SYSTOUMB (CONFIG.SYS directive)
| Company type | Configuration file |
|---|---|
| Industry | Computer software |
| Founded | 1983 |
| Products | System configuration commands for DOS and OS/2 |
CONFIG.SYS serves as the essential configuration file for DOS and OS/2 operating system platforms. This plain text file, written in ASCII, holds configuration commands interpreted by the system's DOS BIOS—usually found in IBMBIO.COM or IO.SYS—during the startup process. CONFIG.SYS was first introduced with DOS version 2.0.
Usage
The directives within CONFIG.SYS set up DOS to manage hardware devices, software applications, and memory allocation. Once CONFIG.SYS has been processed, DOS launches the command interpreter specified by the SHELL directive; if none is specified, it defaults to COMMAND.COM, which subsequently runs AUTOEXEC.BAT.
CONFIG.SYS primarily consists of name=value pairs that adjust system parameters or load components such as device drivers and terminate-and-stay-resident programs (TSRs). In DOS environments, this file is located in the root directory of the boot drive.
Certain DOS variants prioritize alternative configuration filenames over CONFIG.SYS:
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 operating systems support enhanced configuration options stored in binary files like CCONFIG.BIN, commonly edited through CONFIG.EXE.
In DR DOS version 3.31 and later—including PalmDOS 1.0, Novell DOS 7, OpenDOS 7.01, and DR-DOS 7.02+—the file DCONFIG.SYS supersedes CONFIG.SYS. This arrangement facilitates multi-boot environments and disk compression setups where drive letters may shift after compression drivers load. Additionally, OpenDOS 7.01 and DR-OpenDOS 7.02 look for ODCONFIG.SYS, while some iterations of DR-DOS 7.02+ also recognize DRCONFIG.SYS.
Starting with DR DOS 6.0, the SYS /DR:ext command allows customization of default file extensions, enabling multiple system file sets within a single directory selectable via boot managers like LOADER. The SHELL command supports specifying alternate AUTOEXEC.BAT files using /P[:filename.ext], which permits extensions other than ".BAT" (a feature also supported by 4DOS). The CHAIN=filespec directive in CONFIG.SYS enables continuation of configuration through additional files, with optional label jumps (CHAIN=filespec,label) introduced in DR-DOS 7.02+. From version 7.03 onward, DR-DOS includes a SYS /A option to copy CONFIG.SYS and AUTOEXEC.BAT alongside system files.
For other systems: FreeDOS employs FDCONFIG.SYS, RxDOS from version 7.24 uses RXCONFIG.SYS, and PTS-DOS relies on CONFIG.PTS.
While both CONFIG.SYS and AUTOEXEC.BAT are present in the root directories of Windows 95 and 98 boot drives, they are generally left empty. In contrast, Windows Me does not process CONFIG.SYS during startup; instead, it retrieves environment variables from the Windows Registry at 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 at boot time.
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>
:;*?!
- Establishes labels within CONFIG.SYS to serve as jump targets for , , , and commands.
- Functions similarly to , but also facilitates in-line comments following other directives. (Note: MS-DOS/PC DOS does not ignore inline comments enclosed in double quotes.)
- Introduces in-line comments in conjunction with the ; directive.
- Prompts a Yes/No confirmation before executing subsequent CONFIG.SYS commands. (FreeDOS lacks customizable messages but supports conditional execution via boot menus.) DOS 6.0 and later provide a comparable feature by appending after commands.
- Executes the next CONFIG.SYS directive without any condition.
ATemplate:NbndC
- Determines whether programs can be aborted upon encountering errors.
- Configures drives to maintain file access date stamps within FAT file systems.
- Enables conditional execution on AT-compatible hardware.
- Boot manager utility.
- Adjusts verbosity of error messages.
- Alters the default serial device driver assigned to AUX.
- Disables character devices located outside the phantom \DEV directory.
- Loads a base device driver into memory.
- Forces a beep sound even if notifications are disabled.
- Defines which operating system to boot next.
- Enables or disables extended Ctrl+C checking.
- Sets the quantity of disk buffers allocated.
- Similar to BUFFERS but explicitly loads buffers into HMA or UMB memory areas.
- Maximum count of cache blocks allocated from the system pool.
- Maximum duration before unused cache blocks return to the system pool.
- Maximum time before dirty cache blocks are flushed to disk.
