Intel 815 Manuel D’Utilisation

  

107 

used to choose the color data position that is to be written to through the same data port. This 
arrangement allows the same data port to be used for reading from and writing to two different color data 
positions. Reading and writing the color data at a color data position involves three successive reads or 
writes since the color data stored at each color data position consists of three bytes. 

To read a color data position, the index of the desired color data position must first be written to the 
Palette Read Index Register. Then all three bytes of data in a given color data position may be read at the 
Palette Data Register. The first byte read from the Palette Data Register retrieves the 8-bit value 
specifying the intensity of the red color component, while the second and third bytes read are the 
corresponding 8-bit values for the green and blue color components, respectively. After completing the 
third read operation, the Palette Read Index Register is automatically incremented so that the data of the 
next color data position becomes accessible for being read. This allows the contents of all 256 color data 
positions of the palette to be read by specifying only the index of the 0th color data position in the Palette 
Read Index Register, and then simply performing 768 successive reads from the Palette Data Register. 

Writing a color data position entails a very similar procedure. The index of the desired color data 
position must first be written to the Palette Write Index Register. Then all three bytes of data to specify a 
given color may be written to the Palette Data Register. The first byte written to the Palette Data Register 
specifies the intensity of the red color component, the second byte specifies the intensity for the green 
color component, and the third byte specifies the same for the blue color component. One important 
detail is that all three of these bytes must be written before the hardware will actually update these three 
values in the given color data position. When all three bytes have been written, the Palette Write Index 
Register is automatically incremented so that the data of the next color data position becomes accessible 
for being written. This allows the contents of all 256 color data positions of the palette to be written by 
specifying only the index of the 0th color data position in the Palette Write Index Register, and then 
simply performing 768 successive writes to the Palette Data Register. 

In addition to the standard set of 256 color data positions of the palette, there is also an alternate set of 8 
color data positions used to specify the colors used to draw the cursor, and these are also accessed using 
the very same sub-addressing scheme. A bit in the Pixel Pipeline Configuration Register (PIXCONF) 
determines whether the standard 256 color data positions or the alternate eight color data positions are to 
be accessed through this sub-addressing scheme. 

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Pixel Data Mask Register 

I/O (and Memory Offset) Address:  3C6h 
Default: Undefined 
Attributes: Read/Write 

 

7:0 

Pixel Data Mask. In indexed-color mode, the 8 bits of this register are logically ANDed with the 8 bits of 
pixel data received from the frame buffer for each pixel. The result of this ANDing process becomes the 
actual index used to select color data positions within the palette. This has the effect of limiting the 
choice of color data positions that may be specified by the incoming 8-bit data.  

0 =   Corresponding bit in the resulting 8-bit index being forced to 0.  

1 =   Allows the corresponding bit in the resulting index to reflect the actual value of the corresponding 

bit in the incoming 8-bit pixel data.  

In direct-color mode, the palette is not used, and the data in this register is ignored.