The Shifter virus was one of the most unique viruses I have ever tested. Here's the breakdown of how the assembly language works. After writing your assembly code, you save it as a .asm file. It is then compiled by TASM or MASM or similar programs into a .OBJ file. This .OBJ file could then be processed by a program known as a linker to turn it into a .COM or .EXE file, which could then be run by the user. The shifter virus spreads by injecting its code into those compiled .OBJ files, causing it to reside in every valid program made from the .OBJ file.