ABORT (CONFIG.SYS directive)
| Company type | Configuration file |
|---|---|
| Industry | Computer software / Operating systems |
| Founded | 1983 |
| Products | System configuration for DOS and OS/2 |
CONFIG.SYS serves as the chief configuration file for DOS and OS/2 operating system platforms. This plain text file contains setup commands that 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 adjust DOS settings related to hardware devices, software applications, and memory allocation. Once CONFIG.SYS has been processed, DOS initiates the command interpreter specified by the SHELL parameter, defaulting to COMMAND.COM if none is specified; this shell then executes AUTOEXEC.BAT.
Typically, the file is composed of name=value pairs that establish system parameters or load 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.
Certain DOS variants replace CONFIG.SYS with alternative filenames that take precedence. For instance, advanced iterations of Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, and REAL/32 utilize CCONFIG.SYS or CCONFIG.INI. Additionally, REAL/32 supports MCONFIG.SYS. These platforms often extend configuration capabilities through binary files like CCONFIG.BIN, which are commonly managed using CONFIG.EXE.
In the case of DR DOS version 3.31 and later—including distributions such as PalmDOS 1.0, Novell DOS 7, and DR-DOS 7.02 or newer—DCONFIG.SYS overrides CONFIG.SYS to enable disk compression and multi-boot scenarios by permitting multiple configuration files. OpenDOS 7.01 and DR-OpenDOS 7.02 also look for ODCONFIG.SYS, while some DR-DOS editions recognize DRCONFIG.SYS.
Starting with DR DOS 6.0, the SYS /DR:ext command allows renaming of system and configuration files to support several concurrent setups within a single root directory, selectable through boot loaders such as LOADER from Multiuser DOS and DR-DOS 7.x. The SHELL command enhances startup customization by allowing alternative AUTOEXEC.BAT filenames via /P[:filename.ext].
From version 6.0 onward, DR-DOS introduced the CHAIN=filespec instruction to continue processing additional configuration files beyond the root folder, with DR-DOS 7.02 expanding this functionality to include label-specific jumps using CHAIN=filespec,label. Moreover, DR-DOS 7.03 added a SYS /A feature to copy CONFIG.SYS and AUTOEXEC.BAT alongside other system files.
FreeDOS adopts a comparable mechanism with FDCONFIG.SYS, whereas RxDOS version 7.24 and above uses RXCONFIG.SYS. The PTS-DOS system applies the filename CONFIG.PTS.
Within the root directories of Windows 95 and Windows 98, both CONFIG.SYS and AUTOEXEC.BAT are present but generally remain unused due to these systems’ DOS-based architecture. Conversely, Windows Me bypasses parsing CONFIG.SYS during startup, instead retrieving environment variables directly from the Windows Registry at:
- HKLM\System\CurrentControlSet\Control\SessionManager\Environment
For FlexOS, CONFIG.SYS is a binary format file that specifies resource managers and device drivers loaded at boot time.
Example
Below is 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 points for directives like , , , and .
- Functions like , but additionally allows inline comments placed after other CONFIG.SYS commands. (Note that in MS-DOS/PC DOS, inline comments inside double quotes are not disregarded.)
- Used to start inline comments alongside the ; directive.
- Prompts a Yes/No confirmation before executing the subsequent CONFIG.SYS command. (FreeDOS extends this with optional conditions in boot menus.) Starting with DOS 6.0, a similar feature is available by appending after the directive (e.g., ).
- Executes the following directive without any condition.
ATemplate:NbndC
- Determines whether programs can be aborted upon encountering errors.
- Sets which drives record file access timestamps on FAT file systems.
- Enables conditional execution for AT-compatible PCs.
- Acts as a boot manager.
- Adjusts the level of detail shown in error messages.
- Changes the default serial port driver (COM1 through COM4) assigned to the AUX device.
- Prevents character devices from being accessible outside the \DEV directory.
- Loads essential device drivers into memory.
- Produces a beep sound even if notification sounds are turned off.
- Specifies the operating system to boot next.
- Enables or disables extended Ctrl+C checking.
- Sets the quantity of disk buffers allocated.
