LPT4 (CONFIG.SYS directive)

CONFIG.SYS

Company type	Configuration file
Industry	Operating systems
Founded	1983
Products	System configuration for DOS and OS/2

CONFIG.SYS serves as the chief configuration file for the DOS and OS/2 operating system families. This file is a plain ASCII text document containing setup instructions accessible to users, which the operating system's DOS BIOS interprets during the boot sequence. CONFIG.SYS was first introduced with DOS version 2.0.

Usage

The directives within CONFIG.SYS are responsible for configuring DOS components such as hardware devices, software applications, and memory management utilities. Once CONFIG.SYS has been processed, DOS proceeds to load the command interpreter specified by the SHELL directive; if none is specified, it defaults to COMMAND.COM, which then executes commands from the AUTOEXEC.BAT script.

Typically, CONFIG.SYS is composed of name=value pairs that establish configurable settings or load essential files like device drivers and terminate-and-stay-resident programs (TSRs). This file is located in the root directory of the system's boot drive.

Several DOS variants prioritize alternative configuration filenames over CONFIG.SYS:

- Advanced editions of Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, System Manager, and REAL/32 utilize CCONFIG.SYS or CCONFIG.INI (in Multiuser DOS) and MCONFIG.SYS (in REAL/32). These platforms support supplementary configuration data kept in CCONFIG.BIN, commonly edited through CONFIG.EXE.

- In DR DOS 3.31, PalmDOS 1.0, Novell DOS 7, OpenDOS 7.01, and DR-DOS versions 7.02 and later, DCONFIG.SYS overrides CONFIG.SYS priority. This arrangement facilitates multi-boot environments and disk compression techniques where drive letters may be reassigned after loading compression drivers.

- OpenDOS 7.01 and DR-OpenDOS 7.02 also recognize ODCONFIG.SYS, while certain DR-DOS 7.02+ builds check for DRCONFIG.SYS. The command SYS /DR:ext enables changing default file extensions to allow multiple system configuration files within a single root directory, selectable by boot managers like LOADER.

- The SHELL command supports specifying alternative AUTOEXEC.BAT files via /P[:filename.ext], with COMMAND.COM accepting extensions other than .BAT in this mode—a feature also available in 4DOS.

- Starting with DR-DOS 6.0, the CHAIN=filespec directive allows continuation of processing another configuration file, optionally targeting a specific section using a label parameter (CHAIN=filespec,label).

- From DR-DOS 7.03 onward, the SYS /A option facilitates copying CONFIG.SYS and AUTOEXEC.BAT alongside system files during setup.

Other DOS-based systems employ analogous configuration files:

- FDCONFIG.SYS in FreeDOS; - RXCONFIG.SYS in RxDOS version 7.24 and later; - CONFIG.PTS in PTS-DOS.

For Windows 95 and Windows 98 systems—which are built upon DOS—both CONFIG.SYS and AUTOEXEC.BAT exist on boot drives but are generally left empty. In contrast, Windows Me does not process CONFIG.SYS at startup; instead, it retrieves environment variables directly from the Windows Registry (HKLM\System\CurrentControlSet\Control\SessionManager\Environment).

In the case of FlexOS, CONFIG.SYS is a binary file that specifies resource managers and device drivers loaded during system initialization.

Example

Below is a sample CONFIG.SYS for MS-DOS 5: <syntaxhighlight lang="ini" line="1"> DOS=HIGH,UMB DEVICE=C:\DOS\HIMEM.SYS DEVICE=C:\DOS\EMM386.EXE RAM DEVICEHIGH=C:\DOS\ANSI.SYS FILES=30 SHELL=C:\DOS\COMMAND.COM C:\DOS /E:512 /P </syntaxhighlight>

:;*?!

Defines labels within CONFIG.SYS to serve as jump points for , , , , and commands.

Functions similarly to , but additionally allows inline comments after other directives. (In MS-DOS/PC DOS, inline comments enclosed in double quotes are not disregarded.)

Introduces inline comments in conjunction with the ; command.

Presents a Yes/No prompt to confirm execution of subsequent CONFIG.SYS instructions. (FreeDOS does not support optional messages but allows conditions via boot menus.) DOS 6.0 and later versions provide a comparable feature using a trailing after commands, also supported by DR DOS.

Executes the following CONFIG.SYS directive without condition.

ATemplate:NbndC

Determines whether programs may be aborted upon encountering errors.

Sets which drives record file access timestamps within FAT file systems.

Enables conditional execution on AT-compatible hardware.

Manages boot selection.

Adjusts the verbosity level of error messages.

Changes the default serial port driver (COM1 through COM4) assigned to the AUX device.

