XFree86Config bpp depth confusion

["Robin Rowe" <rower@movieeditor.com>]

Hi. I have a little confusion with XFree86Config. What is the difference
between bpp and color depth?

With the DefaultColorDepth set to 24 my screen rate was 115 hz, but at 32 it
is just 85 (max 86). What does choosing 32 or 24 signify? Isn't 24 bits as
much as there actually is?

At 85 hz I can almost (but not quite) get the screen to fit to the edges of
my monitor. Using 85 hz in Win2k/NT/98 booting on the same machine works
fine. In XFree86 3.3.x Potato there is a half inch of black left at the
bottom edge I can't get rid of. Why can't I get the screen to fit using
xvidtune?

Thanks! Relevant sections of XFree86Config below.

Cheers,

Robin

Section "Monitor"
    Identifier  "Primary Monitor"
    VendorName  "ViewSonic"
    ModelName   "E790"
    HorizSync   30-95
    VertRefresh 50-200
# DefaultColorDepth 24:
#   Modeline  "1024x768"  115.50 1024 1104 1296 1440 768 771 781
802 -hsync -vsync
#   Modeline  "1024x768"  115.50 1024 1088 1280 1300 768 768 806
814 -hsync -vsync
# DefaultColorDepth 32:
#   Modeline  "1024x768"  85.00 1024 1104 1296 1440 768 771 781
802 -hsync -vsync
    Modeline  "1024x768"     85.00   1024 1100 1292 1328    768  771  781
794 -hsync -vsync
EndSection

Section "Device"
   Identifier      "Primary Card"
   VendorName      "Unknown"
   BoardName       "Hercules Terminator 128 2X/i AGP"
   VideoRam        8192
    #Option     "no_accel"
    #Option     "sw_cursor"
    #Option     "hw_cursor"
    #Option     "sgram"
    #Option     "sdram"
EndSection

Section "Screen"
   Driver          "SVGA"
   Device          "Primary Card"
   Monitor         "Primary Monitor"
   DefaultColorDepth 32
   SubSection "Display"
      Depth        8
      Modes        "1024x768"
   EndSubSection
   SubSection "Display"
      Depth        15
      Modes        "1024x768"
   EndSubSection
   SubSection "Display"
      Depth        16
      Modes        "1024x768"
   EndSubSection
   SubSection "Display"
      Depth        24
      Modes        "1024x768"
   EndSubSection
   SubSection "Display"
      Depth        32
      Modes        "1024x768"
   EndSubSection
EndSection