LOGO.SYS

LOGO.SYS is a core system file used by Windows 9x operating systems to display its boot-up message.
It is present and used in the Windows 95, Windows 98, and Windows ME products, instead of the Windows NT family of operating systems, such as Windows XP.
There are three variants of the file:
LOGO.SYS is in fact an 8-bit RLE-encoded Windows bitmap file with a resolution of exactly 320×400 pixels at 256 colors. This is displayed in the otherwise little-used 320x400 VGA graphics mode, a compromise to allow the display of a 256-color image with high vertical (but not horizontal) resolution on all compatible systems, even those with plain VGA cards (which could only show 16 colors with high horizontal resolution) and without needing any additional graphics drivers. The mode appears, to any attached monitor, to be identical to the more common 640x400 graphics or 720x400 text modes, and is therefore stretched to a standard 4:3 aspect ratio (meaning the pixels appear to be 1.67x (2/1.2) wider than they are tall, instead of square - as they would be on a full 640x480 VGA display) on a typical 4:3 monitor of the time, and on monitors of other shapes (5:4, 16:9, etc.) when set to display standard video modes in their original aspects with letterbox borders. This lent the startup screens a peculiar, characteristic "feel" and made them more suited to certain subjects (which disguised the horizontal blockiness or made good use of the vertical resolution) than others (which accentuated it), meaning some skill was needed in choosing an image that would still be aesthetically pleasing - or even clear enough to be properly interpreted - once resized.
For LOGO.SYS or the equivalent embedded image in IO.SYS, Windows will also animate the image's color information using palette rotation; the image is static, but may have the illusion of movement as colors are changed.
As the files are standard RLE-compressed .BMPs (with an entirely optional custom tag segment) renamed to ".SYS", they may be opened and edited using image editing tools such as MS Paint, and the contents replaced with user-selected pictures; the only conversion needed is to change the file extension, and to ensure they are in the correct resolution and color depth (with dithering if needed). However, the process is not foolproof: