LOGO.SYS is a core system file used by Windows 9x operating systems to display its boot-up message.

It is present and used on Windows 95, Windows 98 and Windows me products. It is not in the Windows NT-family operating systems such as Windows XP.

There are three options file:

  • LOGOS.SYS: is the "It is now safe to turn off your computer" message. The file is located in the Windows directory. This message is displayed when Windows has successfully shut down to MS-DOS but is not configured to return to the prompt COMMAND.COM again. On systems with proper ACPI support and ATX power supply, the PC may power down instead. If the file cannot be found, the same message is displayed in text mode. No error will be shown if the file cannot be found.
  • LOGOW.SYS: is the "Please wait while Windows is shutting down" message. The file is located in the Windows directory, which by default is C:\WINDOWS. The Windows logo is shown only in Windows 95 and 98. No error will be shown if the file cannot be found.
  • LOGO.SYS: is the "Starting Windows" message, with the Windows logo. The file is located in the root directory of the boot drive. This is usually C:\, but with drive compression, like DriveSpace, this is the host drive often H:\. The default LOGO.SYS file is also stored in IO.SYS and used by MS-DOS during startup if LOGO.SYS could not be found. The display of the logo can be disabled by adding a LOGO=0 setting to the Options section in the MS-DOS 7 configuration file MSDOS.SYS.

Logo.Sys is, in fact, an 8-bit RLE-encoded files, Windows bitmap with a resolution of exactly 320×400 pixels in 256 colors. This icon is displayed otherwise little used 320x400 VGA in graphics mode, compromise, allowing you to display 256-color images with a high vertical, but not horizontal resolution on all compatible systems, even those with normal VGA cards that can only show 16 colors with high horizontal resolution and without any additional graphics drivers. Appear mode, any connected monitor to be identical to more common graphics 640x400 or 720x400 text modes, and therefore stretched to a standard 4:3 aspect ratio, the pixels appear 1.67 x 2 / 1.2 wider than they are high, instead of the square - they are in full 640x480 VGA display) on a normal 4:3 monitor, and monitors other forms of, 5:4, 16:9, etc. when set to display standard modes of video in original aspect with black borders. The post loading screens is peculiar, characteristic "feel" and made them more suitable for certain items that are masked horizontal shades or use a vertical resolution than the others that accentuate it, meaning some skill was required when you select an image that will still be aesthetically pleasing - or even clear enough to be correctly interpreted - somehow changed.

For LOGO.SYS or the equivalent of an embedded image in IO.SYS Windows to animate the image color information using the rotation palette, the image is static, but perhaps the illusion of motion as the colors change.

As the files are just standard RLE compressed.BMP with a completely optional custom segment tag name ".Sys", they can be opened and edited using tools to edit the images like MS Paint, and the contents is replaced with the user-selected images, only conversion you need to change the file extension, and that they desired resolution and color depth with dither if necessary. However, this process is not clear:

  • The loading indicator animation was created using palette rotation. The number of palette entries to rotate is determined by the otherwise seldom used biClrImportant field of the BITMAPINFOHEADER structure. Image editing software usually discarded this data, so it was often not possible to retain it. Some logo creation utilities were specifically created to restore the cycling function and allow creating custom animations.
  • Ensuring the aspect ratio is correct can be confusing, as it is usually displayed in a horizontally compressed form on a screen with square pixels.
  • Just like the bootsplash screens in earlier versions of Windows, there was a hard but poorly documented limit on how large the compressed file could be, because of the very limited memory available during the boot process the very reason that RLE was used in the first place - a plain BMP would have been 125kb and thus entirely too large, the default images are around 10 to 70kb each. If the file was too large, it would either simply fail to display or cause the system to crash, which required the user to reboot and drop into DOS mode before the logo had a chance to load, and either delete it or rename it to prevent the system trying to display it again before it could be fixed or replaced. Staying below, and moreover editing an image to bring it below this limit was an imprecise science that mostly required taking advantage of the particular characteristics of RLE, e.g. ensuring there were sufficient areas of the screen with long horizontal runs of the same color, by reducing the dither quality or color reduction mode, shrinking it slightly and adding a black border a few pixels wide all around, etc. Each run allowed a two byte code to represent a strip many pixels wide, and hence blank spaces or areas of flat color compressed very tightly, whereas regions with no repeated colors at all were at best uncompressed, at worst slightly larger than they would otherwise have been.