Device driver
A device driver, or software driver is a computer program allowing higher-level computer programs to interact with a device.
A driver typically communicates with the device through the computer bus or communications subsystem to which the hardware is connected. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface.
Purpose
A device driver simplifies programming by acting as a translator between a device and the applications or operating systems that use it. The higher-level code can be written independently of whatever specific hardware device it may control. Every version of a device, such as a printer, requires its own specialized commands. In contrast, most applications access devices (such as sending a file to a printer) by using high-level, generic commands, such as PRINTLN. The driver accepts these generic statements and converts them into the low-level commands required by the device.
Design
Device drivers can be abstracted into logical and physical layers. Logical layers process data for a class of devices such as ethernet ports or disk drives. Physical layers communicate with specific device instances. For example, a serial port needs to handle standard communication protocols such as XON/XOFF that are common for all serial port hardware. This would be managed by a serial port logical layer. However, the logical layer needs to communicate with a particular serial port chip. 16550 UART hardware differs from PL-011. The physical layer addresses these chip specific variations. Conventionally, OS requests go to the logical layer first. In turn, the logical layer calls upon the physical layer to implement OS requests in terms understandable by the hardware. Inversely, when a hardware device needs to respond to the OS, it uses the physical layer to speak through the logical layer.
Linux device drivers can be built either as parts of the kernel or separately as loadable modules. Makedev includes a list of the devices in Linux: ttyS (terminal), lp (parallel port), hd (disk), loop (loopback disk device, sound (these include mixer, sequencer, dsp, and audio)...
The Windows .sys files and Linux .ko modules are loadable device drivers. The advantage of loadable device drivers is that they can be loaded only when necessary and then unloaded, thus saving kernel memory.
Kernel-mode vs User-mode
Device drivers, particularly on modern Windows platforms, can run in kernel-mode (Ring 1) or in user-mode (Ring 2). The primary benefit of running a driver in user mode is improved stability, since a poorly written user mode device driver cannot crash the system by overwriting kernel memory.
Device driver applications
Because of the diversity of modern hardware and operating systems, many ways exist in which drivers can be used. Drivers are used for interfacing with:
* Printers
* Video adapters
* Network cards
* Sound cards
* Local buses of various sorts - in particular, for bus mastering on modern systems
* Low-bandwidth I/O buses of various sorts (for pointing devices such as mice, keyboards, USB, etc.)
* computer storage devices such as hard disk, CD-ROM and floppy disk buses (ATA, SATA, SCSI)
* Implementing support for different file systems
* Implementing support for image scanners and digital cameras
Common levels of abstraction for device drivers are
* For hardware:
o Interfacing directly
o Using some higher-level interface (e.g. Video BIOS)
o Using another lower-level device driver (e.g. file system drivers using disk drivers)
o Simulating work with hardware, while doing something entirely different
* For software:
o Allowing the operating system direct access to hardware resources
o Implementing only primitives
o Implementing an interface for non-driver software (e.g. TWAIN)
o Implementing a language, sometimes quite high-level (e.g. PostScript)
Choosing and installing the correct device drivers for given hardware is often a key component of computer system configuration.
Virtual device drivers
A particular variant of device drivers are virtual device drivers. They are used to emulate a hardware device, particularly in virtualization environments, for example when an MS-DOS program is run on a Microsoft Windows computer or when a guest operating system is run on, for example, a Xen host. Instead of enabling the guest operating system to dialog with hardware, virtual device drivers take the opposite role and emulate a piece of hardware, so that the guest operating system and its drivers running inside a virtual machine can have the illusion of accessing real hardware. Attempts by the guest operating system to access the hardware are routed to the virtual device driver in the host operating system as e.g. function calls. The virtual device driver can also send simulated processor-level events like interrupts into the virtual machine.
Virtual devices are also used in a non-virtualized environment. For example a virtual network adapter is used with a virtual private network, while a virtual disk device is used with iSCSI.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.
