TIMEOUT (CONFIG.SYS directive)

CONFIG.SYS
Company type	Configuration file
Industry	Computer software
Founded	1983 (with DOS version 2.0)
Products	Configuration settings for DOS and OS/2

Adapted from the English Wikipedia article TIMEOUT (CONFIG.SYS directive), available under CC BY-SA 4.0Updated 9 August 20251 source

CONFIG.SYS serves as the chief configuration file for DOS and OS/2 operating systems. This plain text file, written in ASCII, holds setup instructions accessible to users, which are interpreted by the operating system's DOS BIOS—commonly found in IBMBIO.COM or IO.SYS—during the system startup. CONFIG.SYS was first introduced with DOS version 2.0.

Usage

The directives within CONFIG.SYS are used to configure DOS settings related to hardware devices, software applications, and memory management. Once CONFIG.SYS is 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.

This file predominantly contains name=value pairs that establish configurable 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 boot drive's root directory.

Certain DOS variants utilize alternate filenames that override CONFIG.SYS:

- Advanced editions of Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, and REAL/32 employ CCONFIG.SYS (derived from "Concurrent") or CCONFIG.INI and CCONFIG.BIN, which are managed via the utility CONFIG.EXE. These platforms offer expanded configuration capabilities beyond those of MS-DOS/PC DOS.

- For DR DOS 3.31, PalmDOS 1.0, Novell DOS 7, and DR-DOS versions 7.02 and later, a file named DCONFIG.SYS (from "DR DOS") takes priority. This arrangement supports multi-boot configurations and disk compression setups where drive letters may change after loading compression drivers.

- OpenDOS 7.01 and DR-OpenDOS 7.02 check for the presence of ODCONFIG.SYS, while certain DR-DOS 7.02+ releases also recognize DRCONFIG.SYS.

Starting with DR-DOS 6.0, users can modify default system file extensions using SYS /DR:ext, enabling multiple sets of system files to coexist within a single root directory and be selected through boot managers like LOADER. The SHELL directive supports specifying alternate AUTOEXEC.BAT files via /P[:filename.ext], and COMMAND.COM can execute files with extensions other than .BAT in this mode (a feature also available in 4DOS). Additionally, the CHAIN=filespec command allows continuation of processing in another configuration file, optionally jumping to a specific label.

From version 7.03 onward, DR-DOS adds a SYS /A switch that copies CONFIG.SYS and AUTOEXEC.BAT alongside other system files.

Comparable features exist in other DOS-like operating systems:

- FDCONFIG.SYS in FreeDOS - RXCONFIG.SYS used by RxDOS 7.24 and later - CONFIG.PTS for PTS-DOS

On Windows 95 and Windows 98 boot volumes, both CONFIG.SYS and AUTOEXEC.BAT are present but generally remain empty due to their reliance on DOS architecture.

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

Within the FlexOS operating system, CONFIG.SYS exists as 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>

:;*?!

: Marks labels within CONFIG.SYS to serve as jump destinations for , , , , and commands.
: Functions similarly to , also used to add inline comments after other directives. (Note: MS-DOS/PC DOS does not ignore inline comments inside quotation marks.)
: Introduces inline remarks in conjunction with the ; directive.
: Prompts the user with a Yes/No confirmation before executing subsequent CONFIG.SYS commands.^[1] (FreeDOS omits optional messages but supports conditional execution via boot menus.) DOS version 6.0 and later allow a comparable feature using a trailing on directives, also implemented in DR DOS.
: Forces unconditional execution of the following CONFIG.SYS command.

ATemplate:NbndC

: Determines whether programs can be interrupted upon encountering errors.
: Specifies which drives record file access timestamps within FAT reserved directory entries.
: Executes conditionally on AT-compatible hardware.
: Manages boot loading functions.
: Adjusts verbosity level of error messages.
: Changes the default serial driver assignment (COM1 through COM4) for the AUX device.
: Deactivates character devices located outside the virtual \DEV directory.
: Loads a fundamental device driver into memory.
: Produces a beep sound even if notification switches are disabled.
: Designates the operating system to boot next.
: Enables or disables enhanced Ctrl+C checking.
: Defines the quantity of disk buffers allocated.
: Similar to BUFFERS but loads buffers into High Memory Area (HMA) or Upper Memory Blocks (UMB).
, CACHETTL, CACHEFLUSH (Embedded DOS 6-XL only): Control cache block allocation and flushing intervals.
: Loads and runs programs within CONFIG.SYS in the foreground, halting processing until completion.
: Sets Caps Lock keyboard status on or off.
, CDDBUFFERS (PTS-DOS only): Specifies CD-ROM driver name along with buffer count.
: Continues processing CONFIG.SYS from another file or label.
, CLS, CODEPAGE, CMOSADDR, COLOR (PTS-DOS or OS/2 specific): Configure screen clearing, OEM codepages, CMOS RAM address, system clock types, and boot menu colors respectively.
, COM2, COM3, COM4 (DR-DOS 7.02+): Define quantity, sequence, I/O addresses, and timeouts for serial COM ports.
, COMMENT(S), COUNTRY, CPOS, CPSW (various DOS versions): Set DMA buffers, comment syntax, internationalization support, cursor positioning, and codepage switching accordingly.
CPU-related directives (CPU88, CPU86, CPU286, CPU386 etc., specific to PTS-DOS and DR-DOS): Conditional execution based on CPU model or clock speed.