- Loads programs specified in CONFIG.SYS and executes them in the foreground, pausing further processing until completion.
- Specifies the status of the keyboard Caps Lock key.
- Indicates the hardware driver name for CD-ROM devices.
- Defines the number of buffers used for CD-ROM access.
- Continues processing CONFIG.SYS commands from a new file, optionally starting at a specified label (available since DR-DOS 7.02).
- Specifies the type of system clock used.
- Clears the screen display.
- Defines OEM codepage(s).
- Specifies CMOS RAM address parameters.
- Sets colors for boot menu displays.
- , ;, ;, ;
- Establishes count, logical sequence, I/O address, and timeout values for COM ports (applicable from DR-DOS 7.02 onwards).
- Defines buffer parameters for DMA operations.
- Redefines lead-in characters for inline comments or ignores the remainder of the line (ignored under DR-DOS 7.02+, Embedded DOS, PTS-DOS).
- Activates international support configuration settings.
- Sets cursor position on configuration menus.
- Supports codepage switching functionality (ignored under DR-DOS 7.02+).
- CPU-related directives (CPU, CPU88, CPU88+, CPU86, CPU86+, CPU286, CPU286+, CPU386, CPU386+, CPU486, CPU486+) enable conditional execution based on processor type (PTS-DOS or DR-DOS).
DTemplate:NbndF
- Configures relocation settings for drive descriptor tables.
- Specifies memory segment address used for multi-sector disk data deblocking processes.
- Enables support for an internal debugger.
- Boot manager utility.
- Loads device drivers into memory space.
- Loads device drivers into upper memory areas (equivalent to in DRDOS 5.0).
- Configures country and codepage information for keyboard, screen, and printer devices (KBD, SCR, LPT).
- Sets cache memory size parameters.
- Defines floppy disk compatibility mode settings.
- Specifies font used for display output.
- Manages loading DOS into high memory area and utilization of upper memory blocks.
- Loads DOS tables such as BUFFERS and FILES into upper memory space.
- Sets alternative search paths for DOS files.
- Defines search paths for data files used by DOS.
- Specifies physical parameters associated with disk drives.
- Defines parameters related to unrecognized drives.
- See .
- Enables error logging and specifies drive location for
PDUMP.nnnlogs. - Displays a message on screen output devices.
- Displays messages including ANSI Escape sequences support.
- Adds extra verification steps when launching programs.
- Defines expected error or return codes for subsequent commands.
- Sets A20 gate state when initiating a program execution.
- Terminates CONFIG.SYS processing before reaching file end (except under certain directive conditions).
- Configures hash table size to accelerate file opening operations (distinct from MS-DOS FASTOPEN driver).
- Specifies maximum number of cached File Control Blocks open simultaneously (shares buffer with FILES under DR-DOS).
- Identical to FCBS but loads into upper memory areas instead of conventional memory.
- Boot manager utility function.
- Defines maximum number of files that can be opened concurrently.
- Same as FILES but loads file handles into upper memory space.
- Includes execution of designated CONFIG.SYS subsections (refer to IGNORE).
GTemplate:NbndI
- Waits for a key press or timeout, then sets the error level to the ASCII value of the key or 1024 if timed out.
- Resumes CONFIG.SYS execution at a designated label.
- Continues nested CONFIG.SYS processing at a specified label, then returns to the previous point.
- Refer to , , or for more details.
- Consult for additional information.
- See for related usage.
- Refer to or for further guidance.
- See or for relevant instructions.
- Consult for context.
- Loads TSR programs into memory following DOS data segment relocation. Compare with INSTCDEX.
- Refer to , , or for more on this topic.
- See for related commands.
- Consult or for further details.
- Activates console and command line history buffers, plus enhanced pattern search capabilities.
- Defines the HMA segment address.
- Configures energy-saving mode using HLT instruction.
- Specifies system scan for boot sector viruses.
- Loads installable file system drivers.
- Prevents execution of certain CONFIG.SYS subsections.
- Specifies programs permitted to execute at ring 2 with direct hardware I/O privileges.
- Incorporates directives from another menu section (simulated via GOSUB/RETURN in DR-DOS).
- Sets keyboard Insert mode status.
- Loads TSR programs into memory.
- Installs TSR programs into upper memory (equivalent to HIINSTALL in DR DOS 5.0).
