MS-DOS 6.22 Console Output Device Driver File

In MS-DOS 6.22, the core system file that contains the default device control drivers for standard console output is IO.SYS. This hidden, read-only system file acts as the primary interface between the system hardware's ROM BIOS and the higher-level MS-DOS operating system. This article explains the architectural role of IO.SYS, how it manages standard console communications, and how it initializes essential hardware routines during the boot sequence.

The Role of IO.SYS

IO.SYS is the first system file loaded into memory by the master boot record when a computer boots into MS-DOS 6.22. It contains the resident hardware-dependent code required to interact directly with the system architecture. While MSDOS.SYS provides the core operating system kernel—handling file management, memory allocation, and software interrupts—IO.SYS provides the physical device drivers necessary to communicate with foundational peripherals.

Standard Console Output and the CON Device

Standard console output in MS-DOS represents text and control codes sent to the primary display monitor, typically referenced by the reserved system device name CON. Built directly into IO.SYS are the low-level routines that handle CON operations, translating software requests into the appropriate video BIOS interrupts (primarily INT 10h) to display characters on the screen.

In addition to console output, the driver within IO.SYS also controls console input via the keyboard (using INT 16h), binding both primary input and output under the single logical CON device.

Default Resident Drivers in IO.SYS

The console driver is not the only interface compiled into IO.SYS. The file contains a complete set of default character and block device drivers designed to ensure basic operational capability before any secondary drivers are loaded. These built-in drivers include:

• CON: The system console (keyboard input and screen display).
• AUX / COM1: The first serial communications port.
• PRN / LPT1: The first parallel printer port.
• CLOCK$: The system real-time clock.
• Default Block Drivers: Interfaces for floppy drives and recognized hard disk partitions.

Extensibility and Replacement

Because the console drivers inside IO.SYS provide only rudimentary, unbuffered text rendering without extended formatting capabilities, MS-DOS allows them to be augmented or overridden. By adding directives such as DEVICE=C:\DOS\ANSI.SYS in the CONFIG.SYS configuration file, users can load advanced console control routines. However, the foundational fallback driver for standard console output remains permanently housed within IO.SYS.