IGNORE (CONFIG.SYS directive)
| Company type | Configuration file |
|---|---|
| Industry | Operating systems |
| Founded | 1983 |
| Products | System configuration for DOS and OS/2 |
CONFIG.SYS serves as the main configuration file for DOS and OS/2 operating systems. This plain text file, written in ASCII, holds setup instructions that the operating system's DOS BIOS—typically found in IBMBIO.COM or IO.SYS—processes during system startup. The CONFIG.SYS file was first introduced with DOS version 2.0.
Usage
The directives within CONFIG.SYS are used to configure DOS for hardware devices, software applications, and memory allocation. Once CONFIG.SYS is executed, DOS initiates the command interpreter specified by the SHELL directive; if none is defined, it defaults to COMMAND.COM, which then runs AUTOEXEC.BAT.
This configuration file primarily contains name=value pairs that set adjustable parameters or load components such as device drivers and terminate-and-stay-resident programs (TSRs) into memory.
In DOS environments, CONFIG.SYS is located in the root directory of the bootable drive.
The same filename is also utilized by (DCP), an MS-DOS variant developed by East-German company VEB Robotron.
Certain DOS variants prioritize alternative filenames over CONFIG.SYS:
Advanced editions of Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, and REAL/32 look for CCONFIG.SYS (derived from "Concurrent") instead of CONFIG.SYS. Some Multiuser DOS releases employ CCONFIG.INI, while REAL/32 checks for MCONFIG.SYS. These platforms support extended configuration through binary files like CCONFIG.BIN, which are generally modified using CONFIG.EXE.
Within DR DOS 3.31, PalmDOS 1.0, Novell DOS 7, and DR-DOS versions 7.02 and later, DCONFIG.SYS (from "DR DOS") takes precedence over CONFIG.SYS. Initially linked to disk compression utilities, this file also assists with multi-boot setups. OpenDOS 7.01 and DR-OpenDOS 7.02 search for ODCONFIG.SYS, whereas some DR-DOS 7.02+ editions look for DRCONFIG.SYS.
Starting with DR DOS 6.0, the SYS /DR:ext command allows users to redefine default system file extensions, enabling multiple sets of system files to coexist within a single root directory and be selected via boot loaders such as LOADER from Multiuser DOS and DR-DOS 7.x. The SHELL directive supports specifying alternative AUTOEXEC.BAT filenames using /P[:filename.ext], and COMMAND.COM accepts extensions beyond .BAT (a feature also supported by 4DOS). Additionally, the CHAIN=filespec directive permits continuation of processing from another configuration file, with optional label jumping in DR-DOS 7.02+.
From DR-DOS 7.03 onward, the SYS /A command copies corresponding CONFIG.SYS and AUTOEXEC.BAT files alongside system files.
Other operating systems use variant filenames: FreeDOS employs FDCONFIG.SYS; RxDOS version 7.24 and above uses RXCONFIG.SYS; and PTS-DOS utilizes CONFIG.PTS.
Although both CONFIG.SYS and AUTOEXEC.BAT exist in the root directories of boot drives for Windows 95 and Windows 98, they are commonly left empty.
Windows Me does not process CONFIG.SYS during startup; instead, environment variables are loaded directly from the Windows Registry at:
- HKLM\System\CurrentControlSet\Control\SessionManager\Environment
In the case of FlexOS, CONFIG.SYS is a binary configuration file that specifies resource managers and device drivers to be 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 destinations for , , , and commands.
- Functions similarly to , but also allows inline comments following other CONFIG.SYS instructions. (Note: In MS-DOS/PC DOS, inline comments inside double quotes are not ignored.)
- Used alongside the ; directive to introduce inline remarks.
- Prompts with a Yes/No question to confirm execution of the next CONFIG.SYS command. (FreeDOS supports conditional execution via boot menus but lacks custom messages.) DOS version 6.0 and later provide a comparable feature by appending after commands (e.g., ). DR DOS supports this functionality as well.
- Executes the subsequent CONFIG.SYS command without condition.
ATemplate:NbndC
- Determines whether programs can be terminated upon encountering errors.
- Controls which drives record file access timestamps on FAT file systems.
- Executes conditionally on AT-compatible hardware.
- Boot management utility.
- Adjusts the level of detail in error reporting.
- Changes the default serial port driver (COM1 through COM4) assigned to the AUX device.
- Prevents character devices from being accessible outside the virtual \DEV directory.
- Loads a fundamental device driver into system memory.
