Home / Certification / CompTIA Certifications

DOS is a important portion of the A+ DOS/Windows Certification exam. You should be familiar with the different portions of DOS and its function in the Windows environment.

DOS boot sequence begins with IO.SYS, then MSDOS.SYS, CONFIG.SYS, COMMAND.COM, and then AUTOEXEC.BAT.

COMMAND.COM contains the internal commands for DOS and is required for DOS, Windows 3.11, and Windows 95. It is required to startup DOS.

Memory

DOS memory is split up into the first 640 KB of RAM, Conventional Memory, the upper memory, 640KB - 1024KB, and everything above 1024KB, extended memory. EMM386.EXE enables the expanded memory and using upper memory for system functions. HIMEM.SYS is a device driver for upper and extended memory areas.

High memory area is the first 64KB of extended memory.

The commands to load DOS into high memory is DOS=HIGH and DEVICE=HIMEM.SYS in the CONFIG.SYS.

File System

DOS uses File Allocation Table (FAT) as its file system. It can hold a maximum of 512 files in the DOS root directory and its filename system is 8.3, or 8 characters for the filename and 3 for the extension.