MSDOS.SYS is a vital system file on MS-DOS and Windows 9x operating systems.

In versions of MS-DOS from 1.1x through 6.22, the file comprised the MS-DOS kernel and is responsible for file access and program management. MSDOS.SYS is loaded by the DOS BIOS IO.SYS as part of the boot procedure.

In Windows 95 (MS-DOS 7.0) through Windows ME (MS-DOS 8.0), the DOS kernel has been combined with the DOS BIOS into a single file, IO.SYS (aka WINBOOT.SYS), while MSDOS.SYS became a plain text file containing boot configuration directives instead. If a WINBOOT.INI file exists, the system will retrieve these configuration directives from WINBOOT.INI rather than from MSDOS.SYS.

Some DOS utilities expect the MSDOS.SYS file to have a minimal file size of at least 1 KB. This is the reason why a large dummy comment is typically found in the MSDOS.SYS configuration file since Windows 95.

By default, the file is located in the root directory of the bootable drive/partition (normally C:\ for hard disks) and has the hidden, read-only, and system file attributes set.

.sys is a filename extension in Microsoft Windows and DOS-type operating system.

Most .sys files are real mode device drivers. Certain files using this extension are not, however: MSDOS.SYS and IO.SYS are core operating system files in MS-DOS and Windows 9x. The CONFIG.SYS text file contains various configuration options and specifies what device drivers will be loaded. COUNTRY.SYS is a binary database containing country and codepage related information for use with the CONFIG.SYS COUNTRY directive and the NLSFUNC driver, whereas KEYBOARD.SYS is a binary database containing keyboard layout related information including short P-code sequences to be executed by an interpreter inside the KEYB keyboard driver.

.sys files are mainly stored in the C:\Windows\system32\drivers directory of Windows computers, and also in the C:\Windows\WinSxS folder, particularly in Windows Vista and its successors.

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JPEG (/ˈdʒeɪpɛɡ/ JAY-peg) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality.

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SYS may indicate:

MS-DOS (/ˌɛmɛsˈdɒs/ EM-es-DOSS; acronym for Microsoft Disk Operating System) is an operating system for x86-based personal computers mostly developed by Microsoft. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid-1990s, when it was gradually superseded by operating systems offering a graphical user interface (GUI), in various generations of the graphical Microsoft Windows operating system by Microsoft Corporation.

MS-DOS resulted from a request in 1981 by IBM for an operating system to use in its IBM PC range of personal computers. Microsoft quickly bought the rights to 86-DOS from Seattle Computer Products, and began work on modifying it to meet IBM's specification. IBM licensed and released it in August 1981 as PC DOS 1.0 for use in their PCs. Although MS-DOS and PC DOS were initially developed in parallel by Microsoft and IBM, in subsequent years the two products diverged and became different from each other, with recognizable differences in compatibility, syntax, and capabilities.