Advanced Graphics Driver Architecture

This invention relates to graphics drivers, and in particular to an architecture for supporting different graphics subsystems and the various drivers usable therewith.

Today even relatively modest personal computer systems have graphics subsystems that facilitate display of elaborate graphic patterns on a video or flat-panel display. The proliferation of applications generating such sophisticated graphics has engendered changes in the way computational responsibility for their actual rendering is allocated. In older systems, the main processor would perform all graphics processing (through a display controller) according to commands supported by the operating system--the software module responsible for managing the processing and transfer of information among various system resources. The desire for greater resolution and rendering capabilities eventually began to impose an excessive computational burden on the main processor, limiting overall speed and ultimately making this simple architecture obsolete.

System designers therefore began transferring graphics-processing tasks to separate graphics hardware boards, called "graphics adapters." These boards contain circuitry dedicated exclusively to drawing graphic patterns on the display, and support both high speed and fine resolution. The fact that their functional repertoires are fixed did not initially prove limiting, since board capabilities were matched to the commands recognized by the main operating system. Application programs requiring graphics operations passed task commands to the operating system, which executed them through the graphics hardware.

The recent advent of graphical user interfaces ("GUIs") has largely rendered this arrangement obsolete as well. GUIs provide a visual metaphor of a real-world scene, such as a desktop, featuring icons that the user can manipulate with a pointing device such as a mouse. GUIs operate at a hierarchical level above the operating system but below applications, from which they accept functional commands that include requests for graphics operations. GUIs offer the user a colorful graphic environment that is preserved across applications designed for the GUI; they also impose substantial graphics-processing demands, typically requiring sophisticated graphics subsystems that include an application-program interface ("API") and a graphics engine. The API layer translates application commands, which are written in a high-level language, to the high-level drawing commands supported by the graphics engine, a specialized component of the operating system. The graphics engine, in turn, processes the high-level drawing commands into lower-level drawing commands supported by a graphics device driver. The device driver, finally, processes the lower-level drawing commands into output signals that drive the graphics adapter, which performs the actual drawing operations on the display. (Typically, the instantaneous appearance of the display is determined by the bitwise or bytewise contents of a display or frame buffer; the graphics adapter draws patterns on the display by modifying bits in the buffer.)

For example, GUI environments such that supplied with the OS/2.RTM. operating system, available from International Business Machines Corp., permit the user to perform various operations in separate rectangular screen windows. The windows have characteristic appearances (colors, borders, shading, etc.), and must also exist alongside other windows and screen artifacts. In order to spare designers of applications intended for use on such a GUI from rewriting the copious graphics instructions associated with windows rendering and management each time these operations become necessary, the API packages these into high-level commands. Thus, the designer need only enter a WinDrawBorder instruction (along with appropriate parameters, such as window size) in order to give the application the ability to draw a window border. The API maintains a library of graphics instructions associated with each high-level command, and imports these into the application program when it is compiled or run.

The GUI must also be compatible with numerous available hardware platforms and graphics adapters. This is facilitated by interposing a device driver, which directly controls the graphics hardware, between that hardware and the graphics engine. Device drivers are a common expedient in current computer systems, and contain processing logic for controlling the low-level or device-specific components of a particular computer resource. Each hardware configuration is provided with a different graphics driver capable of translating the common set of graphics-engine commands, or "primitives," into signals specific to the graphics hardware. This arrangement provides consistency at the operating system and GUI level.

Matters become complicated, however, in systems that support applications written for different GUIs. For example, the OS/2 operating system accepts "native" commands from applications written specifically for OS/2 as well as "foreign" commands directed toward the Windows GUI marketed by Microsoft Corp. This versatility is ordinarily implemented by the simultaneous presence, in computer memory, of the graphics subsystems associated with each of the supported GUIs and the graphics drivers with which they are ordinarily paired.

Without special modification, each driver would operate in its usual fashion, under the assumption that it "owns" the graphics hardware (and without regard to the fact that other drivers also reside in memory and might suddenly become active). Unfortunately, modifying a driver to cooperate with other drivers represents a non-trivial task, not only because of the growing complexity of graphics drivers, but also due to the lack of agreed-upon standard for inter-driver cooperation.

A final complication occurs when graphics boards are updated. These hardware devices are frequently produced by independent vendors, which sell them to computer manufacturers for installation on an original-equipment manufacture (OEM) basis. If the hardware is to run the full repertoire of graphics subsystem functions associated with every supported GUI, the drivers must ordinarily be rewritten with each new equipment release.