- Explicitly loads disk buffers into High Memory Area or Upper Memory Blocks.
- , ;, ; (Embedded DOS 6-XL only)
- Control cache block allocation and flushing timing parameters.
- Loads programs during CONFIG.SYS processing, pausing further processing until they return control.
- Sets the Caps Lock key state on the keyboard.
- , ; (PTS-DOS only)
- Configure CD-ROM driver settings and buffer sizes.
- Continues processing of CONFIG.SYS from another file or label.
- , ;, ;, ;, ;, ;, ;, ;, ;, ;, ;, ;, ;, ;, ; directives related to CPU specify hardware settings, device configurations, internationalization options, display parameters, and conditional execution based on CPU type or system capabilities.
These commands play a crucial role in fine-tuning system efficiency and ensuring hardware compatibility within business environments that depend on DOS-based platforms.
DTemplate:NbndF
- Adjusts relocation settings for drive descriptor tables.
- Defines memory segment addresses used for multi-sector disk data deblocking.
- Activates internal debugger support features.
- Functions as a boot manager.
- Loads device drivers into conventional memory.
- Loads device drivers into upper memory areas (similar to in DRDOS 5.0).
- Configures country and code page settings for keyboard, screen, and printer devices.
- Sets cache memory size parameters.
- Specifies compatibility mode for floppy disks.
- Selects the font used for display output.
- Controls whether DOS loads into high memory and/or utilizes upper memory blocks.
- Loads DOS internal tables (such as BUFFERS, FCBS, FILES, LASTDRIVE, STACKS) into upper memory regions.
- Defines an alternate path for locating DOS files.
- Sets search paths for data files.
- Defines physical disk drive parameters explicitly.
- Specifies parameters for drives that are not recognized automatically.
- See .
- Enables error logging to
PDUMP.nnnfiles and designates the drive used for these logs. - Displays text messages on screen during boot or processing.
- Displays messages using ANSI escape codes for formatting.
- Adds additional validation checks when launching programs.
- Specifies expected error or return codes for subsequent commands.
- Controls the A20 gate state when starting applications.
- Terminates CONFIG.SYS processing prematurely except for certain directives that must still be processed.
- Sets size of hash tables to optimize fast file open operations—functionality differing from MS-DOS/PC DOS FASTOPEN driver behavior.
- Defines how many File Control Blocks can be cached simultaneously (shares buffer space with FILES under DR-DOS).
- Similar to FCBS but explicitly loads FCBs into upper memory areas.
- Functions as a boot manager.
- Determines how many files can be open at once by the system.
- Similar to FILES but forces file handles to load into upper memory explicitly.
- Includes execution of specific CONFIG.SYS subsections (refer to IGNORE).
GTemplate:NbndI
- Waits for a key press or a timeout, then assigns the error level to the ASCII value of the key or 1024 if timed out.
- Resumes processing of CONFIG.SYS at a given label.
- Continues nested CONFIG.SYS processing at a specified label, then returns to the prior point.
- Refer to , , or for further details.
- See related information in .
- Consult for additional context.
- Refer to or for more information.
- See or for related instructions.
- Consult .
- 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 .
- Consult or for reference.
- Activates console and command line history buffers along with enhanced pattern search 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 virus check on the system boot sector.
- Loads installable file system drivers.
- Prevents execution of designated CONFIG.SYS subsections.
- Permits certain programs to operate at ring 2, granting direct hardware I/O access.
- Incorporates directives from another menu block, emulated through GOSUB/RETURN in DR-DOS.
- Sets the Insert mode status of the keyboard.
- 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.
- 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 of the relocated keyboard buffer area.
- Determines the maximum number of drives accessible by the system.
- Functions like LASTDRIVE but additionally loads the Current Directory Structure (CDS) into upper memory.
- Specifies paths for searching Dynamic Link Libraries (DLLs).
- Manages compatibility with legacy software applications.
- Configures the number of file locks allowed simultaneously.
- Enables boot logo support under MS-DOS, facilitating startup messages when using PTS-DOS boot manager.
- Specifies count, order, I/O port address, and timeout settings for the LPT1 device.
