DISPLAY (CONFIG.SYS directive)
| Company type | Configuration file |
|---|---|
| Industry | Computer software |
| Founded | 1983 (introduced in DOS version 2.0) |
| Products | Configuration commands for DOS and OS/2 boot sequence |
CONFIG.SYS serves as the chief configuration file for DOS and OS/2 operating systems. This plain text file contains configuration commands that the operating system's DOS BIOS—typically found in IBMBIO.COM or IO.SYS—interprets during the system's boot sequence. The introduction of CONFIG.SYS dates back to DOS version 2.0.
Usage
The directives within CONFIG.SYS tailor DOS settings for hardware devices, software applications, and memory allocation. Once CONFIG.SYS has been processed, DOS launches the command interpreter designated in the SHELL statement or defaults to COMMAND.COM, which subsequently executes AUTOEXEC.BAT.
Primarily, this file holds name=value pairs that specify adjustable parameters or initiate loading of files such as device drivers and terminate-and-stay-resident programs (TSRs). In DOS environments, CONFIG.SYS is located in the boot drive’s root directory.
The same filename is utilized by (DCP), an MS-DOS variant developed by East-German manufacturer VEB Robotron.
Certain DOS variants check for alternative filenames with higher priority than CONFIG.SYS:
- Advanced editions of Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, and REAL/32 prioritize CCONFIG.SYS, a nod to "Concurrent." Multiuser DOS might employ CCONFIG.INI, while REAL/32 opts for MCONFIG.SYS. These platforms support extended configuration options stored in binary files like CCONFIG.BIN, which are usually edited using CONFIG.EXE.
- In versions such as DR DOS 3.31, PalmDOS 1.0, Novell DOS 7, and DR-DOS 7.02+, DCONFIG.SYS supersedes CONFIG.SYS. Initially linked with disk compression utilities to handle drive letter reassignment, it also enables multi-boot setups. OpenDOS 7.01 and DR-OpenDOS 7.02 check 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 renaming of default system files, facilitating multiple concurrent system file sets within a single root directory. These can be selected through boot loaders like LOADER included with Multiuser DOS and DR-DOS 7.x. The SHELL command supports specifying alternate AUTOEXEC.BAT files using /P[:filename.ext], and COMMAND.COM accepts script extensions beyond ".BAT" under these conditions (also compatible with 4DOS). The CHAIN=filespec directive permits continuation of processing in another file that need not reside in the root directory; from DR-DOS 7.02 onward, chaining to specific labels within files is supported.
- Versions DR-DOS 7.03 and later introduce a SYS /A option to duplicate associated CONFIG.SYS and AUTOEXEC.BAT files alongside system files.
FreeDOS adopts a similar approach with FDCONFIG.SYS; meanwhile, RxDOS version 7.24+ uses RXCONFIG.SYS, and PTS-DOS utilizes CONFIG.PTS.
Within the DOS-based environments of Windows 95 and Windows 98, CONFIG.SYS and AUTOEXEC.BAT remain present in the root of the boot drive but are generally left empty. In contrast, Windows Me ceases to parse CONFIG.SYS during startup, instead loading environment variables directly from the Windows Registry at:
- HKLM\System\CurrentControlSet\Control\SessionManager\Environment
For the FlexOS operating system, CONFIG.SYS is a binary-format file that specifies resource managers and device drivers loaded during 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 like , allowing inline comments after other directives. (Note: MS-DOS/PC DOS does not ignore inline comments enclosed in quotes.)
- Introduces inline commentary alongside the ; directive.
- Presents a Yes/No prompt with an optional message prior to executing CONFIG.SYS commands.[nb 1] FreeDOS accommodates conditional statements in boot menus through and . DOS 6.0 and later versions support similar functionality by appending to directives (for example, ).
- Forces unconditional execution of the subsequent CONFIG.SYS command.
ATemplate:NbndC
- Determines whether programs can be aborted upon encountering errors.
- Configures drives to record file access timestamps within FAT directory entries.
- Enables conditional execution on AT-compatible hardware.
- Acts as a boot manager.
- Adjusts the verbosity level of error messages.
- Alters the default internal serial device driver assigned to the AUX device.
- Prevents character devices from being accessible outside the phantom \DEV directory.
- Loads a base device driver into memory.
- Produces an audible beep even if sound notifications are disabled.
