DOS : Java Glossary

DOS (Disk Operating System). The predecessor to Windows used as an operating system on PCs (Personal Computers). It could run in as little as 64K. It could address a maximum of 640K. It had no GUI (Graphic User Interface). Usually it worked with a 25 × 80 character screen, monochrome or colour. It could fit on a floppy with plenty of room for programs and data. DOS used the FAT-16 file format. With version 2.0 it supported hard disks and directories.

The term also refers to a DOS (Denial of Service attack), when an enemy bombards your website with so many requests there is no bandwidth or computer power left over to handle legitimate traffic. This primitive sort of attack is difficult to defend against other that by having massive reserve in a server farm or cloud. The attacker often uses viruses or Trojans to commandeer PCs all over the globe for the attack, so it difficult to tell attackers from legitimate users. CloudFlare handles DOS attacks by intercepting traffic to your site and passing through only traffic it believes to be legit. This adds 5 seconds or so to the response time. The government of the Philippines uses the service.