IRQPRIORITY (CONFIG.SYS directive)
| Company type | Configuration file |
|---|---|
| Industry | Operating systems |
| Founded | 1983 |
| Products | Configuration commands for DOS and OS/2 |
CONFIG.SYS serves as the chief configuration file for DOS and OS/2 operating systems. This plain text ASCII file holds configuration commands accessible to users, which are interpreted by the operating system's DOS BIOS (commonly within IBMBIO.COM or IO.SYS) during the startup process. The CONFIG.SYS file was first introduced with DOS version 2.0.
Usage
Within CONFIG.SYS, directives set up DOS for hardware devices, software applications, and memory handling. Once CONFIG.SYS is processed, DOS initiates the command interpreter specified in the SHELL directive; if none is specified, it defaults to COMMAND.COM, which subsequently executes AUTOEXEC.BAT.
The file primarily employs name=value pairs to establish adjustable parameters or to load components such as device drivers and terminate-and-stay-resident programs (TSRs). CONFIG.SYS is located in the boot drive’s root directory.
This filename is also utilized by (DCP), an MS-DOS variant developed by the former East German company VEB Robotron.
Certain DOS versions check for alternative configuration filenames that take precedence over CONFIG.SYS:
- CCONFIG.SYS or CCONFIG.INI are used by Concurrent DOS 386, Concurrent DOS XM, Multiuser DOS, and REAL/32. These store settings in a binary file named CCONFIG.BIN, managed through CONFIG.EXE.
- In DR DOS 3.31 and later editions (including PalmDOS, Novell DOS, and OpenDOS), DCONFIG.SYS is prioritized, facilitating multi-boot configurations and disk compression support.
- OpenDOS 7.01 and DR-OpenDOS 7.02 also recognize ODCONFIG.SYS, while some DR-DOS releases search for DRCONFIG.SYS.
- Starting with DR-DOS 6.0, users can modify default configuration file extensions using SYS /DR:ext, allowing multiple system file sets to coexist and be selected via boot loaders like LOADER.
- The SHELL command permits specifying alternative AUTOEXEC.BAT files using /P[:filename.ext], and COMMAND.COM accepts extensions beyond .BAT; these features are also supported by 4DOS.
- From DR-DOS 6.0 onward, the CHAIN=filespec directive enables continuation of configuration processing from another file, with label jumping supported in DR-DOS 7.02 and newer.
- DR-DOS 7.03 and later add a SYS /A option to duplicate CONFIG.SYS and AUTOEXEC.BAT along with system files.
Other operating systems use similar files: FDCONFIG.SYS for FreeDOS, RXCONFIG.SYS for RxDOS, and CONFIG.PTS for PTS-DOS.
On Windows 95 and 98 boot volumes, both CONFIG.SYS and AUTOEXEC.BAT are present but often remain unused. Windows Me does not parse CONFIG.SYS during startup; instead, environment variables are loaded from the Windows Registry at:
- HKLM\System\CurrentControlSet\Control\SessionManager\Environment
In contrast, under FlexOS, CONFIG.SYS is a binary format file that specifies resource managers and device drivers to be loaded during boot.
Example
Sample CONFIG.SYS content 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 that serve as jump points for , , , , and directives.
- Functions similarly to , also used to add inline comments following other commands. (Note: MS-DOS/PC DOS does not ignore inline comments enclosed in double quotes.)
- Introduces inline comments as an alternative to ;.
- Prompts a Yes/No confirmation with an optional message before executing subsequent CONFIG.SYS commands. (FreeDOS supports conditional execution with boot menus but lacks optional messages.)
- Forces unconditional execution of the next CONFIG.SYS directive.
ATemplate:NbndC
- Determines whether programs can be terminated upon encountering errors.
- Sets which drives record file access timestamps in FAT file systems.
- Enables conditional execution on AT-compatible hardware.
- Manages boot process.
- Adjusts the detail level of error message output.
- Changes the default serial port driver (from COM1: to COM4:) for the AUX: device.
- Prevents access to character devices outside the virtual \DEV directory.
- Loads the fundamental device driver into system memory.
- Produces a brief beep sound even if audible alerts are disabled.
- Defines which operating system to boot next.
- Enables or disables extended Ctrl+C monitoring.
- Specifies how many disk buffers should be allocated.
