ANSI.SYS is a device driver in the DOS family of operating systems that provides extra console functions through ANSI escape sequences.
As it was not installed by default, and was notoriously slow, little software took advantage of it and instead resorted to directly manipulating the IBM PC hardware.
[independent source needed] To use ANSI.SYS under DOS, a line is added to the CONFIG.SYS (or CONFIG.NT under Windows NT based versions of Windows) file that reads: where drive: and path are the drive letter and path to the directory in which the file ANSI.SYS is found, and options can be a number of optional switches to control the behaviour.
[5] An interesting (mis)feature of ANSI.SYS is the ability to remap any key on the keyboard in order to perform shortcuts or macros for complex instructions.
A number of products were released to protect users against this: ANSI.SYS appeared in MS-DOS 2.0, the first version of the operating system supporting device drivers.