DTemplate:NbndF

: Configures relocation settings for drive descriptor tables.
: Sets memory segment address used for multi-sector disk data deblocking operations.
: Activates internal debugger capabilities.
: Functions as a boot manager.
: Loads device drivers into conventional memory.
: Loads device drivers into upper memory area (equivalent to in DRDOS 5.0).
: Sets country and codepage parameters for keyboard, screen, and printer devices.
: Adjusts cache memory size allocations.
: Determines compatibility mode for floppy disk drives.
: Specifies display font settings.
: Controls loading of DOS into high or upper memory areas (similar to functionality in DRDOS 5.0).
: Loads DOS tables such as BUFFERS, FCBS, FILES, LASTDRIVE, and STACKS into upper memory segments.
: Defines an alternative directory path for DOS system files.

: Sets search paths for data files during operation.
: Specifies physical parameters for disk drives.
: Defines configuration parameters for unrecognized or unknown drives.
: Refer to .
: Enables PDUMP.nnn error logging and specifies target drive for logs.
: Displays text messages on screen output.
: Shows messages incorporating ANSI escape sequences for formatting.
: Adds extra verification steps when launching programs.
: Defines expected error or return codes following execution steps.
: Controls the A20 gate state upon program startup.
: Terminates CONFIG.SYS processing prematurely except for SWITCHES and SHELL commands.
: Sets hash table size to optimize file open operations differently than MS-DOS/PC DOS FASTOPEN driver implementation.
: Configures number of cached File Control Blocks (FCBs) that can be opened simultaneously (shares buffer with FILES in DR-DOS).
: Similar to FCBS but explicitly loads these into upper memory areas.
: Functions as a boot manager utility.
: Specifies maximum number of concurrently open files allowed by the system.
: Same as FILES directive but loads file handles into upper memory explicitly.
: Includes specified CONFIG.SYS subsections during processing (see IGNORE).

GTemplate:NbndI

: Waits for a key press or timeout, then assigns 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, returning afterward to the prior point.
: Refer to , , or for more details.
: See additional information in .
: Consult for further reference.
: For more information, see or .
: Refer to or for extended details.
: Additional guidance available in .
: Loads a terminate-and-stay-resident (TSR) program into memory following DOS data segment relocation.
: See , , or for related commands.
: Consult for further instructions.
: Refer to or as needed.
: Activates console and command line history with enhanced pattern search capabilities.
: Defines the address of the High Memory Area (HMA) segment.
: Configures energy-saving settings related to the HLT instruction.
: Enables scanning for boot sector viruses during startup.
: Loads installable file system drivers.
: Prevents execution of specified CONFIG.SYS subsections.
: Permits designated programs to operate at ring 2, allowing direct hardware input/output access.
: Incorporates directives from another menu block, simulated through GOSUB/RETURN in DR-DOS environments.
: Sets the status of the keyboard Insert mode.
: Installs a TSR program into memory.
: Loads a TSR program into upper memory, similar to HIINSTALL in DR DOS version 5.0.
: Installs a TSR program after relocating the DOS data segment.
: Adjusts PIC interrupt priorities by rotating them to assign the highest priority to a specified IRQ number.