Prevents access to character devices outside the virtual \DEV directory.

Loads a fundamental device driver into system memory.

Produces a brief beep even if audible alerts are disabled.

Indicates the subsequent operating system to boot.

Enables or disables extended Ctrl+C detection.

Defines the quantity of disk buffers to allocate.

Equivalent to BUFFERS but allocates buffers in HMA or UMB areas.

, CACHETTL, CACHEFLUSH (Embedded DOS 6-XL only)

Manage cache block allocation and timing parameters.

Loads programs during CONFIG.SYS processing and pauses until they return control.

Sets the Caps Lock keyboard state.

, CDDBUFFERS (PTS-DOS only)

Configures CD-ROM driver buffer allocation and parameters.

Continues processing of CONFIG.SYS from another file or label.

, CLS, CODEPAGE (various OS-specific)

Configure system clock type, screen clearing behavior, and OEM codepage support.

, COLOR (PTS-DOS only)

Sets CMOS RAM address and boot menu color schemes.

, COM2, COM3, COM4 (DR-DOS 7.02+)

Configures serial port quantity, sequence, I/O addresses, and timeout values.

, COMMENT(S) (various OS-specific)

Specifies DMA buffer sizes and redefines inline comment syntax or line ignoring behavior.

(DOS 3.0+, DR DOS 3.31+, OS/2)

Activates and configures international language support.

, CPSW (DR DOS/MS-DOS versions)

Enables cursor position display and codepage switching functionality.

, CPU88, CPU86, CPU286, CPU386, CPU486 (+ variants) (PTS-DOS/DR-DOS)

Executes commands conditionally based on CPU model or generation.

This summary provides an essential overview of key CONFIG.SYS directives influential for system setup and performance tuning across various DOS derivatives and OS/2 platforms. It highlights commands affecting device drivers, memory handling, internationalization, and boot management—elements critical for enterprise IT operations.

DTemplate:NbndF

Adjusts relocation settings for drive descriptor tables.

Defines memory segment address for multi-sector disk data deblocking operations.

Activates internal debugger support features.

Manages boot selection processes.

Loads specified device drivers into memory.

Loads device drivers into upper memory areas (equivalent to in DRDOS 5.0).

Configures country codes and codepages for keyboard, screen, and printer devices.

Sets cache memory size parameters.

Defines floppy disk compatibility mode settings.

Specifies font used for display output.

Controls loading of DOS into high memory and/or usage of upper memory blocks.

Loads DOS internal tables (BUFFERS, FCBS, FILES, LASTDRIVE, STACKS) into upper memory regions.

Specifies an alternate directory path for DOS system files.

Defines search paths for data files used by DOS utilities.

Provides physical disk drive configuration parameters.

Sets parameters for drives not natively recognized by the system.

See .

Enables error logging and designates the drive where log files are stored.

Displays a text message on the screen during CONFIG.SYS processing.

Shows a message including ANSI escape sequences for formatting effects.

Specifies additional verification checks when launching programs.

Defines expected error or return codes for subsequent commands.

Controls the A20 line state when programs start running to manage memory access above 1MB.

Terminates CONFIG.SYS processing prematurely except for certain exceptions.

Sets size of hash tables used to speed up file opening operations—differs from MS-DOS/PC DOS FASTOPEN driver behavior.

Determines number of cached File Control Blocks (FCBs) that can be open simultaneously (shares buffer space with FILES under DR-DOS).

Similar to FCBS but loads FCBs into upper memory areas instead of conventional memory.

Boot manager utility command.

Specifies maximum number of files that may be opened concurrently by the system.

Same as FILES but loads file handles into upper memory blocks instead of conventional memory locations.

Includes execution of designated subsections within CONFIG.SYS files (refer to IGNORE directive).

GTemplate:NbndI

Waits for a key press or timeout, then assigns the error level to the ASCII value of the key pressed or 1024 if timed out.

Resumes processing of CONFIG.SYS at a designated label.

Continues nested CONFIG.SYS execution at a specified label, with the ability to return to the previous point.

Refer to , , or for additional details.

Consult for further information.

See for related functionality.

Refer to or for more context.

See or for related commands.

Consult for additional reference.

Loads a TSR (Terminate and Stay Resident) program into memory following DOS data segment relocation. Compare with INSTCDEX.

See , , or for related usage.

Refer to for more details.

Consult or for further instructions.

Activates console and command line history buffer with enhanced pattern searching capabilities.

Defines the address of the High Memory Area (HMA) segment.

Configures energy-saving settings related to the HLT instruction.

Specifies whether to perform a boot sector virus scan during system startup.

Loads installable file system drivers.