- Installs TSR program after relocating DOS data segment (compare INSTCDEX).
- Rotates PIC interrupt priorities, assigning highest priority to the chosen IRQ number.
KTemplate:NbndM
- Defines the keyboard layout configuration.
- Sets the address of the relocated keyboard buffer.
- Determines the maximum number of accessible drives.
- Functions like LASTDRIVE but loads Current Directory Structure (CDS) into upper memory area.
- Specifies search paths used to locate DLL files.
- Controls behavior for legacy application compatibility.
- Sets the maximum number of allowable file locks.
- Configures support for boot logos and startup messages display.
- Establishes count, logical sequence, I/O port address, and timeout parameters for LPT1 device.
- Applies same settings as LPT1 for LPT2 device configuration.
- Mirrors LPT1 settings for LPT3 device setup.
- Enables and configures LPT4 printer port device.
- Sets delay before elevating process priority level.
- Defines strategy for memory allocation management.
- Specifies default method for DOS memory allocation operations.
- Declares menu options available in CONFIG.SYS menus.
- Sets menu color schemes (can be emulated via ECHO command in DR-DOS).
- Defines default menu block selection (emulatable through TIMEOUT and SWITCH commands in DR-DOS).
- Creates a menu block structure (emulated via ECHO and SWITCH commands in DR-DOS).
- Enables support for multitrack disk drive operations.
- Continues CONFIG.SYS processing from a new file after loading a device driver and setting %NEWFILE% environment variable.
NTemplate:NbndQ
- Sets default response character for No () in Yes/No/Resume prompts.
- Controls keyboard [[Num Lock|Template:Keypress]] state (on or off).
- Enables conditional execution based on previous error code, e.g., .
- Displays a message and waits for key input, similar to and commands under DR-DOS environment.
- Requires user confirmation of errors occurring within CONFIG.SYS processing.
- Configures buffer size parameters for LPT ports output buffering.
- Enables conditional execution on PC-compatible systems only.
- Adjusts scheduler priority settings.
- Sets disk I/O program priority levels accordingly.
- Changes default printer device driver assignment among LPT1 through LPT4 ports.
- Specifies whether DOS applications are allowed to execute in the current environment.
- Loads user interface shell program during startup sequence.
- Enables conditional execution on IBM PS/1-compatible machines specifically.
- Allows conditional execution on IBM PS/2-compatible systems only.
- Suppresses standard output display on STDOUT stream.
RTemplate:NbndT
- Activates automatic system reboot upon encountering errors.
- Permits inclusion of comments within CONFIG.SYS files.
- Allocates a drive letter exclusively for local block device usage.
- Establishes the default Resume () response character in Yes/No/Resume prompts.
- Returns nested CONFIG.SYS processing control to the previous GOSUB or SWITCH command, optionally passing a return code.
- Specifies the size of real mode memory available.
- Enables concurrent loading of programs in CONFIG.SYS, functioning similarly to CALL or INSTALL commands found in other DOS variants.
- Saves the initial character of deleted files within the reserved FAT area.
- Determines video display mode (FreeDOS) or screen access methods (PTS-DOS).
- Sets the state of the keyboard Scroll Lock key to either on or off.
- Defines environment variables for system use.
- Loads the command interpreter shell.
- Functions like SHELL but includes a SIZE parameter to pre-load COMMAND.COM into HMA or UMBs, optimizing memory usage particularly with third-party shells.
- Specifies keyboard modifier key states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
- Controls whether files are closed upon pressing Ctrl+Alt+Del.
- Determines if POST is bypassed during reboot sequences.
- , STACKSIZE, STACKSHIGH
- Set hardware interrupt stack sizes and buffer locations across multiple DOS versions.
- , SYSBOOT, SYSGEN, SYSLOAD, SYSTOUMB
- PTS-DOS specific commands managing boot procedures, memory management, and system loading paths.
- , SUBMENU, SUPPRESSPOPUPS, SWAPPATH
- CONFIG.SYS options unique to certain OSes for internal functions including menu blocks, error message suppression, and swap file directory specification.
- , SWITCHES, SWITCHAR
- Manage nested command processing flow, special options handling, and define default switch characters across DOS platforms.
- , THREADS, TIMESLICE
- OS/2 settings addressing hardware fixes, thread management, and scheduler time slice allocation.
