AUX (CONFIG.SYS directive)
| Company type | Configuration file |
|---|---|
| Industry | Computer software |
| Founded | 1983 |
| Headquarters | N/A |
| Products | Configuration commands for DOS and OS/2 |
CONFIG.SYS serves as the principal configuration file utilized by DOS and OS/2 operating systems. This specialized ASCII text document contains setup commands accessible to users, which the operating system's DOS BIOS—commonly found in IBMBIO.COM or IO.SYS—interprets during the boot sequence. CONFIG.SYS made its debut with DOS version 2.0.
Usage
The directives within CONFIG.SYS tailor DOS to manage hardware devices, software applications, and memory management utilities. Once CONFIG.SYS has been processed, DOS launches the command interpreter specified by the SHELL directive or defaults to COMMAND.COM, which subsequently executes the AUTOEXEC.BAT script.
Primarily, this file is composed of name=value entries that configure adjustable settings or load components such as device drivers and terminate-and-stay-resident programs (TSRs).
In DOS environments, CONFIG.SYS is located in the root directory of the system’s boot drive.
The filename is also employed by (DCP), an MS-DOS variant developed by the former East German company VEB Robotron.
Certain DOS derivatives support alternative configuration filenames that supersede CONFIG.SYS:
- Advanced editions of Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, and REAL/32 prioritize CCONFIG.SYS (derived from "Concurrent") instead of CONFIG.SYS. Occasionally, Multiuser DOS uses CCONFIG.INI, while REAL/32 may utilize MCONFIG.SYS. These platforms accommodate enhanced configuration parameters stored in binary files like CCONFIG.BIN, which are generally managed using CONFIG.EXE.
- In versions of DR DOS 3.31 and later (including PalmDOS 1.0, Novell DOS 7, OpenDOS 7.01, and DR-DOS 7.02+), DCONFIG.SYS (short for "DR DOS") takes precedence over CONFIG.SYS. Initially associated with disk compression drivers and multi-boot configurations, these alternative filenames facilitate maintaining multiple system setups.
- OpenDOS 7.01 and DR-OpenDOS 7.02 also recognize ODCONFIG.SYS, while some DR-DOS releases check for DRCONFIG.SYS as well.
- Starting with DR DOS 6.0, the command SYS /DR:ext enables renaming system files and CONFIG.SYS extensions to support multiple parallel configuration sets within a single root directory, selectable through boot loaders such as LOADER in Multiuser DOS and DR-DOS 7.x.
- The SHELL directive allows specifying different AUTOEXEC.BAT files using /P[:filename.ext], with COMMAND.COM accepting extensions beyond ".BAT" in these cases—functionality also provided by 4DOS.
- From DR-DOS 6.0 onward, the CHAIN=filespec command permits continuation of processing another configuration file anywhere on the boot drive; starting with DR-DOS 7.02, an optional label argument (CHAIN=filespec,label) enables jumping directly to designated sections.
- DR-DOS 7.03 and later introduced a SYS /A option to copy CONFIG.SYS and AUTOEXEC.BAT alongside other system files.
Additional DOS variants offer comparable capabilities:
- FreeDOS employs FDCONFIG.SYS; - RxDOS version 7.24 and above uses RXCONFIG.SYS; - PTS-DOS utilizes CONFIG.PTS.
In DOS-based systems like Windows 95 and Windows 98, CONFIG.SYS and AUTOEXEC.BAT remain present in the root of the boot drive but are generally devoid of content.
Windows Me, however, does not process CONFIG.SYS during startup; instead, it loads environment variables directly from the registry path:
- HKLM\System\CurrentControlSet\Control\SessionManager\Environment
Within the context of FlexOS, CONFIG.SYS exists as a binary file that specifies resource managers and device drivers loaded during system initialization.
Example
A sample CONFIG.SYS configuration 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>
:;*?!
- Marks labels within CONFIG.SYS to serve as jump targets for directives such as , , , and .
- Functions similarly to , but additionally allows inline commentary following other CONFIG.SYS instructions. (Note that in MS-DOS/PC DOS, inline comments inside double quotes are not ignored.)
- Used at the start of inline comments alongside the ; directive.
- Prompts a Yes/No confirmation before executing subsequent CONFIG.SYS commands. (FreeDOS supports conditional execution with boot menus but lacks customizable prompt messages.) DOS version 6.0 and later offer a comparable feature by appending to directives.
- Forces unconditional execution of the next CONFIG.SYS command.
ATemplate:NbndC
- Determines whether programs can be aborted when errors occur.
- Controls storing file access date stamps on FAT file systems.
- Executes commands conditionally on AT-compatible hardware.
- Boot management utility.
- Adjusts the level of detail in error message reporting.
- Alters the default serial driver assigned to the AUX device.