KTemplate:NbndM

: Defines the keyboard layout in use.
: Specifies the address of the relocated keyboard buffer.
: Sets the maximum number of accessible drives allowed.
: Functions like LASTDRIVE but additionally loads the Current Directory Structure into upper memory.
: Lists paths for searching Dynamic Link Libraries (DLLs).
: Manages compatibility handling for legacy software.
: Determines the quantity of file locks permitted.
: Configures boot logo display utilizing the PTS-DOS boot manager.
: Sets parameters including count, logical order, I/O address, and timeout for LPT1 device configuration.
: Applies similar settings for LPT2 device configuration.
: Configures LPT3 device with equivalent parameters.
: Enables and sets up LPT4 device similarly to other parallel ports.
: Defines wait duration before increasing process priority levels.
: Establishes strategy for memory allocation management.
: Specifies default DOS memory allocation strategy preferences.
: Defines menu options available within the system interface.
: Sets color schemes for menus, with emulation possible in DR-DOS environments.
: Assigns default menu block, supporting emulation in DR-DOS systems.
: Specifies menu block definitions, emulatable under DR-DOS conditions.
: Enables or disables operations involving multitrack disks.
: Continues CONFIG.SYS processing from an external file, optionally loading a device driver and setting the %NEWFILE% environment variable.

NTemplate:NbndQ

: Determines default "No" character used in Yes/No/Resume prompts.
: Controls keyboard [[Num Lock|Template:Keypress]] state, toggling it on or off at startup.
: Executes commands conditionally based on previous error codes, such as .
: Displays text and awaits key press, functioning similarly to and commands in DR-DOS systems.
: Requires user confirmation when errors occur during CONFIG.SYS processing.
: Configures buffer sizes for LPT1 through LPT3 devices.
: Enables conditional execution specifically on PC-compatible hardware platforms.
: Sets priority levels within the system scheduler.
: Defines priority settings for disk input/output related programs.
: Changes default printer device driver assignment among LPT1 to LPT4 ports.
: Specifies whether DOS applications are permitted to run under current configuration settings.
: Loads a user interface component during system startup or operation.
: Allows conditional execution on IBM PS/1-compatible machines exclusively.
: Enables conditional command execution tailored for IBM PS/2-compatible systems only.
: Suppresses output directed to standard output (STDOUT).

RTemplate:NbndT

: Activates automatic system reboot upon encountering errors.
: Permits inclusion of comments within CONFIG.SYS files.
: Allocates a drive letter specifically for local block device drivers.
: Establishes the default Resume () response character in Yes/No/Resume prompts.
: Returns control from nested CONFIG.SYS processing to the prior GOSUB or SWITCH command, optionally passing back a return code.
: Sets the size of real mode memory available.
: Initiates concurrent loading of programs in CONFIG.SYS, functioning similarly to INSTALL in DOS environments.
: Saves the initial character of deleted files within the FAT reserved area.
: Determines video mode under FreeDOS or selects screen access methods for PTS-DOS.
: Configures keyboard Scroll Lock status to be enabled or disabled.
: Establishes environment variables for the system session.
: Loads the command interpreter shell into memory.
: Functions like SHELL but allows pre-allocating space for relocating COMMAND.COM into HMA or UMBs, reducing fragmentation—beneficial when using third-party shells.
: Defines keyboard modifier states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
: Indicates whether files should close automatically upon pressing Ctrl+Alt+Del.
: Bypasses POST during system reboot sequences.
, STACKSIZE, STACKSHIGH: Manage hardware interrupt stack sizes and buffer locations across various DOS versions, with certain parameters ignored in specific releases for compatibility or memory efficiency.
, SYSBOOT, SYSGEN, SYSLOAD: Directives unique to PTS-DOS that handle boot manager configurations, alternative operating system boot files, DOS memory layout adjustments, and system loading from non-standard disk areas.
, SUPPRESSPOPUPS: Defines submenu blocks (from DOS 6.0 onward) and specifies drives for logging errors instead of displaying popups (as in OS/2).
, THREADS, TIMESLICE: OS/2-exclusive settings that determine swap file paths, concurrent thread counts, and scheduler time slice durations.
, SWITCHES: Enables conditional nested CONFIG.SYS processing (introduced in DR DOS 6.0) and includes special options available since DOS 4.0.
, TMPEXT: Sets the default switch character ('/' or '-') and temporary file extension specific to PTS-DOS environments.
, TRACEBUF: Controls global or local enabling of CONFIG.SYS single-step execution (supported in DR-DOS 7.02+ and PTS-DOS), alongside log buffer size configuration in OS/2 systems.
, SXFAKEHWFPU: OS/2 directives aimed at error logging and correcting FPU rounding issues.

This segment details configuration commands applicable to various DOS derivatives such as DR-DOS, PTS-DOS, OS/2, Novell DOS, and FreeDOS. These directives are essential for managing environment variables, error recovery mechanisms, boot procedures, memory allocation, device handling, and command interpreter operations—crucial elements for optimizing legacy systems within professional computing infrastructures.