- Specifies which operating system to boot next.
- Enables or disables extended Ctrl+C checking.
- Defines the quantity of disk buffers to allocate.
- Functions like BUFFERS but loads buffers into HMA or UMB memory areas.
- , CACHETTL, CACHEFLUSH (Embedded DOS 6-XL only)
- Manage cache size and timing parameters.
- Executes programs within CONFIG.SYS, pausing further processing until completion.
- Sets the initial Caps Lock status when booting.
- , CDDBUFFERS (PTS-DOS only)
- Configures CD-ROM driver name and buffer settings.
- Continues processing CONFIG.SYS from a new file or label location.
- , CLS, CODEPAGE, CMOSADDR, COLOR (PTS-DOS or OS/2 specific)
- Control system clock, screen clearing, OEM codepages, CMOS RAM address, and boot menu color schemes respectively.
- , COM2, COM3, COM4 (DR-DOS 7.02+)
- Define count, order, I/O addresses, and timeout parameters for COM ports.
- , COMMENT(S) (various DOS versions)
- Specify DMA buffer sizes and redefine inline-comment lead characters or ignore specific lines.
- (DOS 3.0+, DR DOS 3.31+, OS/2)
- Activates international support configuration options.
- , CPSW (DR DOS or MS-DOS specific)
- Sets cursor position for menus and supports codepage switching.
- , CPU88, CPU86, CPU286, CPU386, CPU486 (PTS-DOS or DR-DOS)
- Conditional execution based on CPU type or processor speed.
DTemplate:NbndF
- Manages relocation of drive descriptor tables in memory.
- Defines memory segment address for multi-sector disk data deblocking operations.
- Activates internal debugger support features.
- Functions as a boot manager utility.
- Loads a device driver into system memory.
- Loads a device driver into upper memory blocks (similar to in DRDOS 5.0).
- Sets country and codepage parameters for keyboard, screen, and printer devices.
- Configures cache memory sizes for improved performance.
- Defines floppy disk compatibility modes.
- Specifies display font settings for the console.
- Controls loading of DOS into high memory and/or usage of upper memory areas (analogous to in DRDOS 5.0).
- Loads DOS internal tables such as BUFFERS, FCBS, FILES into upper memory segments.
- Specifies an alternative search path for DOS files.
- Defines search paths for data files used by DOS applications.
- Establishes physical parameters for disk drives connected to the system.
- Sets configuration parameters for unrecognized or unknown drives.
- See .
- Enables error logging by specifying drive allocation for
PDUMP.nnnfiles. - Displays a message on the screen during boot or processing stages.
- Outputs messages containing ANSI escape sequences for enhanced formatting.
- Adds extra verification checks when launching programs.
- Defines an anticipated error or return code value for program execution results.
- Controls the state of the A20 gate when starting applications to manage memory access above 1MB.
- Terminates CONFIG.SYS processing prematurely in most cases except some exceptions.
- Configures hash table size to accelerate file open operations (differs from MS-DOS/PC DOS FASTOPEN driver behavior).
- Sets the number of cached File Control Blocks that can be open simultaneously (shares buffer space with FILES in DR-DOS).
- Similar to FCBS but loads these into upper memory areas instead of conventional memory.
- Functions as a boot manager component within CONFIG.SYS processing flow.
- Specifies the maximum number of files that can be open concurrently by DOS programs.
- Same as FILES but loads file handles into upper memory blocks for better conventional memory management.
- Includes execution of specified subsections within CONFIG.SYS using IGNORE functionality.
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 the specified label.
- Continues nested CONFIG.SYS processing at a given label, then returns to the previous point.
- Refer to , , or for additional details.
- Consult for further information.
- See for related commands.
- Refer to or for context.
- See or for relevant instructions.
- Consult for more information.
- Loads a TSR program into memory following DOS data segment relocation. Compare with INSTCDEX.
- See , , or for related functions.
- Refer to for additional context.
- See or for further details.
- Activates console and command line history buffers with enhanced pattern searching capabilities.
- Defines the address of the HMA segment.
- Configures power-saving mode using HLT instruction.
- Performs boot sector virus scanning during startup.
- Loads installable file system drivers.
- Prevents execution of designated CONFIG.SYS subsections.
- Permits certain programs to execute at ring 2 level, allowing direct hardware access.