- Similar to BUFFERS but loads buffers into HMA or UMB areas.
- , CACHETTL, CACHEFLUSH (Embedded DOS 6-XL only)
- Manage cache block allocation and timing for cache flushing.
- Loads programs listed in CONFIG.SYS and pauses processing until they finish.
- Sets the Caps Lock keyboard state.
- , CDDBUFFERS (PTS-DOS only)
- Defines CD-ROM driver name and buffer quantity.
- Continues processing CONFIG.SYS from a new file or label.
- , CLS, CODEPAGE (various OS-specific)
- Configure system clock settings, screen clearing behavior, and OEM codepages.
- , COLOR (PTS-DOS only)
- Sets CMOS RAM address and colors for the boot menu interface.
- , COM2, COM3, COM4 (DR-DOS 7.02+)
- Specify count, sequence, I/O addresses, and timeouts for serial ports.
- , COMMENT(S) (various DOS versions)
- Define DMA buffer settings and modify inline-comment characters or ignore lines entirely.
- , CPOS, CPSW, CPU*, CPU386+ (various OS versions)
- Enable international language support, cursor positioning, codepage switching, RAM refresh rates, and conditional execution based on CPU model.
DTemplate:NbndF
- Controls relocation of drive descriptor tables within memory.
- Sets the memory segment limit for multi-sector disk data deblocking operations.
- Activates internal debugger functionality.
- Handles boot management tasks.
- Loads a device driver into memory space.
- Loads a device driver into upper memory blocks (equivalent to in DRDOS 5.0).
- Configures country-specific and codepage settings for keyboard, screen, and printer devices.
- Adjusts cache memory size allocations.
- Defines floppy disk compatibility modes.
- Specifies the font used for display output.
- Determines whether DOS loads into high memory area and/or upper memory blocks.
- Controls loading of DOS tables such as BUFFERS, FCBS, FILES, LASTDRIVE, STACKS into upper memory regions.
- Sets an alternative directory path for DOS system files.
- Specifies search paths for data files used by the system.
- Defines physical characteristics of disk drives connected to the system.
- Establishes parameters for drives that are not automatically recognized by the system.
- Refer to for additional details.
- Enables error logging via PDUMP.nnn files and designates the drive for storing these logs.
- Presents a message on screen during processing.
- Displays messages incorporating ANSI escape codes for formatting.
- Adds extra verification steps when launching programs.
- Assigns an expected error or return code value for program execution monitoring.
- Sets the A20 gate status when initiating a program run.
- Terminates CONFIG.SYS processing prematurely except when SWITCHES or SHELL directives require continuation under certain conditions.
- Configures hash table size to optimize file open operations differently from MS-DOS/PC DOS FASTOPEN drivers.
- Determines the number of cached File Control Blocks (FCBs) allowed open simultaneously (note that FCBS and FILES share buffers under DR-DOS).
- Functions like FCBS but explicitly loads FCBs into upper memory areas.
- Handles boot management functions again here as well.
- Sets the maximum number of files that can be open concurrently.
- Similar to FILES but explicitly loads file handles into upper memory segments.
- Executes specified CONFIG.SYS subsections as defined by IGNORE.
GTemplate:NbndI
- Waits for a key press or timeout, setting the error level to the ASCII code or 1024 if timed out.
- Resumes CONFIG.SYS processing at the indicated label.
- Continues nested CONFIG.SYS processing at a specified label and then returns to the previous position.
- Refer to , , or for details.
- See also .
- Refer to .
- Consult or for more information.
- Check or for related commands.
- See .
- Loads a TSR program after relocating the DOS data segment; compare with INSTCDEX.
- See , , or for additional context.
- Refer to .
- Consult or for further details.
- Activates console and command line history buffer with enhanced pattern searching capabilities.
- Defines the address of the High Memory Area (HMA) segment.
- Configures energy-saving mode using the HLT instruction.
- Enables scanning of boot sector viruses.
- Loads installable file system drivers.
- Prevents execution of designated CONFIG.SYS subsections.
- Permits specified programs to operate at ring 2, allowing direct hardware I/O access.
- Incorporates directives from another menu block (emulated by GOSUB/RETURN in DR-DOS).
- Sets the keyboard Insert mode state.
- Installs TSR programs into conventional memory.