Advantages of the Invention

An advantage the present invention is simultaneous support of a plurality of graphics device drivers, any of which may be selected for immediate implementation of a desired graphics function and without special programming to prevent unwanted interaction.

Another advantage of the invention is reduction of driver complexity through reduction in the required number of supported graphics operations.

A further advantage of the invention is reduction in the number of drivers required to adapt a given graphics-processing board to a plurality of GUIs.

Still a further advantage of the invention is the ability to support a variety of operating systems and GUIs.

The present invention provides a support architecture that facilitates use of device drivers containing a minimum of hardware-specific software code without compromising overall graphics functionality. This approach is particularly useful in systems employing a plurality of graphics subsystems, since the driver need only support the set of graphic function calls common to all subsystems. These relatively few common functions act as building blocks for the larger, more complex operations typically requested by graphics engines. In order to mediate between the limited-instruction-set device driver and the various higher-level graphics engines, the invention includes a series of translation modules that simplify engine-originated instructions into simpler graphic components. A video manager supervises routing of instructions to the specific drivers they designate, and serializes access to hardware components so that graphic commands execute atomically (i.e., without interruption).

In addition, the invention includes a graphics library containing device-level instruction sets, as well as the on-board capability to execute those commands, for a broad range of graphic operations. In this way, if the translation module is unable to decompose an engine-originated instruction into operations the driver is capable of performing, the invention can utilize the library as a default; this ensures that all graphics requests will ultimately be serviced.

Since a device driver need support only a relatively few functions in order to operate successfully in the environment of the present invention, that environment is highly useful in driver design and development.

The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which:

FIG. 1 schematically illustrates a representative hardware environment for the present invention; and

FIG. 2 schematically illustrates the components and operation of the present.

Refer first to FIG. 1, which illustrates the hardware environment in which the present invention operates, The depicted computer system includes a central-processing unit 15, which performs operations on and interacts with a main system memory 17 and components thereof. System memory 17 typically includes volatile or random-access memory (RAM) for temporary storage of information, including portions of the computer's basic operating system and graphical user interface (denoted collectively by reference numeral 19). The system typically also includes read-only memory (ROM) for permanent storage of the computer's configuration and additional portions of the basic operating system, and at least one mass storage device 21, such as a hard disk and/or CD-ROM drive. All components of the system communicate over a bidirectional system bus 23.

The user ordinarily interacts with the system as it runs one or more application programs 25, at least portions of which reside in system memory 17. As used herein, the term "application program" refers broadly to any body of functionality for processing information of interest to the user and presenting that information in a manner that involves graphics operations. While graphics processing is the primary focus of design applications such as drawing and computer artwork packages, even text and numerically oriented applications such as word processors and spreadsheets can have significant graphic components. This is especially true in the context of systems that include a graphically sophisticated operating system and GUI that make use of a dedicated graphics subsystem 30. (Although, as noted previously, the GUI ordinarily mediates between applications and the basic operating system, the functionality of graphic subsystems can be distributed between these components, and so the two are merged in the figure for simplicity of presentation.)

Graphics subsystem 30 includes an application program interface (API) 32, which translates high-level application instructions into commands within the repertoire of a graphics engine 34. Graphics engine 34, in turn, processes these commands into output signals conveyed to a display driver 36, which is matched to and drives a graphics adapter 38. This latter component, finally, directly affects the contents of a video display 39 (e.g., by setting the bitwise contents of a display buffer in memory 17). A system may support more than one GUI or at least accommodate more than one set of API commands. This may be accomplished using multiple APIs and multiple graphics engines, or a single engine 34 capable of processing the different outputs.

The user interacts with the system using a keyboard 40 and a position-sensing device (e.g., a mouse) 42. The output of either device can be employed to designate information or select particular areas of video screen display 39 when the user interacts with applications 25.

The present invention provides a facility for the development of display drivers and accommodation of drivers having limited output instruction sets by interposing a series of modules between graphics engine 34 and display driver 36. Such modules are illustrated in FIG. 2.

Essentially, the invention comprises an architecture that coordinates communication between each of what may be several graphics engines and the available graphics hardware via one or a plurality of device drivers representatively denoted by reference numerals 50a, 50b; like other elements of the invention, only a small number are actually illustrated in the drawing, the ellipsis notation indicating the possibility of additional such elements. Each device driver 50a, 50b generates output signals that drive an associated graphics adapter 38a, 38b. The graphics adapters are hardware video or graphics accelerator boards, well known in the art and undergoing constant development and improvement. A device driver is typically matched to a particular graphics adapter.