- Produces an audible beep even if sounds are disabled via SWITCHES=/Q.
- Specifies the next operating system to boot.
- Enables or disables extended Ctrl+C detection.
- Sets the number of disk buffers allocated.
- Similar to BUFFERS but explicitly loads buffers into HMA or UMB areas.
- , , (Embedded DOS 6-XL only)
- Configures cache block allocation and flushing intervals.
- Loads applications within CONFIG.SYS, pausing further processing until completion.
- Defines keyboard Caps Lock state.
- , (PTS-DOS only)
- Specifies CD-ROM driver name and buffer quantity.
- Continues processing CONFIG.SYS in a separate file or label.
- , , and other PTS-DOS-specific commands control system clock type, clear screen functions, and optimize boot/configuration routines.
- , , and related directives enable internationalization support and OEM codepage settings across various DOS platforms.
Additional directives cover COM port setup (COM1–COM4), COMMENT handling, CPU type-based conditional execution (CPU386, CPU486+), and buffer management, enhancing system efficiency and hardware compatibility for business environments utilizing legacy DOS systems.
DTemplate:NbndF
- Manages relocation of drive descriptor tables in memory.
- Sets the memory segment limit 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).
- Configures locale and codepage settings for keyboard, screen, and printer devices.
- Defines cache memory size parameters.
- Sets floppy disk compatibility mode options.
- Selects display font settings.
- Controls loading of DOS into high memory area and/or use of upper memory (similar to in DRDOS 5.0).
- Specifies loading of DOS tables such as BUFFERS, FCBS, FILES into upper memory segments.
- Defines an alternative directory path for DOS files.
- Establishes search paths for data files.
- Describes physical disk drive characteristics.
- Sets parameters for drives not recognized by the system.
- Refer to .
- Enables PDUMP.nnn error log creation on specified drives.
- Displays a message on the screen during boot or configuration.
- Displays messages incorporating ANSI escape sequences.
- Determines whether extra checks are performed when launching programs.
- Sets an error or return code for subsequent operations within CONFIG.SYS processing.
- Controls the A20 line gate state when starting programs.
- Terminates CONFIG.SYS processing before reaching the end of the file (except for certain directives).
- Defines hash table size used for accelerated file opening, differing from MS-DOS/PC DOS FASTOPEN driver behavior.
- Specifies how many File Control Blocks (FCBs) may be open simultaneously (shares buffer with FILES under DR-DOS).
- Similar to FCBS but explicitly loads FCBs into upper memory segments.
- Boot management utility.
- Sets maximum number of files that can be opened concurrently by the system.
- Same as FILES but loads file handles explicitly into upper memory areas.
- Includes execution of specified subsections within CONFIG.SYS (see IGNORE).
GTemplate:NbndI
- Waits for a key press or timeout, setting the error level to the ASCII code of the key or 1024 if timed out.
- Resumes processing of CONFIG.SYS at a designated label.
- Continues nested CONFIG.SYS processing at a specified label, returning to the previous point afterward.
- Refer to , , or for additional information.
- See related commands , , and .
- Consult the entries for and for further details.
- Additional reference found in or .
- Also see or for context.
- Consult for more information.
- Loads a TSR program into memory following relocation of the DOS data segment. For comparison, see INSTCDEX.
- Refer to , , or for related commands.
- Additional details available in .
- See , or for further explanation.
- Activates console and command line history buffers along with enhanced pattern searching capabilities.
- Defines the address of the High Memory Area (HMA) segment.
- Configures power-saving mode using HLT instruction.
- Performs boot sector virus checks.
- Loads installable file system drivers.
- Prevents execution of specified sections within CONFIG.SYS.
- Permits programs to operate at ring 2, allowing direct hardware input/output access.
- Incorporates directives from another menu block, emulated via GOSUB/RETURN in DR-DOS environments.
- Sets the status of keyboard Insert mode.
- Installs a TSR program into memory.
- Loads a TSR program into upper memory, equivalent to HIINSTALL in DR DOS 5.0.
- Installs TSR after relocating the DOS data segment. See also INSTCDEX for comparison.
- Adjusts PIC interrupt priorities by rotating them to prioritize a specified IRQ.
KTemplate:NbndM
- Defines the keyboard layout configuration.
- Sets the relocated address for the keyboard buffer.
- Determines the maximum number of accessible drives.
- Similar to LASTDRIVE but loads the Current Directory Structure into upper memory space.
- Lists directories used for DLL search paths.
- Manages backward compatibility with legacy applications.
- Configures the number of file locks allowed simultaneously.
