The Shifter virus also known as Shifter Objective was one of the most unique viruses in the DOS operating system. Here's the breakdown of how the assembly language works. After writing the user's assembly code, the user saves 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.It was made by Stormbringer.
Inject code to .obj files
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.