- , TRACE, TRACEBUF
- Configure timeouts and single-step execution logging within CONFIG.SYS for DR-DOS, PTS-DOS, and OS/2 environments.
- , TRAPDUMP
- Designates temporary file extensions (PTS-DOS) and detailed error log settings (OS/2).
UTemplate:NbndZ
- Assigns a specific segment for use as Upper Memory Block (UMB) memory under DOS management.
- Specifies which UMB server is utilized by the system.
- Defines address ranges allocated as UMB RAM regions.
- Enables or disables verification of disk writes during operations.
- Sets the DOS version number reported to applications.
- Configures VGA display status parameters.
- Indicates current video mode similar to FreeDOS's SCREEN setting.
- Determines whether virtual DMA functionality is permitted.
- Controls processor Virtual Mode Extensions (VME) settings.
- Configures Windows VxD driver parameters.
- Sets system support options related to Windows environments.
- Manages relocation of the Extended BIOS Data Area (XBDA).
- Executes conditionally on XT-compatible hardware systems.
- Enables built-in Year 2000 compliance features.
- Defines default affirmative character used in prompt responses.
- Configures tracking behavior for file deletions.
Issues
Although booting can occur without CONFIG.SYS or AUTOEXEC.BAT files present, these configuration files are crucial for comprehensive DOS customization and meeting software requirements. Any corruption or damage necessitates diagnostic efforts.
If a SHELL directive is absent from CONFIG.SYS or if the file itself is missing or corrupted, DOS attempts to locate COMMAND.COM in the root directory of the boot drive. Versions prior to 6.0 cannot boot without this file. MS-DOS 6.0/PC DOS 6.1 and Novell DOS 7 and later prompt users to specify an alternate command processor path if COMMAND.COM is not found, allowing limited shell functionality without requiring a reboot.
COMMAND.COM from MS-DOS 7.0 and newer is incompatible with DR-DOS systems. Consequently, DR-DOS versions 7.02 and above enforce SHELL directives during skip and trace modes to prevent conflicts in dual-boot configurations. If no valid shell is detected while skipping CONFIG.SYS processing, users are prompted to provide one before COMMAND.COM loads.
The maximum size of CONFIG.SYS is restricted to a few kilobytes under MS-DOS/PC DOS (with recent versions supporting up to 64 KB), whereas DR-DOS imposes no effective size limit because it interprets lines sequentially rather than tokenizing them. This approach allows DR-DOS to dynamically manage driver and TSR loading order using DEVICE and INSTALL commands, enhancing troubleshooting capabilities and facilitating integration of temporary external utilities.
Limits on SHELL command line length differ by version: MS-DOS/PC DOS releases from 2.0 through 4.01 allow up to 31 characters, later versions extend this limit to 128 characters, while DR-DOS accepts lines up to 255 characters long. It should be noted that CONFIG.SYS directives do not support long filenames.
Dual booting DOS and Windows 9x
During the installation of Windows 95 over a pre-existing DOS or Windows environment, the system renames CONFIG.SYS and AUTOEXEC.BAT to CONFIG.DOS and AUTOEXEC.DOS, enabling a dual boot setup between Windows 9x and DOS. When the system boots into DOS mode, these files are temporarily restored to their original names, CONFIG.SYS and AUTOEXEC.BAT. Meanwhile, backup copies of the Windows 95 versions are preserved under the names CONFIG.W40 and AUTOEXEC.W40.
In the case of installing Caldera DR-DOS 7.02 or 7.03 alongside Windows 95, the Windows CONFIG.SYS and AUTOEXEC.BAT filenames remain unchanged, whereas DR-DOS employs DCONFIG.SYS and AUTODOS7.BAT as its startup configuration files.
OS/2 / Windows NT
OS/2 depends extensively on CONFIG.SYS to set up drivers and environment variables prior to launching its graphical user interface.
Within the OS/2 subsystem on Windows NT, CONFIG.SYS configurations are stored in the system registry instead of being maintained as a physical file.
There exists a variety of undocumented CONFIG.SYS commands specifically for OS/2 setups.
CONFIG.SYS continues to play a vital role in OS/2-based operating 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
- Documentation for FreeDOS CONFIG.SYS directives
- PTS-DOS 2000 Pro User Manual featuring CONFIG.SYS directives (archived)
- Unofficial German PTS-DOS FAQ including CONFIG.SYS directives (archived)