- Enables boot logo display and startup messages using PTS-DOS boot manager features.
- Specifies count, sequence, I/O port address, and timeout for LPT1 device configuration.
- Defines count, order, I/O address, and timeout parameters for LPT2 device setup.
- Sets count, sequence, I/O port address, and timeout values for LPT3 device operation.
- Enables and configures LPT4 device with similar parameters as other LPT ports.
- Establishes wait time before elevating process priority level.
- Configures memory allocation methods and strategies.
- Sets default DOS memory allocation policy parameters.
- Defines options available within menu configurations.
- Specifies color schemes for menus (can be emulated in DR-DOS).
- Determines default menu block selection (emulatable in DR-DOS).
- Specifies individual menu blocks (also emulatable in DR-DOS).
- Enables multitrack disk drive operations support.
- Continues processing of CONFIG.SYS from a new file after loading particular drivers and setting the %NEWFILE% environment variable.
NTemplate:NbndQ
- Sets the default character representing No () responses in Yes/No/Resume prompts.
- Controls keyboard Num Lock state by enabling or disabling it at startup.
- Executes commands conditionally based on the last error code returned, e.g., .
- Displays text and pauses awaiting a key press, functioning similarly to and commands in DR-DOS systems.
- Requires user acknowledgment upon encountering errors during CONFIG.SYS processing.
- Configures buffer sizes assigned to LPT ports for improved performance.
- Enables conditional execution on PC-compatible hardware platforms only.
- Sets priority levels within the system scheduler for processes.
- Assigns priority specifically to disk input/output programs to optimize performance.
- Changes default printer device driver assignment among LPT1 through LPT4 ports.
- Specifies whether DOS applications are permitted to execute under current configuration settings.
- Loads a user interface shell environment during system startup.
- Allows conditional execution on IBM PS/1-compatible machines exclusively.
- Permits conditional execution tailored for IBM PS/2-compatible systems only.
- Suppresses output directed to STDOUT by muting display messages.
RTemplate:NbndT
- Activates automatic system reboot when errors occur.
- Permits inclusion of comments within CONFIG.SYS (ignored by PTS-DOS).
- Allocates a drive letter for local block device drivers.
- Sets the default Resume () response character in Yes/No/Resume prompts.
- Returns control from nested CONFIG.SYS processing to the prior GOSUB or SWITCH location, optionally passing a return code.
- Defines the size of real mode memory.
- Allows simultaneous loading of programs within CONFIG.SYS, functioning similarly to CALL or INSTALL commands in DOS.
- Stores the initial character of deleted files in the FAT reserved area.
- Determines video mode under FreeDOS or screen access methods in PTS-DOS.
- Controls the keyboard Scroll Lock state, turning it on or off.
- Assigns environment variables.
- Loads the command interpreter shell.
- Functions like SHELL but enables pre-allocation of COMMAND.COM size into HMA or UMBs to prevent fragmentation, improving performance with third-party shells.
- Defines keyboard modifier states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
- Specifies whether files close upon Ctrl+Alt+Del key combination.
- Configures skipping of POST during system reboot.
- , STACKSIZE, STACKSHIGH, STACK, THREADS (various OS versions)
- Set hardware interrupt stack sizes, thread counts, or stack parameters to enhance system efficiency based on platform requirements.
- , SYSBOOT, SYSGEN, SYSLOAD, SYSTOUMB (PTS-DOS)
- Control boot manager configurations, alternative operating system boot files, memory locations for DOS components, loading from alternate disk areas, and placing DOS code/data into UMBs for optimized memory utilization.
- , STUBA20 (PTS-DOS), SUBMENU (DOS 6+), SUPPRESSPOPUPS (OS/2), SWAPPATH (OS/2), SWITCH (DR DOS), SWITCHAR (multiple DOS variants), SWITCHES (DOS variants), SXFAKEHWFPU (OS/2 Warp)
- Various internal system options including A20 gate management, menu handling, error logging preferences, swap file directory settings, conditional processing based on key inputs or switches, and fixes for FPU bugs enhancing system stability and adaptability.
- , TIMESLICE (various OS versions)
- Adjust timeouts for single-step execution in CONFIG.SYS and configure scheduling time slices to improve responsiveness.
- , TRACE, TRACEBUF (various OS versions)
- Define temporary file extensions and enable or configure tracing logs for debugging CONFIG.SYS and AUTOEXEC.BAT execution.