- Prevents use of character devices outside the virtual \DEV directory.
- Loads a fundamental device driver into memory.
- Produces a beep sound even if audible alerts are disabled.
- Specifies which operating system to boot next.
- Enables or disables extended Ctrl+C signal checking.
- Defines the quantity of disk buffers allocated.
- Similar to BUFFERS but allocates buffers within HMA or UMB memory areas.
- , ,
- Parameters for cache block management.
- Loads programs during CONFIG.SYS processing, pausing further processing until they return.
- Sets the Caps Lock key status on the keyboard.
- ,
- Names CD-ROM drivers and configures their buffer settings.
- Continues processing CONFIG.SYS from another file or label.
- , , , etc.
- Various system setup directives mostly specific to certain DOS variants or OS/2 environments.
Additional directives manage serial port settings (COM1-COM4), keyboard configuration, memory buffer allocation, CPU-specific execution conditions, and device drivers, all aimed at enhancing system performance across diverse DOS platforms and hardware setups.
DTemplate:NbndF
- Adjusts relocation settings for drive descriptor tables.
- Defines memory segment address used for multi-sector disk data deblocking operations.
- Activates internal debugger functionality.
- Boot management utility.
- Loads a device driver into conventional memory.
- Loads a device driver into upper memory blocks (equivalent to in DRDOS 5.0).
- Sets country and codepage parameters for KBD, SCR, LPT devices.
- Configures cache memory sizes for improved performance.
- Sets compatibility mode for floppy disk drives.
- Specifies the display font used by the system.
- Controls loading DOS into high memory area and/or utilization of upper memory (similar to in DRDOS 5.0).
- Directs loading of DOS tables (, , , etc.) into upper memory regions.
- Defines an alternative search path for DOS system files.
- Specifies search paths for data files used by the system.
- Sets physical parameters for disk drives.
- Defines characteristics for unrecognized drive types.
- Refer to .
- Enables logging of error reports as
PDUMP.nnnfiles on a designated drive. - Displays a simple message on screen during startup or processing.
- Shows a message including ANSI escape sequences for enhanced formatting.
- Determines whether additional validation checks occur when launching programs.
- Sets an assumed error or return code value used by the system.
- Configures A20 gate status when initiating a program environment.
- Terminates CONFIG.SYS processing prematurely except for certain allowed directives.
- Defines hash table size to speed up file open operations, differing from MS-DOS/PC DOS FASTOPEN driver behavior.
- Sets maximum number of cached File Control Blocks (FCBs) that can be open simultaneously (shares buffer space with FILES under DR-DOS).
- Similar to FCBS but allocates FCBs in upper memory areas.
- Boot management utility feature.
- Specifies maximum number of files that can be opened concurrently.
- Same as FILES but loads file handles into upper memory blocks.
- Includes execution of specified subsections within CONFIG.SYS (see also IGNORE).
GTemplate:NbndI
- Waits for a key press or timeout, setting the error level to the ASCII value of the key or 1024 if timed out.
- Resumes processing of CONFIG.SYS at a specified label.
- Continues nested CONFIG.SYS processing at a given label and returns afterward.
- Refer to , , or for further details.
- Refer to for additional information.
- See for related commands.
- Consult or for more context.
- Consult or for further guidance.
- Refer to for additional instructions.
- Loads a terminate-and-stay-resident (TSR) program into memory after relocating the DOS data segment. Compare with INSTCDEX.
- Refer to , , or for more details.
- See for related information.
- Consult or as needed.
- Activates console and command line history buffering along with enhanced pattern searching capabilities.
- Defines the address of the High Memory Area (HMA) segment.
- Configures energy-saving behavior related to the HLT instruction.
- Specifies system checks for boot sector virus protection.
- Loads installable file system drivers.
- Prevents execution of specific CONFIG.SYS subsections.
- Permits designated programs to execute at ring 2 privilege level, enabling direct hardware input/output access.
- Includes directives from an alternative menu block, emulated through GOSUB/RETURN in DR-DOS environments.
- Sets the status of the keyboard Insert mode.
- Loads a TSR program into memory.
- Installs a TSR program into upper memory, equivalent to HIINSTALL in DR DOS 5.0.
- Installs a TSR program post DOS data segment relocation. Compare with INSTCDEX.
- Adjusts Programmable Interrupt Controller (PIC) interrupt priorities, elevating a specified IRQ number to highest priority.
KTemplate:NbndM
- Defines the keyboard layout configuration.
- Specifies the address of the relocated keyboard buffer area.
- Sets the maximum number of accessible drives in the system.
- Functions like LASTDRIVE but additionally loads Current Directory Structure into upper memory.
- Lists directories for DLL search paths.
- Manages legacy application loading procedures.
