 Many of the programs effectively hijack the operating system in varying documented or undocumented ways, often causing systems to crash on their activation or deactivation when used with particular application programs or other TSRs.As explained above, some viruses and other malware were coded as TSRs, and are deliberately troublesome.Additionally, all program code in DOS systems, even those with large amounts of physical RAM, have to be loaded into the first 640 KB of RAM (the conventional memory).