Mode X an undocumented video graphics display mode on the IBM VGA graphics hardware that was popularized by Michael Abrash, a writer for Doctor Dobb's Journal.
Mode-X involves tweaking IBM's standard Mode 13h 256 colour graphics mode such that all the video memory is made available to programmer in a planar fashion, at the expense of extra complexity in writing to video memory. For many programmers this expense is well worth the effort as the extra video memory available makes possible functions paramount to producing high quality 2D games or demos, something this mode was primarily used for.
The main uses of the extra memory are:
- Higher resolutions: up to 360x480 in 256 colours is possible
- Double Buffering for flicker free animation
- Hardware smooth scrolling
- Graphics store 'off-screen' in VRAM can quickly be moved around in VRAM using the VGA latches
- Planar mode allows up to 4 pixels to be modified in one byte write operation, ideal for solid filling of objects such as polygons, rectangles, lines etc
