Input/output (I/O) modes determine how character devices process input and output. MS-DOS has two I/O modes: ASCII and binary. (These are sometimes called “cooked” and “raw” modes, respectively.)
The chief difference between these two modes is the way in which MS-DOS processes control characters. In ASCII mode, MS-DOS checks for control characters as it processes input or output for a device. If it encounters a control character, it removes the character from the input and carries out its corresponding action, described in the following table:
| Control character | Action in ASCII mode |
| CTRL+C | Passes control to the CTRL+C exception handler. Subsequent actions depend on the current handler; the default handler terminates the program. |
| CTRL+P | Copies all subsequent input characters, up to the next CTRL+P, to the printer device. |
| CTRL+S | Suspends further output to the device. The next input character restores output. |
| CTRL+Z | Marks the end of the file. Subsequent calls to Read File or Device (Interrupt 21h Function 3Fh) return zero bytes. |
In binary mode, no action is carried out and control characters remain as input until they are read by a program.
ASCII mode also may affect the way characters are displayed. For example, a screen device expands tab characters to space characters in ASCII mode, but not in binary mode.
By default, the MS-DOS I/O mode is ASCII. A program can determine the current I/O mode for a device by using Get Device Data (Interrupt 21h Function 4400h). This function takes a device handle as a parameter and returns a value indicating the device status. If bit 5 is set, the device is in binary mode. Otherwise, the device is in ASCII mode. Set Device Data (Interrupt 21h Function 4401h) changes the mode for a device.
The I/O mode is a property of the device handle and affects the input and output of only those programs that own the handle.