AH = 0FFh
           AL = 0 add installable command
             BL = mode - bit 0 = 1 callable from DOS prompt
                         bit 1 = 1 callable from application
             DS:SI pointer to CR-terminated command name
             ES:DI pointer to FAR routine entry point
           AL = 1 remove installable command
             DS:SI pointer to CR-terminated command name
           AL = 2 reserved, may be used to test for CED installation
Return: CF set on error
            AX = 01h invalid function
                 02h command not found (subfunction 1 only)
                 08h insufficient memory (subfunction 0 only)
                 0Eh bad data (subfunction 0 only)
        AH = 0FFh if CED not installed
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