HP Vectra Reference Manual page 109

Table 7.8
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Registers
Register
I/O Address Read
Write
3DO CRTC undefined CRTC Register Index
3D1 CRTC Data Register CRTC Data Register
3D2 CRTC undefined Duplicate CRTC Register Index
3D3 CRTC Data Register Duplicate CRTC Data Register
3D4 CRTC undefined Duplicate CRTC Register Index
3D5 CRTC Data Register Duplicate CRTC Data Register
3D6 CRTC undefined
Duplicate CRTC Register Index
3D7
CRTC Data Register Duplicate CRTC Data Register
3D8
Undefined* Mode Control Register
3D9 Undefined* Color Select Register
3DA Status Register Not allowed***
3DB Clear LP Flag * * Clear LP Flag * *
3DC
Set LP Flag * * Set LP Flag * *
3DD
Undefined* Extended Control Register
3DE
Undefined* Unused
3DF Board ID* Unused
*Reading an undefined register will cause its contents to become
undefined.
**Reading or writing will cause function. No data is written and value
returned via read is meaningless.
***Do not write to this register.
92 Multi-Mode Video Adapter Card