I picture

See intra picture.

I/O control code


I/O request packet (IRP)

A data structure that kernel-mode device drivers use to communicate with each other. An IRP is an important data structure for the Microsoft Windows Driver Model (WDM), which features multiple layered drivers. An IRP, therefore, presents every input/output (I/O) request. The IRP is passed from one driver layer to another until the request is complete. When a driver receives an IRP, it performs the operation that the IRP specifies. Then, the driver either passes the IRP back to the I/O manager for disposal or to an adjacent driver layer.


Index-Alpha 4-4. A video surface subtype used by decoders that implement DirectX VA. IA44 has a bits-per-pixel value of 8. The character "I" is a 4-bit index into a 16-entry YUV palette. The character "A" represents 4 bits of transparency information (also known as per-pixel alpha). An IA44 surface allows for 16 different colors at 16 different transparency values, or 256 different pixel representations. In IA44 the alpha is stored in the low-nibble.


See installable client driver (ICD).


See image color management (ICM).


See interrupt command register (ICR).


See interrupt dispatch block (IDB).


See Inverse Discrete Cosine Transform (IDCT).


See integrated device electronics (IDE).


See interrupt dispatch table (IDT).

IEEE 1284 Bidirectional Parallel Peripheral Interface

An interface standard that adds bidirectional communication capabilities to a peripheral device that is connected to a computer's parallel port.


Internet Engineering Task Force.


Installable file system.

See FS.


See independent hardware vendor (IHV).


See interface identifier (IID).

image color management (ICM)

A technology developed by Microsoft that ensures that a color image, graphic or text object is rendered as close as possible to its original intent on any device, despite differences in imaging technologies and color capabilities between devices.

independent configuration

See ad hoc mode.

independent hardware vendor (IHV)

An individual or an organization that creates computer hardware.

independent software vendor (ISV)

An individual or an organization that creates computer software.

INF File

A text-based file containing information required by the system to install a device's software components.

For more information, see the topic Creating an INF File.

infrastructure configuration

An infrastructure configuration is an 802.11 LAN configuration that makes it possible for stations to communicate with access points. The access points communicate through portals that are part of a distribution system. Each access point serves the stations in a Basic Service Set (BSS), and the set of BSSs defines an Extended Service Set (ESS).

INI file

An initialization file. Commonly used in 16-bit Windows, INI files have been used by both the operating system and individual applications to store persistent settings related to an application, driver, or piece of hardware. In current Windows operating systems, INI files are supported for backward compatibility, but the registry is the preferred location for storing such settings.


Describes an operation that occurs directly on input data, without use of a secondary buffer for temporary storage.

input class

The class of filters that provide an interface for HID hardware, including USB and legacy devices, as well as proprietary and other HID hardware, under the WDM HID architecture.

installable client driver (ICD)

An OpenGL driver model in which the driver is responsible for implementing the entire OpenGL pipeline. Intended for high-end graphics cards that implement most of the OpenGL pipeline in hardware.

installation root

During an installation, the directory (on the distribution medium) that contains the setup information (INF) file currently in use. After an INF file is copied to the system's INF directory, the INF file's installation root continues to reference the distribution medium, unless an installation application copies the INF file by calling SetupCopyOEMInf.

All directory paths specified within an INF file are specified relative to the installation root.

instance ID

A device identification string that distinguishes a device from other devices of the same type on a machine.

For more information, see the topic Device Identification Strings.

See also device ID, hardware ID, compatible ID, and device instance ID.

integral subsystem

A protected subsystem (server) that performs an essential operating system task. The integral subsystems include the local security authority, the security accounts manager, the session manager, and the network server.

See also protected subsystem.

integrated device electronics (IDE)

A type of disk-drive interface where the controller electronics reside on the drive itself, eliminating the need for a separate adapter card.

Integrated Services Digital Network (ISDN)

A set of standards for integrating services such as voice data and video. There are two types of ISDN interfaces, the basic rate interface and the primary rate interface. Windows supports the basic rate interface as a modem class device.