This segment outlines sophisticated CONFIG.SYS directives utilized across different DOS versions that provide critical flexibility for system configuration aimed at maximizing performance and compatibility within business computing settings.
UTemplate:NbndZ
- Directs DOS to designate a particular segment as an Upper Memory Block (UMB).
- Identifies the UMB server component.
- Sets address boundaries for UMB RAM allocation.
- Toggles disk write verification on or off.
- Defines the DOS version number reported to applications.
- Configures VGA display status parameters.
- Specifies the current video mode similarly to FreeDOS's SCREEN command.
- Enables use of virtual Direct Memory Access (DMA).
- Controls processor Virtual Mode Extensions (VME) functionality.
- Configures VxD driver behavior for Windows environments.
- Manages overall Windows support within the system.
- Relocates the Extended BIOS Data Area (XBDA).
- Executes conditionally on XT-class compatible machines.
- Sets up built-in Year 2000 compliance support mechanisms.
- Establishes default Yes response character in prompt dialogs.
- Monitors driver deletions.
Issues
Although the system can start without CONFIG.SYS or AUTOEXEC.BAT, these files are crucial for tailoring the DOS environment and loading essential software components. When these files become damaged or corrupted, troubleshooting is necessary.
If CONFIG.SYS is missing a SHELL directive or is absent/corrupted, DOS attempts to locate COMMAND.COM in the root directory. Versions prior to 6.0 will fail if COMMAND.COM is not found. Starting with MS-DOS 6.0, PC DOS 6.1, and Novell DOS 7 or later, users are prompted to specify a command processor path when the shell fails to load or exits unexpectedly. This feature facilitates runtime shell replacement without requiring a system reboot.
Because of compatibility issues between MS-DOS 7.x’s COMMAND.COM and DR-DOS, versions 7.02 and above of DR-DOS no longer permit bypassing SHELL directives during skip, trace, or step modes. This change improves stability in dual-boot environments. Recovery options include prompts that request valid shell paths when CONFIG.SYS processing is skipped.
The size of CONFIG.SYS is restricted to a few kilobytes (up to 64 KB) in MS-DOS and PC DOS but remains unrestricted under DR-DOS, limited only by the file system’s maximum file size. MS-DOS processes CONFIG.SYS by tokenizing it into memory before executing directives in a fixed sequence (loading drivers prior to TSRs). Conversely, DR-DOS interprets CONFIG.SYS line-by-line, enabling exact control over the loading order through DEVICE and INSTALL commands. This approach resolves conflicts during loading and supports conditional execution based on hardware presence or error conditions, thereby improving customization options and debugging.
Additionally, DR-DOS allows non-resident programs to be loaded using INSTALL, integrating temporary software into the CONFIG.SYS workflow.
Early MS-DOS editions limited the length of the SHELL command line to 31 characters; later releases expanded this limit to 128 characters, whereas DR-DOS permits up to 255 characters. It should be noted that CONFIG.SYS directives do not support long filenames.
Dual booting DOS and Windows 9x
When Windows 95 is installed over an existing DOS/Windows configuration, the original CONFIG.SYS and AUTOEXEC.BAT files are renamed CONFIG.DOS and AUTOEXEC.DOS, enabling dual-boot functionality. During a DOS boot, these files are temporarily restored to their original names. Windows 95 system backups are saved as CONFIG.W40 and AUTOEXEC.W40.
In setups where Caldera DR-DOS versions 7.02 or 7.03 coexist with Windows 95, Windows’ CONFIG.SYS and AUTOEXEC.BAT retain their original filenames. Instead, DR-DOS uses DCONFIG.SYS and AUTODOS7.BAT for its startup routines.
OS/2 / Windows NT
OS/2 depends extensively on CONFIG.SYS for driver configuration and environment setup prior to launching its graphical user interface.
Within the OS/2 subsystem of Windows NT, what OS/2 applications recognize as CONFIG.SYS is actually maintained within the system registry.
There exist numerous undocumented CONFIG.SYS directives specific to OS/2.
CONFIG.SYS continues to be utilized 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
- Information on AUTOEXEC.BAT and CONFIG.SYS available at Computer Hope
- Overview of CONFIG.SYS directives for Windows 98 / MS-DOS 7.10 (archived)
- Caldera's CONFIG.SYS Command Reference (archived) – summary of DR-DOS 7 directives
- Details on FreeDOS CONFIG.SYS directives
- PTS-DOS 2000 Pro User Guide featuring CONFIG.SYS directives (archived)
- Unofficial German FAQ for PTS-DOS including CONFIG.SYS directives (archived)