path: root/README

Copyright (C) 1999-2002 VMware, Inc.
All Rights Reserved

The code here may be used/distributed under the terms of the standard
XFree86 license.


	VMware SVGA Device Interface and Programming Model
	--------------------------------------------------


Include Files
-------------

svga_reg.h
    SVGA register definitions, SVGA capabilities, and FIFO command definitions.

svga_limits.h
    Included by svga_reg.h, defines maximum frame buffer and memory region
    sizes.

guest_os.h
    Values for the GUEST_ID register.

vm_basic_types.h
    Common type definitions.

vm_device_version.h
    PCI vendor ID's and related information.


Programming the VMware SVGA Device
----------------------------------

1. Reading/writing a register:

    The SVGA registers are addressed by an index/value pair of 32 bit
    registers in the IO address space.
    
    The 0710 VMware SVGA chipset (PCI device ID PCI_DEVICE_ID_VMWARE_SVGA) has
    its index and value ports hardcoded at:
    
    	index: SVGA_LEGACY_BASE_PORT + 4 * SVGA_INDEX_PORT
    	value: SVGA_LEGACY_BASE_PORT + 4 * SVGA_VALUE_PORT
    
    The 0405 VMware SVGA chipset (PCI device ID PCI_DEVICE_ID_VMWARE_SVGA2)
    determines its index and value ports as a function of the first base
    address register in its PCI configuration space as:

    	index: <Base Address Register 0> + SVGA_INDEX_PORT
    	value: <Base Address Register 0> + SVGA_VALUE_PORT

    To read a register:
	Set the index port to the index of the register, using a dword OUT
	Do a dword IN from the value port

    To write a register:
	Set the index port to the index of the register, using a dword OUT
	Do a dword OUT to the value port

    Example, setting the width to 1024:

	mov	eax, SVGA_REG_WIDTH
	mov	edx, <SVGA Address Port>
	out	dx, eax
    	mov	eax, 1024
	mov	edx, <SVGA Value Port>
	out	dx, eax

2. Initialization
    Check the version number
     loop:
      Write into SVGA_REG_ID the maximum SVGA_ID_* the driver supports.
      Read from SVGA_REG_ID.
       Check if it is the value you wrote.
	If yes, VMware SVGA device supports it
	If no, decrement SVGA_ID_* and goto loop
     This algorithm converges.

    Map the frame buffer and the command FIFO
	Read SVGA_REG_FB_START, SVGA_REG_FB_SIZE, SVGA_REG_MEM_START,
	SVGA_REG_MEM_SIZE.
	Map the frame buffer (FB) and the FIFO memory (MEM)

    Get the device capabilities and frame buffer dimensions
	Read SVGA_REG_CAPABILITIES, SVGA_REG_MAX_WIDTH, SVGA_REG_MAX_HEIGHT,
	and SVGA_REG_HOST_BITS_PER_PIXEL / SVGA_REG_BITS_PER_PIXEL.

	Note: The capabilities can and do change without the PCI device ID
	changing or the SVGA_REG_ID changing.  A driver should always check
	the capabilities register when loading before expecting any
	capabilities-determined feature to be available.  See below for a list
	of capabilities as of this writing.

	Note: If SVGA_CAP_8BIT_EMULATION is not set, then it is possible that
	SVGA_REG_HOST_BITS_PER_PIXEL does not exist and
	SVGA_REG_BITS_PER_PIXEL should be read instead.

    Report the Guest Operating System
    	Write SVGA_REG_GUEST_ID with the appropriate value from <guest_os.h>.
	While not required in any way, this is useful information for the
	virtual machine to have available for reporting and sanity checking
	purposes.

    SetMode
	Set SVGA_REG_WIDTH, SVGA_REG_HEIGHT, SVGA_REG_BITS_PER_PIXEL
	Read SVGA_REG_FB_OFFSET
	(SVGA_REG_FB_OFFSET is the offset from SVGA_REG_FB_START of the
	 visible portion of the frame buffer)
	Read SVGA_REG_BYTES_PER_LINE, SVGA_REG_DEPTH, SVGA_REG_PSEUDOCOLOR,
	SVGA_REG_RED_MASK, SVGA_REG_GREEN_MASK, SVGA_REG_BLUE_MASK

	Note: SVGA_REG_BITS_PER_PIXEL is readonly if
	SVGA_CAP_8BIT_EMULATION is not set in the capabilities register.  Even
	if it is set, values other than 8 and SVGA_REG_HOST_BITS_PER_PIXEL
	will be ignored.

    Enable SVGA
	Set SVGA_REG_ENABLE to 1
	(to disable SVGA, set SVGA_REG_ENABLE to 0.  Setting SVGA_REG_ENABLE
	to 0 also enables VGA.)

    Initialize the command FIFO
	The FIFO is exclusively dword (32-bit) aligned.  The first four
	dwords define the portion of the MEM area that is used for the
	command FIFO.  These are values are all in byte offsets from the
	start of the MEM area.

	A minimum sized FIFO would have these values:
	    mem[SVGA_FIFO_MIN] = 16;
	    mem[SVGA_FIFO_MAX] = 16 + (10 * 1024);
	    mem[SVGA_FIFO_NEXT_CMD] = 16;
	    mem[SVGA_FIFO_STOP] = 16;

	Set SVGA_REG_CONFIG_DONE to 1 after these values have been set.
	
