Modes like 320x240x256 etc. are Mode X-like by default. vga_modeinfo(3) lists which modes have Mode X memory layout.
A mode X like memory layout means that memory is organized in four pages (set_page(3)) and that the top most pixel 0,1,2,3 are the first pixel in pages 0,1,2,3. Pixels 4,5,6,7 are than the second pixels in pages 0,1,2,3.
The other, packed pixel, mode will have pixel 0,1,2,3 as pixel 0,1,2,3 in page 0 and so on. It will only use the next pages if the size of the first 64K page is exceeded. Most SVGA 256 color modes work this way.
This is not guaranteed to work. Probably a new mode should be defined for planar 320x200x256.
svgalib(7), vgagl(7), libvga.config(5), vgatest(6), vga_getmodeinfo(3), vga_setmode(3), vga_setpage(3),
This manual page was edited by Michael Weller <firstname.lastname@example.org>. The exact source of the referenced function as well as of the original documentation is unknown.
It is very likely that both are at least to some extent are due to Harm Hanemaayer <H.Hanemaayer@inter.nl.net>.
Occasionally this might be wrong. I hereby asked to be excused by the original author and will happily accept any additions or corrections to this first version of the svgalib manual.