File Extension SYS
File extension SYS is widely used on MS-DOS and Windows platforms by SYStem files. Usually these files contain executable code for hardware device drivers (for example 1394bus.sys). SYS files run in the kernel-mode which is the highest possible privilege on Microsoft Windows systems. This means that errors in these files can cause great harm and data loss. If these files are infected by viruses, trojans or rootkits infection may be very hard to remove. We strongly recommend to install drivers only from trusted sources like official hardware manufacturer websites or digital media that came with the device.
Two other notable file extension SYS files are config.sys and msdos.sys. These files are different from the ones that are described above because they contain configuration parameters in text form rather than executable code. These files were widely used in MS-DOS days along with autoexec.bat file to control the way system boots up. However these files are obsolete on modern Windows systems and are available for backward compatibility purposes only.