Each graphics adapter directly modifies the contents of a frame buffer 52 (generally through a dedicated graphics processor chip). Frame buffer 52 is typically a partition of memory 17 dedicated exclusively to video output, the instantaneous bitwise or (more typically) bytewise contents of which determine the pixel-by-pixel appearance of video display 39. As indicated by dashed lines, device drivers 50a, 50b can be configured to occasionally bypass graphics adapters 38a, 38b and write directly to frame buffer 52 (e.g., to quickly display a stored pixelmap).

Overall management of the invention architecture is controlled by a video manager module 55. This module recognizes a characteristic set of video-manager interface ("VMI") commands, which it utilizes to take various actions and generate graphics hardware interface ("GHI") commands recognizable to a selected device driver. The VMI commands thus represent a set of functions that include commands common to a variety of graphics engines.

If a device driver has not been provided with the ability to execute a particular GHI command, it can send manager 55 a "simulation request," causing it to perform the designated graphics function directly. Manager 55 carries out simulation requests by accessing a stored library 60 of graphics functions. Library 60, retained permanently on a mass storage device 21 but cached in system memory 17 during operation for rapid access, represents a generic library of standard graphic functions (along with the instructions necessary to execute those operations) that may be called by manager 55 for direct execution. For example, library 60 may contain routines that perform basic bit-transfer and line-dravving operations, as well as graphic elements stored in bitmap form; in response to a simulation request, manager 55 can perform the requested operation directly or locate the proper bitmap and cause its transfer directly to frame buffer 52.

Alternatively or in addition, manager 55 can be provided with command templates, stored in partitions of system memory 17, which specify the set of commands recognized by each of device drivers 50a, 50b. If an incoming function request does not correspond to a command in the template of a selected device driver, manager 55 can be configured to implement the function directly (by accessing an appropriate entry of library 60) without waiting for a simulation request from the device driver. (As discussed below, manager 55 can also use the template to select the optimal one of a variety of device drivers to execute the command.)

Requests for graphic functions originate with one or more graphics engines 62a, 62b, 62c. Each of these may represent a separate operating system supported by the computer, or emulations of different operating systems. For example, an operating system may utilize a "microkernel" that supports operation of various "personalities," each of which replicates the behavior of a different operating system (such as AIX) or GUI (such as XWINDOWS), each of which may have its own unique device-driver model.

Accordingly, each graphics engine may be paired with a specific one of the device drivers, or, instead, multiple engines may instead issue commands for ultimate execution by a single device driver (in conjunction with the support function provided by library 60) or any of multiple drivers. This configuration effectively renders the architecture operating-system independent, and is facilitated by a translation module 64a, 64b, 64c associated with each graphics engine. The translation modules convert function calls from the graphics engines into VMI commands recognizable by manager 55. Translation modules 64a, 64b, 64c may operate, for example, by simple table lookup.

For example, a graphics engine may issue generic function calls such as bit-block transfer ("bitblt," which causes transfer of a rectangular array of bitmap or pixelmap data) or specialized calls such as "paint window," "draw text," or "draw line." While primitive function calls can be passed through to manager 55 merely by converting them into the standard VMI format, the translation modules decompose higher-level engine commands into constituent elements drawn from the VMI command set. The repertoire of commands recognized by manager 55 is a matter of design choice. A large set of recognized commands requires less translation overhead but greater processing capacity by manager 55; on the other hand, virtually all graphic functions can ultimately be reduced to sequences of bitbit and line-drawing instructions, simplifying the required capabilities of manager 55 but requiring greater processing time for translation.

Manager 55 serializes communications between translation layers 64a, 64b, 64c and each device driver to accommodate concurrent processing requirements. When it receives a VMI command, manager 55 requests a semaphore (if necessary) from operating system 19, and sends a corresponding GHI command to a designated device driver only when it receives the semaphore. This prevents more than one program thread from accessing a device driver at a given time, and ensures that rendering commands will be executed atomically.

The various components of the invention will now be described in greater detail.

1. Video Manager 55

Manager 55 receives requests from the various translation layers according to a special protocol, the video manager interface, which consists of a small set of operations each identified by a function number. Preferably, manager 55 is implemented as an object in an object-oriented program environment, and exports an entry point for access by the translation layers. A translation layer furnishes the following information to the entry point:

The ID parameter identifies the device driver, the FUNCTION parameter is a number that identifies the requested VMI operation, and the P.sub.in and P.sub.out parameters are pointers to input and output data structures, respectively, that are relevant to the requested operation. Manager 55 passes most of the requested operations directly to the device driver, changing only the prefix of the command from VMI to GHI. The following calls are common to both the VMI and GHI, and are passed through in this fashion:

When manager 55 receives a request from a translation module, it examines the function number and either handles the operation directly or forwards it to the designated device driver after changing the prefix. The device driver, upon receiving the command, responds with a character string or signal indicating success, error, or a request for simulation. Accordingly, the only required features of a compatible device driver are (i) the ability to recognize GHI-prefixed commands; (ii) the ability to process the following "base function" set of commands, and (iii) the ability to return success, error, or simulate call to manager 55 in response to a command received from manager 55.

