I/O System may refer to:
DOS BIOS (Basic Input/Output System), a low-level component in DOS operating systems, including PC DOS and DR-DOS I/O System (86-DOS), the DOS-BIOS specifically in 86-DOS I/O System (MS-DOS), the DOS-BIOS specifically in MS-DOS
See also BIOS, Basic Input/Output System XIOS, Extended Input/Output System CP/M BIOS, the BIOS in the CP/M family of operating systems BIOS (disambiguation) IOS (disambiguation) IOCS, Input/Output Control System