Thanks Chuck, details are as follows: video card is Avance Logic 2101, monitor is IBM 8513002 (I think). I have attached a jpeg of the display to help clarify my problem. The mouse is visible only in the paler part of the screen, but its range upwards seems to be greater.
XF86Config from Section "Display"
Ok in my XF86Config there is a subsection "Display" in section "Screen"
Section "Screen"
Identifier "Screen1"
Device "Avance Logic 2101"
Monitor "My Monitor"
DefaultDepth 8
Subsection "Display"
Depth 8
Modes "640x480" "800x600" "1024x768"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "640x480" "800x600"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "640x480"
ViewPort 0 0
EndSubsection
EndSection
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<<attachment: xscreen.jpg>>