GHI.sub.-- CMD.sub.-- INIT

GHI.sub.-- CMD.sub.-- QUERYCAPS

GHI.sub.-- CMD.sub.-- QUERYMODES

GHI.sub.-- CMD.sub.-- SETMODE

GHI.sub.-- CMD.sub.-- PALETTE

These and related optional commands, and their VMI counterparts, perform the following functions.

VMI.sub.-- CMD.sub.-- INIT, GHI.sub.-- CMD.sub.-- INIT: Causes manager 55 to recognize a translation module and accept subsequent commands from it. The GHI form of the instruction represents the first instruction a device driver receives from manager 55; in response, the device driver returns to manager 55 the number of function classes (a concept discussed below) that it supports. Manager 55 also assigns the device driver a unique identifier when it passes the GHI.sub.-- CMD.sub.-- INIT command.

VMI.sub.-- CMD.sub.-- INITPROC, GHI.sub.-- CMD.sub.-- INITPROC: Causes manager 55 to recognize a command-issuing process (usually a translation module). The GHI form of the instruction informs the designated device driver of the existence of the process. This command can be used to enforce security, preventing unauthorized processes from obtaining access to the video hardware.

VMI.sub.-- CMD.sub.-- TERM, GHI.sub.-- CMD.sub.-- TERM: Informs manager 55 that a translation module is being terminated. The GHI form of the instruction deactivates the associated device driver.

VMI.sub.-- CMD.sub.-- TERMPROC, GHI.sub.-- CMD.sub.-- TERMPROC: Informs manager 55 and the designated device driver that a process is being terminated.

VMI.sub.-- CMD.sub.-- QUERYCAPS, GHI.sub.-- CMD.sub.-- QUERYCAPS: This command is used to query the capabilities of a designated device driver; a translation module, for example, may utilize the response to learn which graphic commands a driver is capable of processing and thereby determine the range of allowed output. Manager 55 reformats this command and passes it directly to the driver, which responds by filling a data structure with identifiers for (or pointers to other data structures specifying) the function sets the driver supports. For example, all device drivers support the "base function" class, and may support others as well.

VMI.sub.-- CMD.sub.-- QUERYMODES, GHI.sub.-- CMD.sub.-- QUERYMODES: When passed to a driver, causes the driver to return the video modes it is capable of supporting. A video mode specifies the screen resolution and color range. For example, a VGA adapter supports 16 colors and resolutions up to 640.times.480 pixels; super VGA mode supports 256 colors and resolutions of 1024.times.768 pixels and up.

VMI.sub.-- CMD.sub.-- SETMODE, GHI.sub.-- CMD.sub.-- SETMODE: Called to set a graphics adapter to a specific video mode.

VMI.sub.-- CMD.sub.-- PALETTE, GHI.sub.-- CMD.sub.-- PALETTE: Called to query or set the color palette of the graphics adapter. Manager 55 reformats this command and passes it directly to the designated driver, which returns the current palette or causes the adapter to adopt the palette specified in the function call. A flag associated with this function specifies which of these operations the driver is to perform.

A device driver can be configured to process the following commands or to return an RC.sub.-- SIMULATE call to manager 55:

GHI.sub.-- CMD.sub.-- BITBLT

GHI.sub.-- CMD.sub.-- LINE

GHI.sub.-- CMD.sub.-- SETPTR

GHI.sub.-- CMD.sub.-- MOVEPTR

GHI.sub.-- CMD.sub.-- SHOWPTR

VMI.sub.-- CMD.sub.-- BITBLT, GHI.sub.-- CMD.sub.-- BITBLT: Requests rendering of a rectangle or series of rectangles by a bitbit operation according to parameters specified in an input data structure P.sub.in. Manager 55 reformats this command and passes it directly to the designated device driver along with the P.sub.in data structure, which specifies foreground and background colors of the rectangle; the dimensions and color format of the source and destination bitmaps and, if applicable, the pattern bitmap; and pre-clipped source and destination coordinates and extents. Additional parameters, such as graphic patterns, can be specified as well.