A driver typically communicates with the device through the computer bus or communications subsystem to which the hardware is connected. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface.
Purpose
A device driver simplifies programming by acting as a translator between a device and the applications or operating systems that use it. The higher-level code can be written independently of whatever specific hardware device it may control. Every version of a device, such as a printer, requires its own specialized commands. In contrast, most applications access devices (such as sending a file to a printer) by using high-level, generic commands, such as PRINTLN. The driver accepts these generic statements and converts them into the low-level commands required by the device.
Design
Device drivers can be abstracted into logical and physical layers. Logical layers process data for a class of devices such as ethernet ports or disk drives. Physical layers communicate with specific device instances. For example, a serial port needs to handle standard communication protocols such as XON/XOFF that are common for all serial port hardware. This would be managed by a serial port logical layer. However, the logical layer needs to communicate with a particular serial port chip. 16550 UART hardware differs from PL-011. The physical layer addresses these chip specific variations. Conventionally, OS requests go to the logical layer first. In turn, the logical layer calls upon the physical layer to implement OS requests in terms understandable by the hardware. Inversely, when a hardware device needs to respond to the OS, it uses the physical layer to speak through the logical layer.
Linux device drivers can be built either as parts of the kernel or separately as loadable modules. Makedev includes a list of the devices in Linux: ttyS (terminal), lp (parallel port), hd (disk), loop (loopback disk device, sound (these include mixer, sequencer, dsp, and audio)...
The Windows .sys files and Linux .ko modules are loadable device drivers. The advantage of loadable device drivers is that they can be loaded only when necessary and then unloaded, thus saving kernel memory.
Kernel-mode vs User-mode
Device drivers, particularly on modern Windows platforms, can run in kernel-mode (Ring 1) or in user-mode (Ring 2). The primary benefit of running a driver in user mode is improved stability, since a poorly written user mode device driver cannot crash the system by overwriting kernel memory.
Device driver applications
Because of the diversity of modern hardware and operating systems, many ways exist in which drivers can be used. Drivers are used for interfacing with:
* Printers
* Video adapters
* Network cards
* Sound cards
* Local buses of various sorts - in particular, for bus mastering on modern systems
* Low-bandwidth I/O buses of various sorts (for pointing devices such as mice, keyboards, USB, etc.)
* computer storage devices such as hard disk, CD-ROM and floppy disk buses (ATA, SATA, SCSI)
* Implementing support for different file systems
* Implementing support for image scanners and digital cameras
Common levels of abstraction for device drivers are
* For hardware:
o Interfacing directly
o Using some higher-level interface (e.g. Video BIOS)
o Using another lower-level device driver (e.g. file system drivers using disk drivers)
o Simulating work with hardware, while doing something entirely different
* For software:
o Allowing the operating system direct access to hardware resources
o Implementing only primitives
o Implementing an interface for non-driver software (e.g. TWAIN)
o Implementing a language, sometimes quite high-level (e.g. PostScript)
Choosing and installing the correct device drivers for given hardware is often a key component of computer system configuration.
Virtual device drivers
A particular variant of device drivers are virtual device drivers. They are used to emulate a hardware device, particularly in virtualization environments, for example when an MS-DOS program is run on a Microsoft Windows computer or when a guest operating system is run on, for example, a Xen host. Instead of enabling the guest operating system to dialog with hardware, virtual device drivers take the opposite role and emulate a piece of hardware, so that the guest operating system and its drivers running inside a virtual machine can have the illusion of accessing real hardware. Attempts by the guest operating system to access the hardware are routed to the virtual device driver in the host operating system as e.g. function calls. The virtual device driver can also send simulated processor-level events like interrupts into the virtual machine.
Virtual devices are also used in a non-virtualized environment. For example a virtual network adapter is used with a virtual private network, while a virtual disk device is used with iSCSI.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.