NAME
        SystemControlA - Method for selectively disabling OS features. (V40)
        SystemControl - varargs stub for SystemControlA().

   SYNOPSIS
        failTag = SystemControlA(tagList);
        D0                       A1

        ULONG SystemControlA(struct tagitem *);

        failTag = SystemControl(firstTag, ...);

        ULONG SystemControl(Tag, ...);

   FUNCTION
        This function is used to alter the operation of the system. Some of
        the alterations involve controlling what are normally regarded as
        system resources. In order to minimize confusion only one task is
        allowed to control any part of the system resources. This prevents
        the possiblity of two tasks fighting, each controlling a part of the
        system. If a tag is identified as task exclusive, it means that
        only one task can hold (set to TRUE) that tag. If
        another task attempts to set the same tag to TRUE, the call to
        SystemControl() will fail.

        It is important to remember that SystemControl() can fail.

        This is a low level function and certain tags do not fit the normal
        Amiga multitasking model.

   INPUTS
        tagList - pointer to an array of tags listing the features of the
                  system to be enabled/disabled.

   TAGS
        SCON_TakeOverSys (BOOL)
                TRUE -  Takes over the CPU to ensure that a program gets every
                        ounce of CPU time (with the exception of crucial
                        interrupts). When in this mode, the CPU will belong
                        completely to the program. task switching will be
                        disabled and the program will get all CPU cycles. This
                        means any calls to the OS that involve multitasking in
                        some way will not execute correctly. Other tasks will
                        not run until this tag is used with FALSE. However,
                        during a wait() on a signal, multitasking will
                        automatically be turned back on until the signal is
                        received. Once received, multitasking will again be
                        disabled and the CPU will be exclusive to the owning
                        program.
                FALSE - Relinquishes the CPU and reenables multitasking.
                        This tag is task exculsive. This tag nests. A task may
                        take over the CPU several times before relinquishing
                        it.

        SCON_KillReq (BOOL)
               TRUE -   Disables system requesters.  These are the reasons for
                        NOT disabling system requesters:

                            1- No calls in the program will cause a system
                               requester.
                            2- The only thing that could cause a requester
                               to appear is the lack of a CD in the drive and
                               SCON_CDReboot is set to CDReboot_On, therefore a
                               requester can't appear.
                            3- The only disk I/O is via a CD with SCON_CDReboot
                               set to CDReboot_On and/or nonvolatile.library.

                        When requesters should not be disabled.
                        GAME PROGRAMS:
                        No DOS calls are used after loading; or SCON_CDReboot
                        is CDReboot_On; and nonvolatile.library is used for
                        loading and saving user data.

                        This fits the above case since; After loading either
                        DOS calls are not used fitting reason 1, or the game
                        is accessing the CD and has SCON_CDReboot set to
                        CDReboot_On fitting reason 2. The game accesses high
                        scores, game position, etc through nonvolatile.library,
                        fitting reason 3.

                FALSE - Enables requesters for the program.

            This tag nests. Tasks may disable requesters several times before
            enabling them.  However, there must be a matching number of calls.

        SCON_CDReboot (ULONG)
                CDReboot_On - Ejecting the CD will cause a reboot of the
                        system. Use this only if the program cannot deal with
                        error conditions.
                CDReboot_Off - Ejecting the CD will not cause a reboot of the
                        system. Use this if the program needs to insert CDs
                        while running.
                CDReboot_Default - Restore the default reboot behavior for this
                        system. This should be used upon exit, if this tag had
                        been used to change the reboot behaviour. For the CD32
                        this value is synonomous with CDReboot_On. For Amiga
                        computers this value is synonomous with CDReboot_Off.

            Note that the default reboot behavior differs depending on the
            platform. If a program requires a specific behavior it must
            use this function to set the behavior. For example, a CD audio
            mixer would use this tag with the data CDReboot_Off. This will
            allow the changing of audio CDs on the game machine as well as
            Amiga computers.

            If, however, there is no error detection code at all this tag
            should be used with the data CDReboot_On.

            It is hoped that no program will require CDReboot_On. If all
            programs check for error condition and recover gracefully such a
            call should never be necessary. With the default behavior the
            CD32 will always reset on disk ejects, and programs
            run from Amiga computers will not reset. Thus, leaving the
            default will increase the market for a program to include both
            types of platforms.

            This tag does not nest.

        SCON_StopInput (BOOL) - When TRUE, stops input.device from using any
            CPU cycles. Also prevents input.device from passing along any
            events from either the keyboard and/or port 0.

            This tag is task exclusive. This tag is NOT reversible.
            Attempting to reverse will result in confused/garbled input
            events.

        SCON_AddCreateKeys (ULONG) - Starts creating rawkey codes for the
            joystick/game controller on the given unit. The unit value is
            checked for validity and must be either 0 or 1. Each different unit
            used results in some code added to the VBlank interrupt chain.
            This tag nests. The tag SCON_RemCreateKeys is used to undo this
            tag. Tasks may create rawkey codes several times before stopping
            them.

            Note that when operating in an Intuition window, the controller's
            blue button is the equivilent of the mouse menu button. Therefore,
            Intuition will be capturing most blue button events. If
            notificiation of these events is important, review the
            documentation for WFLG_RMBTRAP in the
            intuition.library/openwindow() autodoc.

        SCON_RemCreateKeys (ULONG) - stops rawkey codes for the joystick/game
            controller on the given unit. The unit value is checked for
            validity and must be either 0 or 1.

   RESULT
        failTag - zero if all tags succeeded. A non-zero return indicates a
                  tag that has failed. It is possible that other tags may
                  fail as well.

                  If any tag fails there will be no change in the system due
                  to other tags.

   SEE ALSO
        <libraries/lowlevel.h>