A representative first data structure (BMAPINFO) is as follows:

where ulLenth is the length of the BMAPINFO data structure, in bytes; ulType is a description of the bit-block transfer as discussed below; ulWidth and ulHeight are the width and height, respectively, of the bitmap in pels; ulBpp is the number of bits per pel/color depth; ulBytesPerLine is the number of aligned bytes per line; pBits is a pointer to the bitmap bits; ulXOrg and ulYOrg are the X-origin and Y-origin, respectively, of the destination bit-block transfer; and ulXExt and ulYExt are the X and Y extents, respectively, of the bit-block transfer.

A representative set of define statements for the ulType field of the BMAPINFO data structure is as follows:

where BMAP.sub.-- VRAM refers to bitmaps in video memory and BMAP.sub.-- MEMORY refers to bitmaps in system memory.

A representative second data structure (BITBLTINFO) is as follows:

where ulLength is the length of the BITBLTINFO data structure in bytes; ulBltFlags are miscellaneous flags used by the graphics engine for rendering (and which indicate, for example, the direction of the bit-block transfer, whether the operation includes a source or pattern bitmap, and whether the source, destination and/or pattern bitmaps are transparent); cBlits is a count of bit-block transfers to be performed; ulROP designates a raster operation; ulMonoBackROP specifies a background mix; ulSrcFGColor specifies a monochrome source foreground color; ulSrcBGColor specifies a

whereulStyleStep specifies the value to be added to ulStyleValue on each pel stepped along the style major direction; ulStyleValue specifies the style value (composed of an error value and a mask position) at the current pel; ulAbsDeltaX and ulAbsDeltaY specified clipped Bresenham Delta X and Y values, respectively; ptlClipStart and ptlClipEnd are pointers to starting and endling locations, respectively, for execution of the Bresenham algorithm; ptlStart and ptlEnd are starting and ending locations, respectively, for the line; IClipStartError is the standard Bresenham error at the clipped start point; and ulFlags are flags used for the LINEPACK data structure, as follows:

LINE.sub.-- DO.sub.-- FIRST.sub.-- PEL (draws the first pel)

LINE.sub.-- DIR.sub.-- Y.sub.-- POSITIVE (indicates line direction is bottom-to-top)

LINE.sub.-- HORIZONTAL (indicates line is horizontal; no Bresenham algorithm)

LINE.sub.-- X.sub.-- MAJOR (line is XMAJOR)

LINE.sub.-- DIR.sub.-- X.sub.-- POSITIVE (line direction is right-to-left)

LINE.sub.-- VERTICAL (line is vertical; no Bresenham algorithm)

LINE.sub.-- STYLE.sub.-- X.sub.-- MAJOR (line style is XMAJOR)

LINE.sub.-- SO.sub.-- LAST.sub.-- PEL (draws the last pel) monochrome source background color and transparent color; ulPatFGColor and ulPatBGColor specify monochrome pattern foreground and background colors, respectively; abColors is a pointer to a color translation table; pSrcBmapInfo and pDstBmapInfo are pointers to BMAPINFO data structures for source and destination bitmaps, respectively; aptlSrcOrg and aptlPatOrg are pointers source and pattern origins, respectively; abrDst is a pointer to an array of bit-block transfer rectangles (specified by data structures including X and Y origins and extents); prclSrcBounds is a pointer to a source bounding rectangle of source bit-block transfers; and prclDstBounds is a pointer to a destination bounding rectangle of destination bit-block transfers.

VMI.sub.-- CMD.sub.-- LINE, GHI.sub.-- CMD.sub.-- LINE: Requests rendering of a line according to parameters specified in an input data structure P.sub.in. Manager 55 reformats this command and passes it directly to the designated device driver along with the P.sub.in data structure, which specifies data such as line length, destination position and orientation, foreground and background colors, and whether the line is solid or patterned.

A representative first data structure (LINEPACK), which contains line information on a per-line basis, is as follows:

A representative second data structure (LINEINFO) is as follows:

where ulLength specifies the length of the LINEINFO data structure; ulType defines the line type, as follows:

LINE.sub.-- SOLID (line will be solid in foreground color)

LINE.sub.-- INVISIBLE (line is not drawn)

LINE.sub.-- ALTERNATE (line will be alternative foreground and

ulStyleMask specifies a 32-bit style mask; cLines is a count of the lines to be drawn; ulFGColor and ulBGColor specify line foreground and background colors, respectively; ulForeROP and ulBackROP are line foreground and background mixes, respectively; pDstBmapInfo is a pointer to a destination surface bitmap; alpkLinePack is a pointer to the LINEPACK data structure; and prclBounds is a pointer to a bounding rectangle of a clipped line.

