Цитата man xorg.conf

Virtual xdim ydim
This optional entry specifies the virtual screen resolution to be used. xdim must be
a m Virtual xdim ydim
This optional entry specifies the virtual screen resolution to be used. xdim must be
a multiple of either 8 or 16 for most drivers, and a multiple of 32 when running in
monochrome mode. The given value will be rounded down if this is not the case.
Video modes which are too large for the specified virtual size will be rejected. If
this entry is not present, the virtual screen resolution will be set to accommodate
all the valid video modes given in the Modes entry. Some drivers/hardware combina‐
tions do not support virtual screens. Refer to the appropriate driver-specific docu‐
mentation for details.

ViewPort x0 y0
This optional entry sets the upper left corner of the initial display. This is only
relevant when the virtual screen resolution is different from the resolution of the
initial video mode. If this entry is not given, then the initial display will be
centered in the virtual display area.ultiple of either 8 or 16 for most drivers, and a multiple of 32 when running in
monochrome mode. The given value will be rounded down if this is not the case.
Video modes which are too large for the specified virtual size will be rejected. If
this entry is not present, the virtual screen resolution will be set to accommodate
all the valid video modes given in the Modes entry. Some drivers/hardware combina‐
tions do not support virtual screens. Refer to the appropriate driver-specific docu‐
mentation for details.