Booting KNOPPIX_V3.6-2004-08-16-EN from CD-ROM with "screen=1280x1024" gives no display in X11, depite the graphics card and monitor being capable. The video card is ATI Technologies Inc Rage 128 Pro Ultra TF rev 0 (0000:01:00.0 0300: 1002:5446) which can generate any resolution and depth desired here. The display is Planar PL1700, which supports 1280x1024 in other Linux; Vsync is 49.0-75.0 [Hz] and Hsync is 24.0-80.0 [kHz] from the manufacturer's manual, and these are the numbers reported by DDC according to /var/log/XFree86.0.log:
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(II) VESA(0): VESA VBE DDC read successfully
(II) VESA(0): Manufacturer: PNR Model: 5780 Serial#: XXXXX
...
(II) VESA(0): Supported VESA Video Modes:
(II) VESA(0): 1280x1024@75Hz
...
(WW) VESA(0): config file hsync range 28-96kHz not within DDC hsync range 24-80kHz
(II) VESA(0): Monitor0: Using hsync range of 28.00-96.00 kHz
(II) VESA(0): Monitor0: Using vrefresh range of 49.00-75.00 Hz
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According to /etc/X11/XF86Config-4:
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# Auto-generated by KNOPPIX mkxf86config

Section "Monitor"
Identifier "Monitor0"
Option "DPMS" "true"
VendorName "PNR"
ModelName "PNR5780"
# HorizSync 28.0 - 78.0 # Warning: This may fry very old Monitors
HorizSync 28.0 - 96.0 # Warning: This may fry old Monitors
VertRefresh 49 - 75 # DDC-probed
...
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So KNOPPIX mkxf86config has made an error in allowing 96kHz HorizSync. The error is compounded because booting with "screen=1280x1024 vsync=75 hsync=80" does not work either! The same wrong XF86Config-4 is generated. [This monitor nicely tells you that the input frequency is out-of-range.] Booting with "screen=1024x768" works.

So, why has KNOPPIX ignored the DDC, and why doesn't the boot command line parameter hsync work? Both of these sources should be trusted more than any internal table.