Using these data structures and definitions, a line starts from ptlStart and ends at ptlEnd (inclusive). The IClipStartError parameter represents the standard Bresenham error at the clipped start point, and is calculated from the initial error at the start point and the error increments for major and diagonal steps. Horiztonal and vertical lines are drawn from the clipped start to the clipped end. Lines are styled using the ulStyleMask, ulStyleStep and ulStyleValue; the ulStyleMask is a 32-bit style mask, ulStyleValue is the style value at the current pixel (pel), and ulStyleStep is the value to be added to ulStyleValue on each pixel stepped along the style major direction.

Manager 55 also supports a set of utility commands:

VMI.sub.-- CMD.sub.-- VRAMALLOC, GHI.sub.-- CMD.sub.-- VRAMALLOC: Called to allocate off-screen video memory.

VMI.sub.-- CMD.sub.-- REQUESTHW, GHI.sub.-- CMD.sub.-- REQUESTHW: An application (or translation module) uses this call to request access to the video hardware. This allows, for example, direct manipulation of frame buffer 52. In response to this command, manager 55 obtains a semaphore to ensure uninterrupted access to the video hardware, and establishes a data path between the requesting process and the appropriate graphics adapter or the frame buffer. This capability may be used, for example, by a multimedia application, which for performance reasons writes directly to frame buffer 52 when presenting software motion video.

VMI.sub.-- CMD.sub.-- BANK, GHI.sub.-- CMD.sub.-- BANK: Called to designate a specific video memory bank. VMI.sub.-- CMD.sub.-- EXTENSION, GHI.sub.-- CMD.sub.-- EXTENSION: Called to send an extension command (discussed below) to a driver. The preferred data structure associated with this command is:

where ulLength is the size, in bytes, of the HWEXTENSION data structure; ulXSubFunction is an extension-specific function number; cSrcChangeRects is a count of screen rectangles affected by the extension; arectlScreen specifies an array of screen rectangles affected by the extension; ulXFlags, in the current embodiment, is a single flag facilitating hardware serialization; and pExtPI specifies an extension-specific input packet,

VMI.sub.-- CMD.sub.-- QUERYCHAININFO: Called to obtain a pointer to a data structure containing capability and mode information for each device driver. The preferred data structure for each driver is:

where ID is the driver identifier; pszDriverName is the driver name; cModes is a count of available graphics modes supported by the driver; pModeInfo points to a data structure specifying the modes supported by the driver; pCapsInfo points to a data structure specifying the capabilities of the driver; and pNextDriverInfo points to the next data structure in the driver chain (discussed below).

In an illustrative embodiment, manager 55 is responsible for pointer management. Generally, the pointer will ultimately overwrite other on-screen material--if rendered objects are thought of as occupying planes arranged along a Z-axis, so that objects on higher-order planes occlude those on lower-order planes, the pointer generally occupies the highest Z-order plane--but will be the last object rendered; accordingly, the pointer is itself temporarily overwritten as lower-order objects are rendered.

When it receives a VMI command, manager 55 first determines whether execution of the command will affect the pointer. If so, it clears the pointer, passes the command to the designated device driver (or to graphics library 60), and replaces the pointer when the driver (or graphics library 60) has finished executing the command.

Manager 55 can also perform pointer operations in response to commands issued by applications 25. Manager 55 either executes these commands directly or passes them for execution to a designated device driver (equipped to handle pointer operations). The pointer commands supported are:

VMI.sub.-- CMD.sub.-- MOVEPTR, GHI.sub.-- CMD.sub.-- MOVEPTR: In response to this command, which includes a reference to a data structure indicating the pointer destination, manager 55 either performs the move operation or reformats this command and passes it to a designated device driver. This arrangement applies to the remaining commands as well.

VMI.sub.-- CMD.sub.-- SETPTR, GHI.sub.-- CMD.sub.-- SETPTR: Called to set the pointer AND, XOR masks and, for color pointers, to designate a bitmap.

VMI.sub.-- CMD.sub.-- SHOWPTR, GHI.sub.-- CMD.sub.-- SHOWPTR: Called to set the visibility state of the pointer.