In Microsoft DirectX video acceleration, the representation of picture content by first encoding a prediction of an area of the picture using some previously-decoded picture and then optionally adding a signal that represents the deviation from that prediction.


A set of method and properties exported by an object type.

interface identifier (IID)

A GUID that uniquely identifies a particular COM interface.

inter-integrated circuit (I2C)

The I2C bus is a standard protocol two-wire (clock and data) serial data bus.

inter-integrated circuit (IIC)

See inter-integrated circuit (I2C).

  1. A scanning method that divides the screen into two fields, alternately drawing odd-numbered and even-numbered scan lines.
  2. A scheme to display a video image by displaying alternate scan lines in two discrete fields. Interlaced signals are used in broadcast video and are required for video to be compliant with NTSC.
intermediate driver

An intermediate driver processes I/O requests "between" the highest-level driver (frequently a file system driver) and the lowest-level driver that controls the hardware device for which the request is bound.

Examples of intermediate drivers include the ftdisk driver, which supports fault tolerance, other Filter Drivers, and many class drivers, particularly those layered over mass-storage device drivers.

See also class driver and layered driver.

International Commission on Illumination (CII)

The developer of color-matching systems.


The process of averaging pixel information when scaling an image. When reducing the size of an image, pixels are averaged to create a single new pixel; when an image is scaled up in size, additional pixels are created by averaging pixels of the smaller image.

interprocess communication (IPC)

The Windows NT-based system has two defined, message-passing mechanisms for IPC:

  • LPC, defining the port object type with a server process that supplies system (port) services to local (using the same physical memory on a single system) client processes

  • RPC, with a system-supplied run-time library that implements such RPC capabilities as binding a client process in one address space with a server process in another address space and sending necessary messages to make a remote procedure call possible


An asynchronous hardware signal that is detected by the processor. An interrupt usually indicates that a peripheral device needs service. An interrupt causes the processor to save the current state, to branch to a fixed location, and to resume execution at that location.

See also interrupt dispatch table (IDT).

interrupt command register (ICR)

An APIC feature.

interrupt dispatch block (IDB)

An internal structure used by the kernel.

interrupt dispatch table (IDT)

A kernel-defined call table with a platform-dependent number of first-level entries (some for DIRQL ISRs) and second-level entries for interrupt transfer routines (dependent on a first-level ISR). For symmetric multiprocessor platforms, the kernel sets up an IDT for each processor.

The kernel reserves eight first-level entries per IDT for its own use; the remaining first-level entries can be connected to a platform-specific I/O bus interrupt by the HAL or to a device interrupt. The kernel's reserved entries (in low-to-high IRQL priority) are defined by the following constants:

  • PASSIVE_LEVEL—execute thread
  • APC_LEVEL—execute special kernel APC; page fault
  • DISPATCH_LEVEL—dispatch (execute DPC)
  • WAKE_LEVEL—debugger execution
  • CLOCK2_LEVEL—interval-timer execution
  • REQUEST_LEVEL—interprocessor request
  • POWER_LEVEL—power failure notification
  • HIGH_LEVEL—machine checks or bus errors

The set of software and hardware interrupt vectors mapped to these constants is platform-dependent, but none can be connected to a driver's interrupt object. Note that the constants PASSIVE_LEVEL through DISPATCH_LEVEL and WAKE_LEVEL correspond to software interrupt vectors. DIRQLs for device drivers usually have hardware priority higher than DISPATCH_LEVEL but lower than CLOCK2_LEVEL.

See also multiprocessor machine.

interrupt object

A kernel-mode-only, kernel-defined control object type, used to connect a hardware interrupt source (see DIRQL) and an ISR to an IDT entry, or to connect an ISR and an interrupt transfer routine (dependent on the ISR at its IRQL) to a second-level IDT entry.

