MS DOS (or Microsoft Disk Operating System) is a single user, single task operating system. Time, design limitations and lack of support have relegated MD DOS and all its variations to almost oblivion. Nowdays, only industrial PCs and embedded processors use any of DOS' versions.
Being an Operating System means that MS DOS was the first program loaded on RAM by the BIOS after the Power On Self Tests (POST). All other programs would have to be loaded manually after the MS DOS files, including the venerable Windows 3.1.
MS DOS had two hidden system files containing the operating system itself (msdos.sys and io.sys) and one command line interpreter program (command.com). The command.com, once loaded on RAM, would present a prompt where the user would type in commands and parameters, similar to those used in Linux.
To load Windows on startup, the line "win.com" had to be included in the autoexec.bat file, a text file whose with DOS comands that were automatically read and interpreted by the command.com interpreter.
Apart from being natively single user and single task, limitations that could be circunvented with third party applications, MS DOS had serious memory management limitations, by which it would could not handle directly more than the first 640 KB of the PC's RAM (lower memory) out of the total 1 MB accessible by the microprocessor (8088 or 8016). The portion of RAM between the 640 KB and the 1 MB (upper memory) was dedicated by the processor to access peripheral devices, such as video cards (mono and color), LAN cards, printer, HDD, floppies, serial ports, etc.
The first approach to expand the avalable memory was to use an ISA card that would be assigned a segment of the upper memory. This expanded memory card had memory chips with a capacity that exceeded the assigned upper memory segment. Therefore, the total expanded memory had to be accessed in segments known as pages. The first program (that I remember) that made use of the Expanded Memory was Lotus 123, an spreadsheet application.
Newer microporcessors (286, 386 and 486) were then able to access RAM memory beyond the 1 MB limit. This way, third party applications could mimic the behavior of the Expanded RAM using segments of the Extended Memory (anything above 1 MB).
When Windows 95 was released, MS DOS finally faced its impending demise. However, many industrial applications still use MD DOS because of its stability and simplicity. Many embedded O/S computers, like those used in gas pumps, ATMs or other applications, still use some versions of DOS. Microsoft, howeer, suspended any further support for MS DOS, whose last version was "6.22.", several years ago.
Not bad for an O/S derived from the old CP/M operating system first released for the Apple II around 1980.