- Includes directives from another menu block (simulated via GOSUB/RETURN in DR-DOS).
- Sets the status of keyboard Insert mode.
- Loads TSR programs into memory.
- Installs TSR utilities into upper memory, equivalent to HIINSTALL in DR DOS 5.0.
- Loads TSR programs after DOS data segment relocation, similar to INSTALL command.
- Adjusts PIC interrupt priorities to assign highest priority to a specified IRQ.
KTemplate:NbndM
- Defines the keyboard layout configuration.
- Specifies the address of the relocated keyboard buffer.
- Sets the maximum number of accessible drives.
- Similar to LASTDRIVE but also loads Current Directory Structure (CDS) into upper memory area.
- Lists paths for DLL search operations.
- Manages loading of legacy applications.
- Specifies the count of file locks allowed.
- Configures boot logo support when using PTS-DOS boot manager.
- Defines count, order, I/O port address, and timeout settings for LPT1 device.
- Defines count, order, I/O port address, and timeout parameters for LPT2 device.
- Sets count, order, I/O port address, and timeout values for LPT3 device.
- Enables and configures LPT4 device similarly to other parallel ports.
- Determines wait time before elevating process priority level.
- Specifies memory allocation strategy parameters.
- Sets default DOS memory allocation method.
- Defines menu option entries within CONFIG.SYS menus.
- Specifies color schemes for menus (emulated through ECHO commands in DR-DOS).
- Sets default menu block selection (emulated using TIMEOUT and SWITCH in DR-DOS).
- Defines individual menu blocks (emulated with ECHO and SWITCH under DR-DOS).
- Enables multitrack disk operations support.
- Continues CONFIG.SYS processing from a new file after loading a device driver and sets %NEWFILE% environment variable accordingly.
NTemplate:NbndQ
- Sets default character representing No () in Yes/No/Resume prompts.
- Controls state of keyboard Num Lock key at startup.
- Enables conditional execution based on previous error code (e.g., ).
- Functions similarly to and , displaying text and awaiting key input.
- Requires user confirmation upon errors encountered in CONFIG.SYS processing.
- Configures buffer sizes assigned to LPT ports.
- Enables conditional execution when running on PC-compatible hardware platforms.
- Sets scheduler priority level for processes or tasks.
- Adjusts disk input/output priority settings.
- Changes default printer device driver assignment among LPT1 through LPT4 ports.
- Controls whether DOS applications are permitted to execute under current environment.
- Loads a user interface shell program during startup sequence.
- Conditional execution tailored for IBM PS/1 systems.
- Conditional execution designed specifically for IBM PS/2 machines.
- Suppresses output directed to standard output (STDOUT).
RTemplate:NbndT
- Activates automatic system reboot upon encountering errors.
- Permits inclusion of comments within CONFIG.SYS.
- Allocates drive letters reserved by local block device drivers.
- Defines the default Resume () prompt character.
- Enables returning from nested CONFIG.SYS processing to prior GOSUB or SWITCH points, optionally with a return code.
- Specifies the size of real mode memory.
- Allows simultaneous loading of programs in the background during CONFIG.SYS execution.
- Designates storage of the initial character of deleted files in the FAT reserved area.
- Sets video mode for FreeDOS or screen access methods for PTS-DOS.
- Manages the Scroll Lock key status on the keyboard.
- Establishes environment variables.
- Loads the command interpreter shell.
- Functions like SHELL but supports pre-allocating space for loading COMMAND.COM into HMA or UMBs, optimizing memory utilization with third-party shells.
- Defines keyboard modifier states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
- Determines whether files close upon pressing Ctrl+Alt+Del.
- Bypasses POST during reboot sequences.
- , STACKSIZE, STACKSHIGH
- Control hardware interrupt stack sizes and buffer locations across various DOS versions and derivatives to enhance system performance.
- , SYSBOOT, SYSGEN, SYSLOAD, SYSTOUMB (PTS-DOS)
- Manage boot procedures, system loading addresses, and memory allocation strategies crucial for system stability and efficiency.
- , SUPPRESSPOPUPS (DOS/OS2)
- Configure submenu blocking and error logging preferences to improve user experience and error handling processes.
- , SWITCHES, SWITCHAR
- Facilitate conditional processing and specify system switch characters to tailor command parsing behavior across DOS variants.
