Compiled virus

A compiled virus is a virus that has had its source code converted by a compiler program into a format that can be directly executed by an operating system. Compiled viruses typically fall into three categories:


 * File infector virus. A file infector virus attaches itself to executable programs, such as word processors, spreadsheet applications, and computer games. When the virus has infected a program, it propagates to infect other programs on the system, as well as other systems that use a shared infected program. Jerusalem and Cascade are two of the best known file infector viruses.


 * Boot sector virus. A boot sector virus infects the master boot record (MBR) of a hard drive or the boot sector of a hard drive or removable media, such as floppy diskettes. The boot sector is an area at the beginning of a drive or disk where information about the drive or disk structure is stored. Boot sectors contain boot programs that are run at host startup to boot the operating system. The MBR of a hard drive is a unique location on the disk where a computer's basic input/output system (BIOS) can locate and load the boot program. Removable media, such as floppy disks, need not be bootable to infect the system; if an infected disk is in the drive when the computer boots, the virus could be executed. Boot sector viruses are easily concealed, have a high rate of success, and can harm a computer to the point of making it completely inoperable. Symptoms of boot sector virus infection on a computer include an error message during booting or the inability to boot. Form, Michelangelo, and Stoned are examples of boot sector viruses.


 * Multipartite virus. A multipartite virus uses multiple infection methods, typically infecting both files and boot sectors. Accordingly, multipartite viruses combine the characteristics of file infector and boot sector viruses. Examples of multipartite viruses include Flip and Invader.

In addition to infecting files, compiled viruses can reside in the memory of infected systems so that each time a new program is executed, the virus infects the program. Among compiled viruses, boot sector viruses are the most likely to be memory resident. Viruses that are memory resident stay in memory for an extended period of time and therefore are likely to infect more files and to interfere more frequently with normal system operations than non-memory-resident viruses.