Prevents execution of specified CONFIG.SYS subsections.

Permits designated programs to execute at ring 2, enabling direct hardware I/O access.

Incorporates directives from an alternate menu block, emulated via GOSUB/RETURN in DR-DOS environments.

Sets the status of the keyboard Insert mode.

Installs a TSR program into memory.

Loads a TSR program into upper memory, functionally equivalent to HIINSTALL in DR DOS 5.0.

Installs a TSR program following DOS data segment relocation. Compare with INSTCDEX.

Adjusts PIC interrupt priorities by rotating them to assign highest priority to a specified interrupt number.

KTemplate:NbndM

Defines the keyboard layout configuration.

Sets the address for the relocated keyboard buffer.

Specifies the maximum number of accessible drives allowed.

Functions like LASTDRIVE but additionally loads Current Directory Structure (CDS) into upper memory.

Lists directories used when searching for DLL files.

Manages compatibility with legacy programs during loading processes.

Configures the number of allowable file locks.

Sets parameters for boot logo display and startup messages when using PTS-DOS boot manager.

Defines count, sequence, I/O port address, and timeout settings for LPT1 device.

Similar settings as LPT1 applied to LPT2.

Same as LPT1 configuration but for LPT3.

Enables and configures LPT4 device in a manner consistent with previous LPT devices.

Determines wait duration before increasing process priority level.

Specifies strategy for memory allocation management.

Sets default method for allocating DOS memory resources.

Defines available menu options within the system interface.

Specifies color schemes for menus, which can be emulated under DR-DOS systems.

Designates default menu block, with emulation support under DR-DOS environments.

Establishes individual menu blocks, emulatable within DR-DOS implementations.

Enables or disables multitrack disk operations feature.

Continues processing CONFIG.SYS from an external file, optionally loading a device driver and setting the %NEWFILE% environment variable.

NTemplate:NbndQ

Sets default character used as "No" () response in Yes/No/Resume prompts.

Controls status of keyboard [[Num Lock|Template:Keypress]] key upon startup.

Enables conditional command execution based on previous error codes, such as .

Displays text message and waits for user key press, similar in function to and in DR-DOS systems.

Requires user confirmation before proceeding after errors in CONFIG.SYS are encountered.

Defines buffer size parameters for LPT printer devices.

Enables conditional execution on PC-compatible hardware platforms.

Configures scheduler priority levels within the operating system environment.

Sets priority values specifically for disk I/O operations programs.

Changes default printer device driver assignment among LPT1 through LPT4 ports.

Specifies whether DOS applications are permitted to execute on the system.

Loads user interface components during startup sequence.

Enables conditional execution on IBM PS/1-compatible machines only.

Allows conditional execution on IBM PS/2-compatible computer systems exclusively.

Suppresses standard output (STDOUT) display output from commands.

RTemplate:NbndT

Activates automatic system reboot upon encountering errors.

Permits inclusion of comments within CONFIG.SYS.

Allocates a drive letter to prevent conflicts with local block devices.

Establishes the default character for Resume () prompts.

Returns control from nested CONFIG.SYS processing to the previous GOSUB or SWITCH point, optionally passing a return code.

Specifies the size of real mode memory available.

Enables simultaneous loading of programs in CONFIG.SYS (background execution).

Saves the initial character of deleted files in the FAT reserved sector.

Determines video mode under FreeDOS or selects screen access methods for PTS-DOS.

Configures the status of the keyboard Scroll Lock key.

Sets environment variables for the system session.

Loads the command interpreter shell.

Functions like SHELL but supports pre-allocating space to relocate COMMAND.COM into HMA or UMBs, minimizing fragmentation.

Adjusts keyboard modifier keys such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.

Controls whether files are closed upon pressing Ctrl+Alt+Del.

Indicates if POST should be bypassed during reboot.

, STACKSIZE, STACKSHIGH

Define interrupt stack sizes and buffer locations across various DOS flavors and versions.

, SYSBOOT, SYSGEN, SYSLOAD

Specific to PTS-DOS for managing boot processes and system loading configurations.

, SWITCH, SWITCHES

Manage submenu blocks and conditional execution within CONFIG.SYS, depending on DOS variant.

, SWAPPATH

OS/2 parameters for error logging and specifying swap file directories.

, SWAPPATH

Sets system switch characters ('/' or '-') and locations for swap files.

, THREADS, TIMESLICE

OS/2 options addressing FPU fixes, thread concurrency control, and scheduler time slices.

, TRACE, TRACEBUF

Configure timeouts and tracing capabilities within CONFIG.SYS for DR-DOS, PTS-DOS, and OS/2 systems.