UTemplate:NbndZ

: Designates a particular memory segment for use as Upper Memory Block (UMB) space in DOS management.
: Specifies which UMB server is utilized by the system.
: Defines address ranges allocated as UMB RAM areas.
: Toggles disk write verification on or off to ensure data integrity.
: Sets the reported DOS version presented to applications.
: Controls VGA display enablement status.
: Indicates the current video mode setting in use.
: Determines whether virtual DMA functionality is permitted.
: Enables or disables the processor’s Virtual Mode Extensions feature.
: Configures VxD driver parameters for Windows environments.
: Adjusts system settings supporting Windows operation.
: Manages relocation of the Extended BIOS Data Area (EBDA).
: Allows conditional execution depending on XT-compatible hardware detection.
: Configures built-in Year 2000 (Y2K) compliance support mechanisms.
: Specifies the default affirmative response character used in prompts.
: Controls tracking of file deletions within the system.

Issues

The system is capable of booting even if CONFIG.SYS is absent or corrupted; however, both this file and AUTOEXEC.BAT are crucial for comprehensive operating system customization and meeting software prerequisites. If either file becomes damaged, troubleshooting becomes necessary.

When the SHELL directive is missing from CONFIG.SYS or if the file itself is corrupted or unavailable, DOS attempts to locate COMMAND.COM in the root directory of the boot drive. Versions of DOS prior to 6.0 will fail to boot without this file, whereas MS-DOS 6.0, PC DOS 6.1, and Novell DOS 7 and later versions prompt users to enter a path for the command processor, enabling limited shell replacement during runtime without requiring a reboot.

Because MS-DOS 7.0 and later versions’ COMMAND.COM are incompatible with DR-DOS, versions 7.02 and above of DR-DOS prevent bypassing SHELL directives when operating in skip, trace, or step modes to improve boot stability. If no valid shell is found during CONFIG.SYS skipping, users are asked to specify one.

The maximum size allowed for CONFIG.SYS differs: MS-DOS and PC DOS generally restrict it to a few kilobytes (up to 64 KB in newer releases), while DR-DOS imposes no practical size limitation, processing directives sequentially rather than compiling them all at once. This approach offers granular control over loading device drivers and TSR programs, helping resolve conflicts and enabling conditional execution based on hardware detection or error conditions—features particularly beneficial in complex corporate settings.

In MS-DOS/PC DOS versions from 2.0 through 4.01, SHELL directives were limited to 31 characters; later editions expanded this limit to 128 characters, with DR-DOS supporting up to 255 characters. Notably, CONFIG.SYS directives do not support long filenames.

Dual booting DOS and Windows 9x

When Windows 95 is installed on a system already running DOS or an earlier Windows version, the original CONFIG.SYS and AUTOEXEC.BAT files are renamed CONFIG.DOS and AUTOEXEC.DOS respectively. This renaming facilitates switching between Windows 9x and DOS environments. During a DOS boot, these files are temporarily reverted to CONFIG.SYS and AUTOEXEC.BAT. Additionally, Windows 95’s versions are backed up as CONFIG.W40 and AUTOEXEC.W40.

If Caldera DR-DOS 7.02 or 7.03 is installed alongside Windows 95, the Windows configuration files remain unaltered. Instead, DR-DOS employs its own startup scripts named DCONFIG.SYS and AUTODOS7.BAT.

OS/2 / Windows NT

OS/2 relies heavily on CONFIG.SYS for its configuration settings, driver loading, and environment preparation before its graphical user interface initializes.

Within the OS/2 subsystem embedded in Windows NT, what appeared as CONFIG.SYS to OS/2 applications was actually stored within the system registry rather than as a physical file.

There exist numerous undocumented CONFIG.SYS commands specific to OS/2.

CONFIG.SYS continues to be utilized by OS/2-based systems such as eComStation and ArcaOS.

External links

AUTOEXEC.BAT and CONFIG.SYS details at Computer Hope
Overview of Windows 98 / MS-DOS 7.10 CONFIG.SYS commands (archived)
Caldera’s CONFIG.SYS Command Reference (archived) – summary of DR-DOS 7 directives
FreeDOS guide to CONFIG.SYS directives
PTS-DOS 2000 Pro User Manual covering CONFIG.SYS commands (archived)
Unofficial German FAQ for PTS-DOS including CONFIG.SYS details (archived)

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