CONFIG.SYS
CONFIG.SYS loads just after IO.SYS, IBMDOS.COM, or xxxDOS (the hidden boot file). CONFIG.SYS is an important ASCII file that sets configuration limits that are beyond default values and loads drivers and files needed by peripherals. The content of this file is usually established by writing to the file during installation of some peripheral or DOS/Windows applications. Each additional installation may increase the contents of this file. The lines written (commands) direct the system to load drivers and support files for the installed peripherals so that they can operate. A simple system running small simple programs can rely on default limits and does not need the CONFIG.SYS file.
System or peripheral failures can originate in the CONFIG.SYS file when a device driver is loaded into the same memory location as another driver or uses the same I/O address as previously loaded driver. This is not actually a CONFIG.SYS problem but an installation problem with the second installed peripheral. The user can troubleshoot this problem in the CONFIG.SYS file, by editing the file and inserting a semicolon in the first character space of the suspect line. When the system is rebooted, the system ignores the line with the semicolon. Alternatively, press SHIFT+F8 during the boot (just after the memory check), the xxxBIOS will allow the CONFIG.SYS file to be executed line by line with a confirmation request of each step. You can try to eliminate a possible offending load without actually editing the CONFIG.SYS file.
The most outstanding characteristic of the CONFIG.SYS file is the 'equal' sign. Each line will have an equal sign. The lines may have upper or lower case characters. Some spacing and punctuation is critical except around the equal sign. If BIOS cannot find a driver or device, it will issue an error message specific to the device on the screen. The order of the lines determines when the device is loaded. Note that DEVICE=<some drive, some directory>HIMEM.SYS must be first among the device drivers, (put it on line 1) since it will be used to load the other devices into high memory. BIOS will read through CONFIG.SYS file four times, executing everything but DEVICE=xx, INSTALL=xx and SHELL=xx on the first pass, followed by one pass for each of the remaining three commands.