- Determines the count of file locks allowed.
- Enables boot logo display and startup messages using PTS-DOS boot manager settings.
- Defines parameters such as count, sequence, I/O address, and timeout for LPT1 device configuration.
- Applies similar configuration parameters to LPT2 device.
- Sets up LPT3 device with comparable options.
- Enables and configures LPT4 device functionality.
- Establishes wait time before increasing process priority levels.
- Configures memory allocation methods used by DOS.
- Specifies default DOS memory allocation strategy settings.
- Defines available menu options within CONFIG.SYS menus.
- Sets color schemes for menus, which can be emulated under DR-DOS systems.
- Selects the default menu block, emulatable in DR-DOS environments.
- Specifies particular menu blocks, with emulation support in DR-DOS.
- Enables support for multitrack disk operations on compatible devices.
- Continues CONFIG.SYS processing within a new file after loading a driver and sets %NEWFILE% environment variable accordingly.
NTemplate:NbndQ
- Sets the default character representing "No" () in Yes/No/Resume prompts.
- Turns keyboard [[Num Lock|Template:Keypress]] status on or off by default at startup.
- Enables conditional execution based on the last error code returned, e.g., .
- Displays a message and waits for a key press, akin to and commands found in DR-DOS systems.
- Requires user confirmation when errors occur during CONFIG.SYS processing.
- Configures buffer sizes assigned to LPT1 through LPT3 parallel ports.
- Allows conditional execution exclusively on PC-compatible machines.
- Sets process scheduling priority parameters within DOS environment.
- Defines priority levels specifically for disk I/O operations by programs.
- Changes the default printer device driver assignment among LPT1 through LPT4 ports.
- Controls whether DOS applications are permitted to run under current settings.
- Loads an alternative user interface shell program during startup sequences.
- Enables conditional execution only on IBM PS/1-compatible hardware platforms.
- Enables conditional execution restricted to IBM PS/2-compatible systems.
- Suppresses output sent to standard output (STDOUT) during command execution.
RTemplate:NbndT
- Activates automatic system reboot upon encountering errors.
- Permits inclusion of comments within CONFIG.SYS files.
- Allocates drive letters for local block device drivers.
- Establishes the default Resume () character used in Yes/No/Resume prompts.
- Returns control from nested CONFIG.SYS processing to the prior GOSUB or SWITCH point, optionally providing a return code.
- Specifies the size of real mode memory available.
- Enables concurrent background loading of programs from CONFIG.SYS.
- Saves the initial character of deleted files in the FAT reserved area.
- Determines video mode under FreeDOS or screen access protocols under PTS-DOS.
- Sets the keyboard Scroll Lock key state to on or off.
- Assigns environment variables.
- Loads the command interpreter shell.
- Functions like SHELL but includes a SIZE parameter for pre-allocating COMMAND.COM into HMA or UMBs, optimizing memory use with third-party shells.
- Defines keyboard modifier states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
- Manages file closure behavior when Ctrl+Alt+Del is pressed.
- Skips Power-On Self-Test (POST) during system reboot.
- Configures the number of stacks allocated for hardware interrupts.
- Sets the default stack size for newly created threads.
- Similar to STACKS but loads buffers into upper memory blocks.
- Contains boot manager settings.
- Controls behavior of the XMS A20 gate.
- Defines submenu blocks compatible with DR-DOS emulation.
- Redirects error logging to a drive instead of displaying popups.
- Specifies the location path for swap files.
- Allows conditional continuation of nested CONFIG.SYS processing based on key input, supporting RETURN to previous locations.
- Sets the system-wide default switch character, either '/' or '-'.
- Defines special option parameters.
- Enables a fix addressing floating-point unit (FPU) rounding errors.
- Specifies an alternate DOS operating system filename for boot purposes.
- Configures memory locations used by DOS components.
- Loads the system from an alternative disk area.
- Defines the size of the dynamic system memory pool.
- Loads DOS data and code into upper memory blocks (UMBs).
- { { nobold |( OS /2 only ) }}
- Specifies the number of concurrent threads supported.
- { { mono |1 = TIMEOUT } }{ { nobold |(DR - DOS 6 .0 + , PTS - DOS ) } }
- Sets timeouts for CONFIG.SYS commands such as SWITCH, GETKEY, and default prompt answers.
;{ { anchor |TIMESLICE } }{ { mono |1 = TIMESLICE } }{ { nobold |( OS /2 only ) } }: Configures scheduler time slice durations.
;{ { anchor |TMPEXT } }{ { mono |1 = TMPEXT } }{ { nobold |( PTS - DOS only ) } }: Defines temporary file extensions used by the system.
