IO.sys: the real-mode operating system
The IO.sys system file, which Windows 95 uses, replaces the MS-DOS system files (IO.SYS and MSDOS.SYS). This is a real-mode operating system file that contains the information needed to start the computer. CONFIG.SYS and AUTOEXEC.BAT are no longer required by the computer to start Windows 95. However, Windows 95 saves these files for backward compatibility with certain applications and drivers. If the computer is started using the previous operating system, the Windows 95 IO.SYS file is renamed to WINBOOT.SYS
The following drivers are loaded by default into IO.SYS:
- HIMEM.SYS
- IFSHLP.SYS
- SETVER.EXE
- DBLSPACE.BIN or DRVSPACE.BIN (if found on the hard disk)
The following are the common entries in CONFIG.SYS that are now incorporated into IO.SYS and their default values in Windows 95: (Source: Microsoft Corporation, 7/95)
dos=high,umb Specifies that MS-DOS should be loaded in the high memory area (HMA), the default.
himem.sys Enables access to the HMA. This line loads and runs the real-mode Memory Manager and is the default.
ifshlp.sys Installable File System Helper, which loads device drivers. This allows the system to make file system calls. Until this is loaded, only the minimal file system from IO.SYS is used. After this point, the full file system is available.
setver.exe Optional TSR-type device. This file responds to DOS applications that require a DOS version number and sets the version number.
files=60 Specifies the number of file handle buffers to create.
lastdrive=z Specifies the last drive letter available for assignment. If Windows 95 Setup finds this entry, it is moved to the Registry.
buffers=30 Specifies the number of file buffers to create.
stacks=9,256 Specifies the number and size of stack frames. This is not required for Windows 95 but is included for compatibility with older applications.
shell=command.com /p Indicates what command process to use. The /p switch indicates that the command process is permanent and should not be unloaded. If the /p is not specified in CONFIG.SYS, the command process can be unloaded when quitting the operating system.
fcbs=4 Specifies the number of file control blocks that MS-DOS can have open at the same time.
To override the above-listed default values in Windows 95 IO.SYS:
- Place an entry in CONFIG.SYS containing the desired value
Any switches or parameters entered into CONFIG.SYS OVERRIDE those of drivers and settings created by IO.SYS.
NOTE: Entries for files=, buffers=, and stacks= must be set in CONFIG.SYS to at least the default values in IO.SYS.
Default settings corresponding to AUTOEXEC.BAT commands in IO.SYS for Windows 95:
net start Binds the real-mode network components and validates the binding. Any errors received are placed in the NDISLOG.TXT file. (SYSINIT or COMMAND.COM performs the necessary net start.)
set path Sets the path as specified.
IO.SYS does not load EMM386.exe. EMM386 must be defined in CONFIG.SYS for any applications requiring expanded memory.