, TRAPDUMP

Defines temporary file extensions in PTS-DOS and detailed error logging options under OS/2.

UTemplate:NbndZ

Directs DOS to designate a specific memory segment as UMB (Upper Memory Block).

Specifies the UMB manager utilized by the system.

Defines address ranges allocated for UMB RAM usage.

Toggles disk write verification on or off.

Reports a particular DOS version to applications.

Controls VGA display status settings.

Indicates current video mode similarly to FreeDOS's SCREEN command.

Enables virtual DMA functionality.

Activates or deactivates the processor's Virtual Mode Extensions (VME).

Configures Virtual Device Drivers (VxD) for Windows environments.

Sets parameters supporting Windows integration.

Moves the Extended BIOS Data Area (XBDA) to a different location.

Executes conditionally on XT-compatible hardware platforms.

Provides Year 2000 compliance support settings.

Defines the default affirmative response character in prompts.

Tracks file deletion operations.

Issues

Although the system can boot without CONFIG.SYS or if it is corrupted, both this file and AUTOEXEC.BAT are critical for complete DOS startup customization and proper software operation. If these files become damaged, troubleshooting is required.

When CONFIG.SYS lacks a SHELL directive or is missing/corrupted, DOS attempts to locate COMMAND.COM in the root directory of the boot drive. Versions prior to 6.0 fail to start without COMMAND.COM present. In contrast, MS-DOS 6.0/PC DOS 6.1 and Novell DOS 7+ provide a prompt requesting an alternative command processor path if COMMAND.COM is absent or execution is aborted. This prompt enables limited runtime shell replacement without needing a reboot.

COMMAND.COM from MS-DOS 7.0 onward is incompatible with DR-DOS but often resides on drive C: in dual-boot configurations. Starting with DR-DOS 7.02, SHELL directives are strictly enforced during skip, trace, or step modes to prevent bypasses common in earlier releases and improve recovery from invalid shell argument errors.

CONFIG.SYS file size restrictions differ: MS-DOS/PC DOS typically limits size to several kilobytes (up to 64 KB), whereas DR-DOS imposes no practical limit by interpreting directives sequentially rather than compiling them. This method grants enhanced control over driver and TSR loading order and allows conditional processing based on hardware detection, user input, or error codes. Additionally, DR-DOS's INSTALL command facilitates incorporating non-resident software into the CONFIG.SYS execution flow for greater flexibility.

The maximum line length for SHELL directives expanded from 31 characters in MS-DOS/PC DOS versions 2.0 through 4.01 up to 128 characters in later releases; DR-DOS supports lines as long as 255 characters. CONFIG.SYS directives do not accept long filenames.

Dual booting DOS with Windows 9x

During the installation of Windows 95 on a system already running DOS or an earlier Windows version, the files CONFIG.SYS and AUTOEXEC.BAT are renamed to CONFIG.DOS and AUTOEXEC.DOS to enable dual boot functionality between Windows 9x and DOS. When the system boots into DOS, these files are temporarily reverted to their original filenames. Meanwhile, the Windows 95 versions are preserved under the names CONFIG.W40 and AUTOEXEC.W40.

In cases where Caldera DR-DOS 7.02 or 7.03 is installed alongside Windows 95, the Windows configuration files CONFIG.SYS and AUTOEXEC.BAT remain unchanged, whereas DR-DOS utilizes DCONFIG.SYS and AUTODOS7.BAT to manage its startup settings.

OS/2 and Windows NT

The OS/2 operating system extensively depends on CONFIG.SYS for configuring system parameters, loading device drivers, and setting up the environment before launching its graphical user interface.

Within the OS/2 subsystem incorporated in Windows NT, what OS/2 applications recognize as CONFIG.SYS is actually maintained within the system registry.

Numerous CONFIG.SYS commands used by OS/2 are either undocumented or have very limited documentation available.

CONFIG.SYS remains in use by OS/2-based systems such as eComStation and ArcaOS.

See also

• IBMBIO.COM / IO.SYS
• IBMDOS.COM / MSDOS.SYS
• CONFIG (environment variable)
• AUTOEXEC.BAT
• COMMAND.COM
• List of DOS commands

External links

• AUTOEXEC.BAT and CONFIG.SYS details at Computer Hope
• Overview of Windows 98 / MS-DOS 7.10 CONFIG.SYS commands (archived)
• Caldera's CONFIG.SYS Command Reference (archived) – summary of DR-DOS 7 directives
• Explanation of FreeDOS CONFIG.SYS commands
• PTS-DOS 2000 Pro User Guide including CONFIG.SYS command descriptions
• German-language unofficial PTS-DOS FAQ (PTS/FAQD) with extended CONFIG.SYS information