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Whenever possible, display drivers should use the features of the CPU to
optimize their performance. This is particularly important for drivers when
used in multimedia versions of Windows.
 
A display driver can determine what CPU is present and what mode Windows is
operating in by examining the __WinFlags variable or by calling the
GetWinFlags function. If the CPU is at least a 386, the display driver
should take advantage of the CPU's 32-bit registers to manipulate data and
to index huge arrays.
 
Tips for Writing Transparent Text
 
Drivers generating VGA text output can make two significant speedups for
transparent text: don't draw empty bytes or words and use write mode 3.
 
While drawing the character to video memory, avoid copying the "empty" part
of a glyph's bitmap. For example, there is no need to copy the space
character to video memory or the ascender on letters like the lowercase 'p'.
Since accessing video memory is nearly 5 times slower than accessing system
memory, it is cheaper to check for and ignore empty bytes or words in a
glyph bitmap, than it is to store nothing to video memory. In the following
example, the code gives a significant performance increase for many video
adapters:
 
      mov  ax,glyphbits
      or   ax,ax
      jz   around_it
      xchg es:[di],ax
  around_it:
 
Use write mode 3 on VGA hardware. This mode simplifies the output of
transparent text by eliminating the extra step of setting the bitmask
register for each store to video memory. For example, the VGA portion of the
following EGA/VGA code uses write mode 3 for the VGA adapter and is
considerably faster than the equivalent EGA code.
 
        mov     ax,glyphbits
if _EGA
        push    dx
        mov     dx,3cfh
        out     dx,al
        xchg    es:[di],al              ;write ax to screen.
        mov     al,ah
        out     dx,al
        xchg    es:[di+1],ah            ;write ax to screen.
        pop     dx
else
        xchg    es:[di],al              ;write ax to screen.
        xchg    es:[di+1],ah            ;write ax to screen.
endif
 
Tips for Using the Interrupt Flag
 
Display drivers should avoid needless clearing and setting of the interrupt
flag using the cli and sti instructions. Since 386 enhanced-mode Windows
traps these instructions as part of its management of the CPU's virtual
mode, these instructions each take about 600 CPU clocks to execute.
 
 
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