- Defines similar parameters—count, order, I/O address, timeout—for LPT2 device.
- Provides count, order, I/O port address, and timeout configurations for LPT3 device.
- Activates and configures parameters for LPT4 device support.
- Sets the delay before a waiting process is granted higher CPU priority.
- Determines memory allocation methods and strategies used by DOS.
- Establishes default DOS memory allocation behavior.
- Defines available menu options within CONFIG.SYS menus.
- Specifies color schemes for menus, which can be emulated via ECHO commands in DR-DOS environments.
- Sets the default menu block, emulatable using TIMEOUT and SWITCH commands in DR-DOS systems.
- Defines individual menu blocks that can be emulated through ECHO and SWITCH under DR-DOS operation.
- Enables or disables multitrack disk operations functionality.
- Continues CONFIG.SYS processing from a new file after loading a device driver and setting %NEWFILE% environment variable, similar to DR-DOS #CHAIN command.
NTemplate:NbndQ
- Assigns the default "No" character used in Yes/No/Resume prompt dialogs.
- Indicates whether keyboard [[Num Lock|Template:Keypress]] is active or inactive by default.
- Allows conditional execution based on the previous error code, for example .
- Displays specified text and waits until a key is pressed by the user.
- Requires user confirmation before continuing after errors encountered in CONFIG.SYS processing.
- Configures buffer sizes associated with LPT port operations.
- Enables conditional execution based on PC-compatible hardware detection.
- Sets system scheduler priority levels for processes or tasks.
- Defines priority levels 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 run on the system.
- ; Loads an alternative user interface shell program during startup.
- ; Executes conditionally on IBM PS/1-compatible systems only.
- ; Executes conditionally on IBM PS/2-compatible systems only.
- ; Suppresses display output sent to STDOUT.
RTemplate:NbndT
- Activates automatic system reboot upon encountering errors.
- Permits inclusion of comments within CONFIG.SYS files.
- Reserves a drive letter from local block device drivers.
- Configures the default Resume () character displayed in 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.
- Allows concurrent background loading of programs during CONFIG.SYS execution.
- Stores the initial character of deleted filenames within the FAT reserved area.
- Defines video mode settings (FreeDOS) or screen access methods (PTS-DOS).
- Controls the keyboard Scroll Lock key status, enabling or disabling it.
- Establishes environment variables.
- Loads the command interpreter shell.
- Similar to SHELL, but includes a parameter to pre-allocate COMMAND.COM memory in HMA or UMBs to minimize fragmentation.
- Sets keyboard modifier states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
- Determines whether files close automatically upon Ctrl+Alt+Del key combination.
- Specifies if the Power-On Self-Test (POST) is bypassed during reboot.
- , STACKSIZE, STACKSHIGH
- Configure hardware interrupt stack sizes and buffer locations across various DOS versions and Embedded DOS platforms.
- , SYSBOOT, SYSGEN, SYSLOAD, SYSTOUMB
- PTS-DOS-specific commands that manage boot options, system memory allocation, and loading system components into UMBs for enhanced memory efficiency.
- , SWITCH, SWITCHES
- Facilitate conditional processing within CONFIG.SYS through submenu blocks and option switches in multiple DOS variants except DR-DOS and FreeDOS.
- , SWAPPATH, THREADS, TIMESLICE
- OS/2 directives controlling error log suppression, swap file path designation, thread concurrency levels, and scheduler time slice durations.
- , TRACE, TRACEBUF, TRAPDUMP
- Handle temporary file extensions, enable single-step debugging modes for CONFIG.SYS/AUTOEXEC.BAT, configure trace log buffer sizes, and provide detailed error logging mainly under DR-DOS, PTS-DOS, and OS/2 systems.
- , define timeouts for single-step operations in CONFIG.SYS and default responses to prompts under DR-DOS and PTS-DOS.
This segment highlights essential CONFIG.SYS commands across DOS variants that offer critical flexibility for customizing system configurations tailored to business computing environments.
UTemplate:NbndZ
- Directs DOS to utilize a specified segment as Upper Memory Block (UMB) memory.
- Configures the UMB server settings.
- Defines address ranges allocated as UMB RAM.
