Rockwell Brooktree Bt848EVK, Техническое справочное руководство

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Bt848 Software Specification
7
UG848_3A
Bt848
DDI Interface
Software Specification
Brooktree
®
DDI Interface
Direct Draw Interface (DDI) is an extension to Microsoft Windows which allows direct access to
video and graphics hardware in order to accelerate such tasks as displaying decompressed video
under Video for Windows. It is ideally suited to provide direct access to the graphics frame buffer.
The central concept of the DDI interface is the “surface.” In general, a surface is an image plane
or buffer containing part of the display which is shown to the user. Each surface may have different
image or color-space formats; for example, one surface may contain 8 bpp RGB pixels while
another contains YUV 4:1:1 information. The Bt848 manages the necessary conversions before dis-
play. Several surfaces are combined into the final monitor image using either direct writing to the
frame buffer (primary surface) or by off-screen surface methods.
The Bt848 capture driver uses DDI to perform the task of displaying live video in a window.
The Bt848 driver has the option of moving the video image to the graphics frame buffer, or to the
host memory for later display.
Windows95 Plug and Play
As a PCI device, the Bt848’s configuration information is dynamically created at system start-up by
Windows95. The operating system can make dynamic changes to the installed hardware. This is all
accomplished through the Plug and Play Configuration Manager.
Bt848 driver is able to respond to the Windows95 Plug and Play Configuration Manager’s mes-
sages and to dynamically reconfigure the installed hardware as necessary. Under Windows95, a set
of API exist in the VxD that provide the interface for interacting with the Configuration Manager.
RISC Program
The Bt848 driver software generates a RISC program to fill requests from Video for Windows. All
RISC operations are internal to the driver and are triggered indirectly by VfW messages. The RISC
program software can generate all necessary command sequences to support video capture in all
formats, and preview in the RGB modes. The software also performs the necessary linear to physi-
cal address translations.
Bt848 driver supports complex clipping of graphics data. Based on a clip list from the DDI, the
driver generates the RISC program in such a way to block the video information in areas where
graphics are to be displayed instead. The PCI bus is not utilized when the video data is being
occluded.
The RISC program instructions reside in system memory. The VxD allocates a portion of con-
tiguous memory during initialization and uses it for the RISC commands. The amount of memory
required by the RISC instructions ranges from 3 to 12 K bytes.