2. Graphics Library 60

The capabilities of graphics library module 60 (that is, the range of graphic functions it is able to execute) determine the extent to which high-level function calls from graphics engines must be translated. In the simplest and currently preferred approach, graphics library 60 supports the BITBLT and LINE drawing commands using the parameters discussed above. Although this requires translation modules 64a, 64b, 64c to decompose function calls into these primitives before they can be executed (assuming that the associated device drivers are incapable of processing higher-level function calls), it affords simplicity in system design.

3. Filters

The present invention can also include one or more filter modules 70a, 70b interposed between each device driver and manager 55. These modules can enhance the overall functionality of a device driver or preprocess instructions before they reach the driver. For example, a filter can accommodate a device driver to the environment of the invention if the driver is capable of processing the base function command set but not according to the syntax recognized by the present invention. Instead the filter processes these commands, interacting with the device driver and manager 55 as necessary. For example, suppose the invention is to be used to test the capabilities of a device driver not otherwise intended for use with the invention. A filter module can be established with (i) a data structure specifying the driver's capabilities; (ii) the ability to generate output signals to the driver that determine or set its current video mode and palette; and (iii) the ability to process VMI commands and otherwise interact with manager 55. In this way, the device driver is integrated into the invention without special modification. The filter intercepts GHI commands, setting the driver mode and palette according to the instructions of manager 55, responding to GHI.sub.-- CMD.sub.-- QUERYCAPS and GHI.sub.-- CMD.sub.-- QUERYMODES commands, and otherwise converting GHI commands into forms consistent with the device driver's instruction set.

A filter can also view commands and parameter information without modifying them. One application of such a filter is use as a performance monitor. For example, the filter can receive commands from manager 55 and timestamp them before transferring the commands to the associated device driver, then record the time when the operation is completed. The difference between these timestamps represents the time taken by the device driver to execute the instruction. For example, in the preferred embodiment, a filter calls the driver with which it is associated as a process, thereby registering its completion automatically.

A filter can also screen commands, rejecting commands that the associated device driver is incapable of processing and signaling manager 55 accordingly. For example, the filter can be associated with a command template stored in a partition of memory 17, and configured to pass to the device driver only those commands conforming to a template entry. If a command fails to so conform, the filter can return a simulation request to manager 55. Once again, this approach eases compatibility between the invention and a device driver not specifically designed for operation with the invention.

This screening function can also be combined with processing capability. For example, a filter can analyze function parameters associated with a command (e.g., the data structures of the LINE or BITBLT instructions) for conformance to a template prior to transferring it to a device driver. The template can contain, for example, data specifying parameters that must contain arguments, as well as the form of argument (e.g., numerical) or its proper scope (e.g., a numerical range). In this way, the filter can act as a real-time debugger, rejecting commands that fail to satisfy basic structural requirements or which specify impossible arguments (e.g., a BITBLT larger than the frame buffer) and alerting manager 55 thereto.

Filters are also useful in conjunction with accessory modules such as remote screen monitoring utilities (such as the IBM Distributed Console Access Facility, or "DCAF"). These facilities operate over wide-area or local-area computer networks, conforming a target computer screen to the appearance of a base screen. DCAF transmits an initial pixelmap to the frame buffer of the target computer. Instead of retransmitting the entire screen each time it changes, however, DCAF instead transmits only rectangle changes. A receiving DCAF module on the target computer modifies the display based on the transmitted changes.

A filter can be used to monitor the flow of commands to a device driver and to transmit those affecting screen appearance to a DCAF accessory module 80 as well as to the device driver. Accessory module 80 sends these commands to the DCAF receiving module on the target computer, either directly or after processing them to determine the precise rectangle changes they produce.

4. Extensions

The architecture of the present invention allows for extensions to accommodate or take the fullest possible advantage of future hardware. Particularly if the components of the invention are used as part of an operating system (and not solely for driver design and development), the invention should allow for future enhancement of a driver command set or addition to a driver of an extension module that provides advanced capabilities.

In the former case, the entire driver and its associated function class(es) can simply be replaced. More typically, however, designers of drivers will make expansion modules available. The user of an existing driver may purchase the expansion module to enhance graphic capabilities, or may instead choose to retain the original driver. This approach not only avoids replacement of an entire driver, but also avoids the potential code corruption that might occur were only a single module of the driver replaced.

The invention facilitates driver expansion by providing for "virtual" or extension drivers within the above-described component architecture. Specifically, each enhancement, having a distinct repertoire of executable graphic functions, is treated as a separate function class. As mentioned above, a driver, in response to a GHI.sub.-- CMD.sub.-- INIT command, returns to manager 55 the function classes the driver supports. Manager 55 allocates one INFO data structure for each supported class, and assigns each class an identifier as if the class were itself a separate driver. Thus, as shown in FIG. 2, each driver 50a, 50b can be associated with an extension driver 74a, 74b. Extension drivers 74a, 74b each have separate a associated ID and therefore respond to commands specifying the ID as a separate driver module. However, each remains associated with the same graphics adapter 38a, 38b as the original driver.

