Intel manual Intel 815 Chipset Graphics Controller PRM, Rev

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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monochrome BLT operand following the BLT_TEXT command. The BLT engine will receive this data through the command stream and use it as the source data for this BLT operation. The BLT engine will be set to the same color depth as the graphics system  8 bits per pixel, in this case. Since the source data in this BLT operation is monochrome, color expansion must be used to convert it to an 8 bpp color depth. To ensure that the gray background behind this letter “f” is preserved, per-pixel write masking will be performed, using the monochrome source data as the pixel mask.

The BLT Setup and Text command packets are used to select the features to be used in this BLT operation. Only the fields required by these two command packets must be programmed carefully. The BLT engine ignores all other registers and fields. The source select field must be set to 1, to indicate that the source data is provided by the host processor through the IMMEDIATE_BLT command packet. Finally, the raster operation field should be programmed with the 8-bit value CCh to select the bit-wise logical operation that simply copies the source data to the destination. Selecting this bit-wise operation in which no pattern data is used as an input, causes the BLT engine to automatically forego reading pattern data from the frame buffer.

The Setup Pattern/Source Expansion Foreground Color Register to specify the color with which the letter “f” will be drawn. There is no Source address. All scan lines of the glyph are bit packed and the clipping is controlled by the ClipRect registers from the SETUP_BLT command and the Destination Y1, Y2, X1, and X2 registers in the TEXT_BLT command. Only the pixels that are within (inclusive comparisons) the clip rectangle are written to the destination surface.

The Destination Pitch Register must be programmed with a value equal to the number of bytes in the interval between the first bytes of each adjacent scan line’s worth of destination data. Since the color depth is 8 bits per pixel and the horizontal resolution of the display is 1024 pixels, the value to be programmed into these bits is 400h, which is equal to the decimal value of 1024. Since the source data used in this BLT operation is monochrome, the BLT engine will not use a byte-oriented pitch value for the source data.

Since the source data is monochrome, color expansion is required to convert it to color with a color depth of 8 bits per pixel. Since the Setup Pattern/Source Expansion Foreground Color Register is selected to specify the foreground color of black to be used in drawing the letter “f”, this register must be programmed with the value for that color. With the graphics system set for a color depth of 8 bits per pixel, the actual colors are specified in the RAMDAC palette, and the 8 bits stored in the frame buffer for each pixel actually specify the index used to select a color from that palette. This example assumes that the color specified at index 00h in the palette is black, and therefore bits [7:0] of this register should be set to 00h to select black as the foreground color. The BLT engine ignores bits [23:8] of this register because the selected color depth is 8 bits per pixel. Even though the color expansion being performed on the source data normally requires that both the foreground and background colors be specified, the value used to specify the background color is not important in this example. Per-pixel write-masking is being performed with the monochrome source data as the pixel mask, which means that none of the pixels in the source data that will be converted to the background color will ever be written to the destination. Since these pixels will never be seen, the value programmed into the Pattern/Source Expansion Background Color Register to specify a background color is not important.

The Destination Width and Height Registers are not used. The Y1, Y2, X1, and X2 are used be program with values that describe to the BLT engine the 8x8 pixel size of the destination location. The Destination Y1 and Y2 address registers must be programmed with the starting and ending scan line address of the destination data. This address is specified as an offset from the start of the frame buffer of the scan line at the destination that will be written to first. The destination X1 and X2 registers must be programmed with the starting and ending pixel offsets from the beginning of the scan line.

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