- Installs TSR programs into upper memory, equivalent to HIINSTALL in DR-DOS 5.0.
- Loads TSR programs after relocating DOS data segment; see also INSTCDEX.
- Adjusts PIC interrupt priorities to elevate a specific IRQ number.
KTemplate:NbndM
- Defines the keyboard layout configuration.
- Specifies the address of the relocated keyboard buffer area.
- Sets the maximum number of accessible drives.
- Functions like LASTDRIVE but loads Current Directory Structure (CDS) into upper memory.
- Lists directories searched for DLL files.
- Manages backward compatibility for legacy software.
- Determines the number of file locks allowed.
- Configures whether a boot logo is displayed during startup.
- Specifies count, logical order, I/O port address, and timeout settings for LPT1 device.
- Provides similar configuration options for LPT2 device.
- Configures LPT3 device with comparable parameters.
- Enables and sets up LPT4 device support.
- Defines delay before increasing process priority level.
- Controls memory allocation strategy used by DOS.
- Sets default DOS memory allocation method.
- Specifies menu options available at boot time.
- Defines menu color schemes (emulatable under DR-DOS).
- Sets default menu block selection (emulatable under DR-DOS).
- Defines individual menu blocks (emulatable under DR-DOS).
- Enables or disables multitrack disk operations support.
- Continues CONFIG.SYS processing from another file, optionally loading a device driver and setting %NEWFILE% environment variable.
NTemplate:NbndQ
- Sets default "No" character in Yes/No/Resume prompts.
- Controls keyboard [[Num Lock|Template:Keypress]] status on startup.
- Enables conditional execution based on previous error code, such as .
- Displays a message and waits for keypress, similar to DR-DOS's and .
- Requires user confirmation when errors occur in CONFIG.SYS processing.
- Sets buffer sizes for LPT ports operation.
- Enables conditional execution on PC-compatible systems only.
- Adjusts scheduler priority settings.
- Sets priority level for disk I/O programs specifically.
- Changes the default printer device driver (e.g., switching from LPT1 to LPT3 or LPT4).
- Determines whether DOS applications are permitted to run.
- Loads a user interface shell environment.
- Enables conditional execution on IBM PS/1-compatible machines only.
- Enables conditional execution on IBM PS/2-compatible machines only.
- Suppresses output sent to STDOUT.
RTemplate:NbndT
- Activates automatic system reboot upon encountering errors.
- Permits inclusion of comments within CONFIG.SYS files.
- Allocates drive letters for local block device drivers to prevent conflicts.
- Establishes the default character for Resume () prompts in Yes/No/Resume queries.
- Facilitates returning from nested CONFIG.SYS processing to the preceding GOSUB or SWITCH instruction, optionally passing a return value.
- Specifies the size of real mode memory available.
- Functions similarly to CALL by loading programs asynchronously during CONFIG.SYS execution.
- Saves the initial character of deleted files within the FAT reserved section.
- Determines video mode settings in FreeDOS or screen access methods in PTS-DOS.
- Controls the status of the Scroll Lock key on the keyboard.
- Assigns environment variable values.
- Loads the command interpreter shell.
- Comparable to SHELL but includes a size parameter allowing COMMAND.COM to load into HMA or UMBs, enhancing memory efficiency with third-party shells.
- Sets keyboard modifier states such as Shift Lock, Num Lock, Scroll Lock, and Insert mode.
- Defines whether files close automatically upon pressing Ctrl+Alt+Del.
- Bypasses POST during system reboot.
- , STACKSIZE, STACKSHIGH
- Configure interrupt stack sizes and buffer placements across different DOS versions, improving system stability and performance.
- , SYSBOOT, SYSGEN, SYSLOAD
- PTS-DOS-specific commands managing boot configurations, memory layout, and alternative operating system loading options for flexible setups.
- , SWITCH
- Enables conditional nested processing and menu-driven CONFIG.SYS structures in DOS variants supporting advanced scripting capabilities.
- , SWAPPATH
- OS/2 directives specifying paths for error logs and swap file locations to enhance system management.
- , SWITCHES
- Define default command switch characters ('/' or '-') and special option flags across DOS platforms to ensure consistent command-line operations.
- , TRACE
- Manages CONFIG.SYS execution timing and activates single-step debugging to increase reliability during startup processes.