Each interrupt object:

  • Can be associated with a single IDT entry (and, therefore, with a single processor in an SMP machine)
  • Is either LevelSensitive or Latched, depending on the platform, I/O bus, or device
  • Can be one of several interrupt objects associated with a given IDT entry if and only if every interrupt object for that entry is of a single type (LevelSensitive or Latched)

For a second-level IDT entry, an ISR (such as an ISR for a bus adapter that is associated with a first-level IRQL) routes the interrupt to the device-specific interrupt transfer routine for service, using its entry in the IDT.

When more than one set of interrupt objects is connected to the same first-level IDT entry, the corresponding ISRs are called in the same order in which they were connected.

For more information, see the topic Servicing Interrupts.

interrupt request level (IRQL)

The hardware priority level at which a given kernel-mode routine runs, masking off interrupts with an equivalent or lower IRQL on the processor. A routine can be preempted by an interrupt with a higher IRQL.

interrupt request line (IRQ)

A hardware line over which a peripheral device, bus controller, other processor, or the kernel signals a request for service to the microprocessor.

interrupt service routine (ISR)

A routine whose purpose is to service a device when it generates an interrupt.

For more information, see the topic Servicing Interrupts.


The representation of picture content without the use of prediction. Intra pictures do not using any previously-decoded picture as a reference.

See also I picture.

intra picture

Intra-coded picture. A picture type that is coded as a stand-alone image. It does not use motion-compensated prediction.

invalid page

A page-sized range of virtual addresses for which a page fault occurs when any address in the range is referenced. For drivers, referencing an invalid page causes a fatal page fault unless the driver is running at an IRQL <= APC_LEVEL when the reference occurs.

Inverse Discrete Cosine Transform (IDCT)

A method to convert macroblocks of DCT coefficients back to pixels. IDCT reverses the DCT process.

See also DCT.


I/O control code. A system-defined or vendor-defined control code that is used to identify I/O control operations. These control codes are contained in I/O request packets, if the IRP code is IRP_MJ_DEVICE_CONTROL or IRP_MJ_INTERNAL_DEVICE_CONTROL.

For more information, see the topic Using I/O Control Codes.


See interprocess communication (IPC).


I/O request block. An IRB is a block of data that describes an operation to perform.


Infrared Data Association.


See I/O request packet (IRP).

IRP code

Short for "IRP function code". See the topic IRP Major Function Codes.

IRP sink pin


IRP source pin



IRP major function code. One of a predefined class of IRPs that a driver can accept.

For more information, see the topic IRP Major Function Codes.


See interrupt request line (IRQ).

See interrupt.


See interrupt request level (IRQL).

Running at on a given processor has no effect on the IRQL of any other processor in a symmetric multiprocessor machine.

For more information, see the topic Managing Hardware Priorities.

See also IDT, interrupt object, IRQ, ISR, multiprocessor machine, and priority.


Industry standard architecture. A standard defining the architecture of the PC I/O bus.

Also called AT bug standard.


See Integrated Services Digital Network (ISDN).

ISDN basic rate interface

ISDN basic rate interface (BRI) consists of one D channel and two B channels. The ISDN D channel is used for signaling. The B channels each provide 64 Kbps of data capacity with a combined capability of 128 Kbps. The second B channel can be configured to support voice or data traffic.

ISDN primary rate interface

In the United States, ISDN primary rate interface (PRI) consists of one D channel and 23 B channels. The D channels are used for signaling. The B channels each provide 64 Kbps of data capacity with a combined capability of 1472 Kbps.

ISDN terminal adapter

An ISDN terminal adapter is a device that is used as a modem in a Windows ISDN installation.


See ISDN terminal adapter.


International Standards Organization.

isochronous transfer

A transfer type that provides a steady, timing-critical data transmission without regard to guaranteed delivery of data.


The Plug and Play process by which cards on an ISA bus are distinguished from each other after system startup.


See interrupt service routine (ISR).


See independent software vendor (ISV).



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