- , TIMESLICE (DR-DOS, PTS-DOS, OS/2)
- Adjust CONFIG.SYS timeouts and scheduler time slices to boost system responsiveness and multitasking capabilities.
- , TRACE, TRACEBUF (various OS)
- Define temporary file extensions and enable tracing features for debugging and system monitoring purposes.
- (OS/2)
- Records detailed error logs to aid troubleshooting.
UTemplate:NbndZ
- Configures DOS to allocate a specific segment as Upper Memory Block (UMB).
- Sets parameters for the UMB server.
- Defines address ranges designated as UMB RAM.
- Enables or disables verification of disk writes.
- Specifies the DOS version reported to applications.
- Configures VGA display status settings.
- Sets the current video mode parameters.
- Determines if virtual DMA usage is permitted.
- Controls deactivation of processor Virtual Mode Extensions.
- Configures VxD driver settings for Windows environments.
- Establishes system support configurations for Windows.
- Manages relocation of the Extended BIOS Data Area (XBDA).
- Allows conditional execution on XT-compatible machines.
- Enables built-in Year 2000 compliance support.
- Defines default affirmative response character in prompts (Yes/Y).
- Configures tracking of file deletions.
Issues
While it is possible for the system to start without CONFIG.SYS or AUTOEXEC.BAT files, these are crucial for complete customization and ensuring software compatibility. Corruption or absence of either file necessitates troubleshooting measures.
If 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 if COMMAND.COM is unavailable. Starting with MS-DOS 6.0/PC DOS 6.1, Novell DOS 7, and later versions, users are prompted at runtime to specify a command interpreter path if COMMAND.COM is missing or aborted. This feature permits shell replacement without requiring a reboot.
COMMAND.COM from MS-DOS 7.0 and newer is incompatible with DR-DOS, impacting dual-boot configurations where COMMAND.COM resides in C:\ root. DR-DOS versions 7.02 and above enforce SHELL directives during skip, trace, or step modes to prevent circumvention, enhancing stability during debugging and recovery operations.
CONFIG.SYS size restrictions differ among operating systems: MS-DOS/PC DOS impose limits up to approximately 64 KB, whereas DR-DOS effectively has no limit due to its line-by-line interpretation approach rather than tokenization. This method grants users greater control over driver and TSR loading sequences, allowing tailored configurations that minimize conflicts—an advantage for enterprises requiring customized environments.
For MS-DOS/PC DOS versions between 2.0 and 4.01, SHELL lines are limited to a maximum length of 31 characters. Later editions extend this allowance up to 128 characters, while DR-DOS supports up to 255 characters. Notably, directives within CONFIG.SYS do not accommodate long filenames.
Dual Booting DOS and Windows 9x
When Windows 95 is installed over a pre-existing DOS/Windows environment, the system renames CONFIG.SYS and AUTOEXEC.BAT to CONFIG.DOS and AUTOEXEC.DOS respectively. This renaming supports dual boot functionality between Windows 9x and DOS. Selecting the DOS boot option temporarily reverts these files to their original names, while the Windows 95 versions are backed up as CONFIG.W40 and AUTOEXEC.W40.
In contrast, installing Caldera DR-DOS versions 7.02 or 7.03 alongside Windows 95 retains the original CONFIG.SYS and AUTOEXEC.BAT filenames used by Windows. Instead, DR-DOS employs startup files named DCONFIG.SYS and AUTODOS7.BAT.
OS/2 / Windows NT
The OS/2 operating system depends extensively on CONFIG.SYS for setting up device drivers and environment variables before launching its graphical user interface.
Within the OS/2 subsystem of Windows NT, CONFIG.SYS functionality is replicated through registry entries rather than physical configuration files.
OS/2 includes a variety of undocumented CONFIG.SYS commands, highlighting the complexity of its configuration process.
CONFIG.SYS continues to play a crucial role in 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 CONFIG.SYS directives for Windows 98 / MS-DOS 7.10 (archived)
- Caldera's CONFIG.SYS Command Reference for DR-DOS 7 (archived)
- Guide to FreeDOS CONFIG.SYS directives
- User Manual for PTS-DOS 2000 Pro including CONFIG.SYS commands
- Unofficial German-language PTS-DOS FAQ (PTS/FAQD) (archived)
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