Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 10h Function 0Ch
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     The range of valid pixel values and (x, y) coordinates depends on
     the current video mode.
 
     If bit 7 of AL is set, the new pixel value will be exclusive-OR'd
     (XOR) with the current contents of the pixel.
 
     Register BH is ignored for display modes that support only one
     page.
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