CED - Installable commands (INT 21h, FFh)

        Entry:
          AH = FFh
              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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