to support the 64-bit DEC Alpha. After Compaq announced they would discontinue support of the processor, Microsoft stopped releasing tests build of Windows 2000 for AXP to the public, stopping with beta 3. Development of Windows on the Alpha continued internally in order to continue to have a 64-bit architecture development model ready until the wider availability of the Intel Itanium IA-64 architecture. The HAL now only supports hardware that is compatible with the Intel x86 architecture.

[modifica] Note

References

[modifica] Collegamenti esterni

• Microsoft's official Windows 2000 site
• Windows 2000 Plug and Play architecture