NAME
	AllocScreenBuffer -- Get a ScreenBuffer for double-buffering. (V39)

    SYNOPSIS
	sbuffer = AllocScreenBuffer( Screen, BitMap, flags )
	D0                           A0      A1      D0

	struct ScreenBuffer *AllocScreenBuffer( struct Screen *,
	    struct BitMap *, ULONG );

    FUNCTION
	Allocates a ScreenBuffer structure and usually a BitMap.  This
	structure can be used for double (or multiple) buffering in
	Intuition screens.  You may use this call to obtain a ScreenBuffer
	structure for the screen's initial BitMap, or for other BitMaps
	you intend to swap in.

	AllocScreenBuffer() also allocates a graphics.library DBufInfo
	structure.  The ScreenBuffer structure this call returns contains
	a pointer to that DBufInfo structure, which is free for your use.
	See graphics.library/AllocDBufInfo() for full details on using
	this structure to achieve notification of when it is safe to
	render into an old buffer and when it is right to switch again.

    INPUTS
	Screen = pointer to the screen to double-buffer
	BitMap = For CUSTOMBITMAP screens, you may pre-allocate
		a BitMap matching the screen's BitMap's properties, and
		pass that in, to get a ScreenBuffer referring to that
		BitMap.  Set this parameter to NULL if you'd prefer
		that Intuition allocate the alternate buffer.  This parameter
		must be NULL for non-CUSTOMBITMAP screens.
	Flags = Set a combination of these flags:
	    For non-CUSTOMBITMAP screens, set SB_SCREEN_BITMAP in order to
		get a ScreenBuffer referring to the screen's actual BitMap.
		(For CUSTOMBITMAP screens, just set the BitMap parameter
		to the BitMap you passed to OpenScreen()).
	    Set SB_COPY_BITMAP if you would like the screen's BitMap
		copied into this ScreenBuffer's BitMap.  This is required
		to get any menu-bar or gadget imagery into each
		ScreenBuffer's BitMap.  If the screen has no Intuition
		rendering, you may omit this flag.  Also, when allocating
		a ScreenBuffer for the screen's initial BitMap, this flag
		should not be set.

    RESULT
	Pointer to a valid ScreenBuffer structure, or NULL if failure.
	You may render into the resulting BitMap as appropriate.
	You may use the sb_DBufInfo field to access graphics.library
	ViewPort-buffering features such as learning when it is safe
	to reuse the previous BitMap.  If not, you risk writing into
	the on-screen BitMap, which can damage menu or gadget rendering!

    NOTES

	The role of SB_SCREEN_BITMAP is to wrap a ScreenBuffer around
	the actual current BitMap used by the non-CUSTOMBITMAP screen.
	The words actual or current are used instead of initial, since
	an application could do some double-buffering, leave the
	non-initial buffer installed into the screen, free up the
	others, then re-allocate ScreenBuffer structures to do more
	double-buffering.  In that case, the actual screen BitMap is
	not necessarily the initial one.

	The role of SB_COPY_BITMAP is to say "please put whatever
	imagery is currently on-screen into this new BitMap too".
	That's how the screen titlebar and other imagery get copied
	over into each buffer.

    BUGS

    SEE ALSO
	FreeScreenBuffer(), ChangeScreenBuffer(),
	graphics.library/AllocDBufInfo(), graphics.library/ChangeVPBitMap()