What is the difference between a non-resident memory virus and a resident virus?

Memory-resident viruses are immediately infected. Every time a virus program is executed, it actively searches the current path and finds executable files that meet the requirements. This type of virus does not modify the interrupt vector, and does not change any state of the system. Therefore, it is difficult to distinguish whether a virus is running or a normal program. Typical viruses include Vienna/648.

The difference between a memory virus and a resident virus is that the former is a virus-infected Host Program that immediately infect other programs during execution, the latter is generally not contagious in the Process of hosting a virus-infected Host Program in the memory. It resident in the system, usually by modifying the EXEC (4BH) of INT 21 H) or find files (11 H, 12 H, 4EH, 4FH) to monitor the programs to be infected.

It is worth noting that the new virus, known as the New Generation virus, will no longer be interrupted by controlling the INT 21 H, but will be monitored and infected by other methods such as controlling the INT 24 H.