- Additional instructions such as THREADS and TIMESLICE configure multitasking parameters under OS/2 environments to optimize resource distribution.
This segment details essential CONFIG.SYS commands across various DOS platforms—vital for enterprises maintaining legacy or embedded systems—highlighting configurable options that improve control over system behavior, boot procedures, memory allocation, and error recovery.
UTemplate:NbndZ
- Specifies a particular memory segment for DOS to use as Upper Memory Block (UMB) space.
- Determines which UMB server is utilized by the system.
- Defines address ranges designated as UMB RAM areas.
- Enables or disables verification of disk writes.
- Sets the DOS version reported to applications.
- Configures VGA display status parameters.
- Indicates the current video mode setting.
- Controls whether virtual DMA functionality is permitted.
- Decides if the processor's Virtual Mode Extensions (VME) should be turned off.
- Sets up VxD driver support for Windows environments.
- Configures overall Windows system support features.
- Adjusts relocation settings for the Extended BIOS Data Area (XBDA).
- Allows conditional execution based on XT-compatible hardware detection.
- Implements built-in Year 2000 (Y2K) compliance support.
- Establishes the default character for affirmative responses in prompts.
- Manages settings related to deletion tracking.
Issues
Although a system may start without CONFIG.SYS or AUTOEXEC.BAT, these files are crucial for comprehensive customization and meeting software demands. When either file becomes damaged or corrupted, diagnostic measures are necessary.
In the absence of a SHELL directive within CONFIG.SYS—or if the file itself is missing or compromised—DOS attempts to locate COMMAND.COM in the root directory of the boot drive. Versions prior to 6.0 will not boot if COMMAND.COM cannot be found. Starting with MS-DOS 6.0, PC DOS 6.1, Novell DOS 7, and subsequent releases, the system prompts users to input the command processor path, offering a limited shell replacement at runtime without needing a reboot.
Because MS-DOS 7+ COMMAND.COM is incompatible with DR-DOS, versions 7.02 and later of DR-DOS prevent skipping SHELL directives during skip, trace, or step modes. These versions require valid shell filenames before loading COMMAND.COM. Users can access recovery options through specific key sequences when invalid SHELL parameters are encountered.
Under MS-DOS and PC DOS, CONFIG.SYS size is restricted to a few kilobytes (up to 64 KB in later editions), whereas DR-DOS imposes no such limits aside from those set by the file system. MS-DOS processes CONFIG.SYS by converting it into a tokenized form in memory before executing directives sequentially—loading drivers prior to TSRs. Conversely, DR-DOS interprets CONFIG.SYS line-by-line, granting precise control over the loading order of drivers and TSRs via DEVICE and INSTALL commands. This method facilitates conflict resolution and supports integrating non-resident software into the startup sequence.
In MS-DOS/PC DOS versions 2.0 through 4.01, the SHELL directive accepts a maximum of 31 characters; later versions extend this limit to 128 characters. DR-DOS permits up to 255 characters for this directive. It should be noted that CONFIG.SYS directives do not accommodate long filenames.
Dual booting DOS and Windows 9x
When Windows 95 is installed over an existing DOS or Windows environment, the original CONFIG.SYS and AUTOEXEC.BAT files are renamed CONFIG.DOS and AUTOEXEC.DOS respectively to support dual-boot functionality between Windows 9x and DOS. During a DOS startup, these renamed files are temporarily reverted to their standard names. Additionally, 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, the Windows CONFIG.SYS and AUTOEXEC.BAT remain untouched. Instead, DR-DOS utilizes its own startup scripts named DCONFIG.SYS and AUTODOS7.BAT.
OS/2 / Windows NT
OS/2 relies heavily on CONFIG.SYS for configuring device drivers and environment variables before launching its graphical user interface.
Within the OS/2 subsystem of Windows NT, what would traditionally be set in CONFIG.SYS files is instead represented as registry entries rather than separate configuration files.
A significant number of OS/2 CONFIG.SYS directives lack official documentation.
CONFIG.SYS continues to be employed in 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 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
- Details on FreeDOS CONFIG.SYS commands
- PTS-DOS 2000 Pro User Manual featuring CONFIG.SYS directives (archived)
- Unofficial German-language PTS-DOS FAQ including CONFIG.SYS information (archived)