/zd       Enables line number information in .OBJ file

          The /zd option causes Turbo Assembler to place line-number
          information in the object file. This lets Borland's stand-alone
          debugger, Turbo Debugger, display the current location in your
          source code, but does not put the information in the object file
          that would allow the debugger to access your data items.

          If you run out of memory when trying to debug your program under
          Turbo Debugger, you can use /zd for some modules and /zi for
          others.