ANSI.SYS is a device driver in the DOS family of operating systems that provides extra console functions through ANSI escape sequences. It is partially based upon a subset of the text terminal control standard proposed by the ANSI X3L2 Technical Committee on Codes and Character Sets (the "X3 Committee"). 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. A number of third-party alternatives that ran at reasonable speed were created, such as ANSI.COM, NANSI.SYS and ANSIPLUS.EXE to attempt to change this.

Usage

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. ANSI.SYS may also be loaded into upper memory via DEVICEHIGH/HIDEVICE.

Functionality

Using this driver, programs that write to the standard output can write escape sequences to make use of the 16 text foreground colors and 8 background colors available in VGA-compatible text mode, make text blink, change the location of the cursor on the screen, and blank the screen. They could also change the video mode from standard 80×25 text mode to a number of different graphics modes (for example, 320×200 graphics mode with text drawn as pixels, though ANSI.SYS is not able to turn individual pixels on and off). The standard ANSI.SYS is very slow as it maps escape sequences to BIOS calls (which managed to be designed so that two calls were needed to put a character on the screen and move the cursor right). Several companies made third-party replacements such as NANSI.SYS that interface directly with the video memory, in a similar way to most DOS programs that have a full-screen user interface. checked if this driver was in use, and changed the CLS command to use an escape sequence instead of a BIOS call.

Keyboard remapping

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. Using special escape sequences, the user can define any keystroke that has a character-code mapping to simulate an arbitrary sequence of such keystrokes. This was used to create simple trojans out of text files laced with nefarious keyboard remaps, known as "ANSI bombs". A number of products were released to protect users against this:

Occurrence

ANSI.SYS appeared in MS-DOS 2.0, the first version of the operating system supporting device drivers. It was supported by all following versions of MS-DOS. It is also present in many non-Microsoft DOS systems, e.g. IBM PC DOS and DR-DOS. ANSI.SYS was required to run some software that used its cursor and color control functions. It could also be used to enable elaborate color codes in the COMMAND.COM prompt. These uses were overshadowed by the use of ANSI.SYS in BBSes; ANSI escape sequences were used to enable BBSes to send text graphics more elaborate than ASCII art, and to control the cursor in ways that were used in a number of online games and similar features. Most versions of Windows did not support ANSI escape codes in any useful way (it could be used by MSDOS emulation in some versions). In Windows 10 support for similar escape sequences was built into the Win32 console (the text terminal window), but must be activated using the Windows API function SetConsoleMode by setting the ENABLE_VIRTUAL_TERMINAL_PROCESSING flag.

Features

CSI (Control Sequence Introducer) is a placeholder for the common two-byte escape lead-in sequence "ESC [" (that is, 0x1B 0x5B). The ANSI standard also defines an alternative single-byte CSI code 0x9B, which is not supported by ANSI.SYS (this code instead drew a cent sign ⟨¢⟩ from CP437). Standard DOS ANSI.SYS drivers support only the following sub-set of ANSI escape sequences: There are also some escape sequences specific to the implementation of ANSI.SYS. They are not generally supported by ANSI consoles in other operating systems. In some DOS implementations, video modes above 7 are not documented. Under Multiuser DOS, the only valid argument in conjunction with PCTERM is 7.