;{ { anchor |TRACE } }{ { mono |1 = TRACE } }{ { nobold |(DR - DOS 7 .02 + , PTS - DOS , OS /2 ) } }: Enables or disables single-step execution for CONFIG.SYS and AUTOEXEC.BAT scripts, with functionality varying by operating system.
;{ { anchor |TRACEBUF } }{ { mono |1 = TRACEBUF } }{ { nobold |( OS /2 only ) } }: Sets the buffer size for TRACE log files.
;{ { anchor |TRAPDUMP } }{ { mono |1 = TRAPDUMP } }{ { nobold |( OS /2 3 .0 + ) } }: Records detailed error diagnostics.
UTemplate:NbndZ
- Sets DOS to manage a specified memory segment as Upper Memory Block (UMB).
- Determines which UMB server is utilized by DOS.
- Defines address ranges allocated as UMB RAM areas.
- Toggles disk write verification on or off.
- Specifies the DOS version reported to applications.
- Configures VGA display status parameters.
- Sets current video mode similarly to FreeDOS's SCREEN command.
- Enables or disables virtual DMA support.
- Controls activation of processor Virtual Mode Extensions (VME).
- Configures Virtual Device Driver (VxD) settings for Windows environments.
- Manages system support features related to Windows operation.
- Handles relocation of the Extended BIOS Data Area (XBDA).
- Executes commands conditionally based on XT-compatible machine detection.
- Configures built-in Year 2000 compliance support mechanisms.
- Sets default affirmative response character in prompt dialogs.
- Controls tracking and management of file deletions.
Issues
Although the system is capable of booting without CONFIG.SYS, this file, together with AUTOEXEC.BAT, is vital for comprehensive DOS customization and meeting software demands. Troubleshooting becomes necessary if these files are damaged or absent.
When CONFIG.SYS is missing a SHELL command or is corrupted, DOS attempts to locate COMMAND.COM in the root directory of the boot drive. Versions prior to 6.0 will fail to boot if COMMAND.COM is not found. In contrast, MS-DOS 6.0, PC DOS 6.1, and Novell DOS 7 and later versions prompt the user for an alternate command processor path and offer a limited runtime shell replacement without requiring a reboot.
Because of compatibility issues between MS-DOS 7.0 and newer versions of COMMAND.COM and DR-DOS, DR-DOS 7.02 and subsequent releases prevent bypassing SHELL directives during skip, trace, or step modes. This measure improves stability when operating in dual-boot environments. When skipping CONFIG.SYS processing, users are prompted to provide valid shell filenames as part of recovery options.
Under MS-DOS and PC DOS, CONFIG.SYS file size is restricted to a few kilobytes (up to 64 KB in later releases), whereas DR-DOS imposes no such limit since it processes directives sequentially rather than compiling them all at once. This approach enables precise management of driver and TSR loading sequences, resolves conflicts, and supports conditional execution based on hardware detection or user input.
The maximum length for SHELL command lines in MS-DOS/PC DOS versions 2.0 through 4.01 is capped at 31 characters; subsequent versions expand this limit to 128 characters. DR-DOS allows up to 255 characters for these lines. It should be noted that CONFIG.SYS directives do not accommodate long filenames.
Dual booting DOS and Windows 9x
During the installation of Windows 95 over an existing DOS/Windows configuration, the original CONFIG.SYS and AUTOEXEC.BAT files are renamed to CONFIG.DOS and AUTOEXEC.DOS, respectively, enabling dual-boot functionality between Windows 9x and DOS environments. When booting into DOS mode, these backups are temporarily restored as CONFIG.SYS and AUTOEXEC.BAT. Additionally, Windows 95 versions of these files are saved under the names CONFIG.W40 and AUTOEXEC.W40.
In setups where Caldera DR-DOS 7.02 or 7.03 coexists with Windows 95, the Windows CONFIG.SYS and AUTOEXEC.BAT files remain unchanged. Instead, DR-DOS employs alternative startup scripts named DCONFIG.SYS and AUTODOS7.BAT.
OS/2 / Windows NT
OS/2 extensively utilizes CONFIG.SYS for driver configuration and environment setup prior to launching its graphical user interface.
Within the OS/2 subsystem on Windows NT, settings traditionally found in CONFIG.SYS for OS/2 applications are instead stored within the system registry rather than as physical files.
A variety of undocumented CONFIG.SYS directives exist specifically for OS/2.
CONFIG.SYS continues to be employed by OS/2 derivatives 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 directives (archived)
- Caldera’s CONFIG.SYS Command Reference (archived) – overview of DR-DOS 7 directives
- FreeDOS CONFIG.SYS directive descriptions
- PTS-DOS 2000 Pro User Manual including CONFIG.SYS directives (archived)
- Unofficial German-language PTS-DOS FAQ (archived) covering CONFIG.SYS directives