- Enables or disables verification of disk writes.
- Reports a specific DOS version to software applications.
- Configures VGA display status parameters.
- Sets the current video mode similar to FreeDOS's SCREEN command.
- Determines whether virtual DMA is permitted.
- Controls deactivation of processor Virtual Mode Extensions.
- Sets up VxD driver parameters for Windows environments.
- Configures system support settings for Windows operating systems.
- Manages relocation of the Extended BIOS Data Area (XBDA).
- Executes conditionally on XT-compatible hardware platforms.
- Enables built-in Year 2000 compliance features.
- Defines the default affirmative response character in prompt queries.
- Configures tracking of file deletions.
Issues
While the system can initiate booting even if CONFIG.SYS is absent or corrupted, this file together with AUTOEXEC.BAT remains crucial for comprehensive operating system customization and meeting software prerequisites. Troubleshooting becomes necessary when either file is damaged.
In cases where CONFIG.SYS does not include 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 will fail to boot without it. MS-DOS 6.0/PC DOS 6.1 and Novell DOS 7 and later versions will prompt users to specify a command processor path. This recovery prompt also appears if the primary command interpreter terminates unexpectedly or exits intentionally, allowing limited shell replacement at runtime without requiring a reboot.
COMMAND.COM from MS-DOS 7.0 onwards is incompatible with DR-DOS but frequently resides in C:\ within dual-boot systems. DR-DOS versions 7.02 and later prevent bypassing SHELL directives during skip/trace/step modes triggered by Ctrl+F5/F7/F8 keys. Subsequent versions introduced Ctrl+F6 to reinstate skipping behavior for recovering from invalid SHELL parameters. If no valid SHELL directive is detected during CONFIG.SYS skipping procedures, users are prompted to input an appropriate shell filename before COMMAND.COM is loaded from the root directory.
CONFIG.SYS file size restrictions differ by operating system version—MS-DOS/PC DOS limits range up to approximately 64 KB. In contrast, DR-DOS imposes no effective size constraints due to its line-by-line interpretation method rather than tokenization. This approach grants DR-DOS users full control over driver and TSR loading order through DEVICE and INSTALL commands, facilitating conflict resolution or debugger integration prior to driver initialization.
DR-DOS’s flexible processing engine supports conditional execution based on hardware detection, key presses, error codes, or return values. It can also incorporate non-resident programs into the CONFIG.SYS sequence via INSTALL.
Earlier MS-DOS/PC DOS editions limited SHELL directive line length to 31 characters, later expanded to 128 characters. DR-DOS accepts lines up to 255 characters in length. None of these CONFIG.SYS directives support long filenames.
Dual Booting DOS and Windows 9x
During the installation of Windows 95 over a pre-existing DOS or Windows system, 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, backups of the Windows 95 versions are stored as CONFIG.W40 and AUTOEXEC.W40.
In setups where Caldera DR-DOS 7.02 or 7.03 is installed alongside Windows 95, the Windows CONFIG.SYS and AUTOEXEC.BAT files remain untouched, whereas DR-DOS utilizes separate startup configuration files named DCONFIG.SYS and AUTODOS7.BAT.
OS/2 / Windows NT
OS/2 depends extensively on CONFIG.SYS for loading device drivers and setting environment parameters prior to launching its graphical user interface.
Within the OS/2 subsystem of Windows NT, CONFIG.SYS does not exist as a physical file but is instead virtualized within the system registry.
Numerous CONFIG.SYS commands in OS/2 lack formal documentation, highlighting their specialized nature for that operating system.
CONFIG.SYS continues to be employed 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
- Information on AUTOEXEC.BAT and CONFIG.SYS at Computer Hope
- Overview of Windows 98 / MS-DOS 7.10 CONFIG.SYS directives (archived)
- Caldera’s CONFIG.SYS Command Reference (archived) – summary of DR-DOS 7 directives
- FreeDOS CONFIG.SYS directive descriptions
- PTS-DOS 2000 Pro User Manual including CONFIG.SYS directives (archived)
- Unofficial German PTS-DOS FAQ (archived) featuring additional CONFIG.SYS commands