MOV     CL,4            ;Needed for shifts
        MOV     [OFFSET],-100H
        MOV     SI,FCB+11H      ;Scan second FCB for offset
        LODB
        CMP     AL," "          ;Check if offset present
        JZ      HAVOFF
        MOV     B,[SIGN],0      ;Assume positive sign for now
        CMP     AL,"+"
        JZ      GETOFF          ;Get a positive offset
        CMP     AL,"-"
        JNZ     GETOFF1         ;If not + or -, then not signed
        MOV     B,[SIGN],1      ;Flag as negative offset
    

When creating an external command, there are two ways to pass arguments from the command line to the external command. First, when passing the file names to the command line as arguments, use 5CH and 6CH in the system scratch area. COMMAND.COM, which always considers the first and second parameters as file names when external commands are executed, expands them to a drive number (1 byte) + file name (8 bytes) + extension (3 bytes) and stores them in 5CH and 6CH. These are in the same format as the first 12 bytes of FCB, so setting these address as first addresses of FCB permits various operatuons.

However, since in this method only 16 bytes differ from the starting addresses of two FCBs, either 5CH or 6CH (only) can be used as a complete FCB. Next, when passing arguments other than file names (numbers, for instance) or creating an external command handling more than three file names, COMMAND.COM stores the whole command line, which invoked the external command, except for the command line itself in the form of number of bytes (1 byte) + command line body, so it can be used by interpreting it in the external command properly. See List 3.3 of section 4 for an example of passing arguments using this DMA area.