Although each extension may theoretically be addressed directly, as a separate driver, practical considerations dictate use of the VMI.sub.-- CMD.sub.-- EXTENSION command, which designates the base driver with which the extension is associated as well as the extension itself. For example, a DCAF filter associated with the base device driver may not be associated with the extension; the cScrChangeRects field of the HWEXTENSION data structure provides the information accessory 80 will need to update the target screen. In addition, if manager 55 is to retain control of the pointer without being programmed to recognize all commands recognized by each extension--a requirement inconsistent with the generality afforded by the invention--it must be furnished with sufficient information (such as that contained in the HWEXTENSION data structure) regarding each extension command to derive necessary pointer operations.

5. Chains of Device Drivers

Each set of associated device drivers and associated graphics adapters and filters represents a "chain." In FIG. 2, driver 50a, adapter 38a and filter 70a are one chain, and driver 50b, adapter 38b and filter 70b a second chain. For example, one chain might be dedicated to graphics support of the IBM Presentation Manager.RTM. ("PM") GUI and the other to support of Microsoft Windows.TM.. Two device drivers can also share a single graphics adapter and exist, in the present invention, as separate chains. Thus, continuing the previous example, device driver 50a might be dedicated to PM command processing and driver 50b to Windows command processing, but both commonly drive graphics adapter 38a.

Chains can be simultaneously resident in an operational sense, being independently actuated by commands designating particular device drivers. Chain selection commands can originate with an application, manager 55 or, preferably, a translation module, which selects the chain of the driver best suited to executing the translated command(s). This arbitration capability is implemented using templates and decision logic.

In particular, a field of the INFO data structure associated with each driver 50a, 50b identifies all function classes (including extension modules) supported by that driver. This structure is returned in response to the VMI.sub.-- CMD.sub.-- QUERYCAPS command, furnishing the requesting process with complete capability information for each driver and its extensions. Preferably, however, manager 55 supports an additional command, VMI.sub.-- CMD.sub.-- QUERYCHAININFO, which returns the INFO data structures associated with all registered device drivers. In the case of multiple active chains, a CHAININFO structure contains pointers to a linked list of all INFO data structures.

These data structures provide the templates according to which a translation module selects driver chains. The decision logic depends on the capabilities of the various drivers, the costs (e.g., time overhead) associated with each, and the needs of particular applications. For example, a GUI might require only a moderate resolution and palette range, so that a chain implementing VGA will suffice; a computer-aided design or image-processing program, by contrast, can require much more extensive--but also slower--graphics capabilities, necessitating use of a driver capable of fine resolution and a large palette.

Driver chain selection can take place serially or in parallel. In a serial configuration, each separate chain performs temporally separate operation on frame buffer 52; in other words, each chain, responding to commands from one or more translation modules, "owns" frame buffer 52 within timeslices allocated by manager 55. As described previously, manager 55 serializes interaction with frame buffer 52 to prevent overlap of graphics operations; a requested operation must be completed before manager 55 will pass another command down a chain or accord a process access to frame buffer 52.

Parallel operation can take different forms. In one implementation, different chains can control different regions (e.g., separate windows) within frame buffer 52. To return to a previous example, a GUI requiring only VGA support can have commands routed down a chain to a VGA adapter, while an image-processing program, operative within a discrete screen window, can have commands routed down a chain to a more powerful adapter. Each chain will operate on different pixel regions of frame buffer 52: the more powerful driver within the geometric confines of the program window, and the VGA driver to the remainder of the frame.

Another parallel operation involves multiple screens, wherein a single image is divided into segments for display on a matrix of contiguous screen displays. Each screen display is driven by a separate screen buffer 52. The use of multiple screens not only enlarges the user's view, but effectively multiplies the obtainable output resolution by the number of separate screens. Multiple-screen output can be implemented by means of a GUI expressly configured therefor or through the use of a filter configured to process the output of a normal-resolution GUI (e.g., by computationally projecting a pixel array onto a larger array, algorithmically adjusting for distortions and filling in dead space).

It will therefore be seen that the foregoing represents a highly extensible and advantageous approach to accommodation of multiple device drivers having varying capabilities and interoperabilities. The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. For example, the various modules of the invention can be implemented on a general-purpose computer using appropriate software instructions, or as hardware circuits.