	Note: Setting SVGA_REG_CONFIG_DONE to 0 will stop the device from
	reading the FIFO until it is reinitialized and SVGA_REG_CONFIG_DONE is
	set to 1 again.

3. SVGA command FIFO protocol
    The FIFO is empty when SVGA_FIFO_NEXT_CMD == SVGA_FIFO_STOP.  The
    driver writes commands to the FIFO starting at the offset specified
    by SVGA_FIFO_NEXT_CMD, and then increments SVGA_FIFO_NEXT_CMD.

    The FIFO is full when SVGA_FIFO_NEXT_CMD is one word before SVGA_FIFO_STOP.

    When the FIFO becomes full, the FIFO should be sync'd

    To sync the FIFO
	Write SVGA_REG_SYNC
	Read SVGA_REG_BUSY
	Wait for the value in SVGA_REG_BUSY to be 0

    The FIFO should be sync'd before the driver touches the frame buffer, to
    guarantee that any outstanding BLT's are completed.

4. Cursor
    When SVGA_CAP_CURSOR is set, hardware cursor support is available.  In
    practice, SVGA_CAP_CURSOR will only be set when SVGA_CAP_CURSOR_BYPASS is
    also set and drivers supporting a hardware cursor should only worry about
    SVGA_CAP_CURSOR_BYPASS and only use the FIFO to define the cursor.  See
    below for more information.

5. Pseudocolor
    When the read-only register SVGA_REG_PSEUDOCOLOR is 1, the device is in a
    colormapped mode whose index width and color width are both SVGA_REG_DEPTH.
    Thus far, 8 is the only depth at which pseudocolor is ever used.

    In pseudocolor, the colormap is programmed by writing to the SVGA palette
    registers.  These start at SVGA_PALETTE_BASE and are interpreted as
    follows:

    	SVGA_PALETTE_BASE + 3*n		- The nth red component
    	SVGA_PALETTE_BASE + 3*n + 1	- The nth green component
    	SVGA_PALETTE_BASE + 3*n + 2	- The nth blue component
    
    And n ranges from 0 to ((1<<SVGA_REG_DEPTH) - 1).
    

Drawing to the Screen
---------------------

After initialization, the driver can write directly to the frame buffer.  The
updated frame buffer is not displayed immediately, but only when an update
command is sent.  The update command (SVGA_CMD_UPDATE) defines the rectangle
in the frame buffer that has been modified by the driver, and causes that
rectangle to be updated on the screen.

A complete driver can be developed this way.  For increased performance,
additional commands are available to accelerate common operations.  The two
most useful are SVGA_CMD_RECT_FILL and SVGA_CMD_RECT_COPY.

After issuing an accelerated command, the FIFO should be sync'd, as described
above, before writing to the frame buffer.

Addendum on 7/11/2000
---------------------

SVGA_REG_FB_OFFSET and SVGA_REG_BYTES_PER_LINE may change after SVGA_REG_WIDTH
or SVGA_REG_HEIGHT is set.  Also the VGA registers must be written to after
setting SVGA_REG_ENABLE to 0 to change the display to a VGA mode.

Addendum on 11/29/2001
---------------------

Actually, after changing any of SVGA_REG_WIDTH, SVGA_REG_HEIGHT, and
SVGA_REG_BITS_PER_PIXEL, all of the registers listed in the 'SetMode'
initialization section above should be reread.  Additionally, when changing
modes, it can be convenient to set SVGA_REG_ENABLE to 0, change
SVGA_REG_WIDTH, SVGA_REG_HEIGHT, and SVGA_REG_BITS_PER_PIXEL (if available),
and then set SVGA_REG_ENABLE to 1 again.


Capabilities
------------

The capabilities register (SVGA_REG_CAPABILITIES) is an array of bits that
indicates the capabilities of the SVGA emulation.  A driver should check
SVGA_REG_CAPABILITIES every time it loads before relying on any feature that
is only optionally available.

Some of the capabilities determine which FIFO commands are available.  This
table shows which capability indicates support for which command.

    FIFO Command			Capability
    ------------			----------

    SVGA_CMD_RECT_FILL			SVGA_CAP_RECT_FILL
    SVGA_CMD_RECT_COPY			SVGA_CAP_RECT_COPY
    SVGA_CMD_DEFINE_BITMAP		SVGA_CAP_OFFSCREEN
    SVGA_CMD_DEFINE_BITMAP_SCANLINE	SVGA_CAP_OFFSCREEN
    SVGA_CMD_DEFINE_PIXMAP		SVGA_CAP_OFFSCREEN
    SVGA_CMD_DEFINE_PIXMAP_SCANLINE	SVGA_CAP_OFFSCREEN
    SVGA_CMD_RECT_BITMAP_FILL		SVGA_CAP_RECT_PAT_FILL
    SVGA_CMD_RECT_PIXMAP_FILL		SVGA_CAP_RECT_PAT_FILL
    SVGA_CMD_RECT_BITMAP_COPY		SVGA_CAP_RECT_PAT_FILL
    SVGA_CMD_RECT_PIXMAP_COPY		SVGA_CAP_RECT_PAT_FILL
    SVGA_CMD_FREE_OBJECT		SVGA_CAP_OFFSCREEN
    SVGA_CMD_RECT_ROP_FILL		SVGA_CAP_RECT_FILL +
					    SVGA_CAP_RASTER_OP
    SVGA_CMD_RECT_ROP_COPY		SVGA_CAP_RECT_COPY +
					    SVGA_CAP_RASTER_OP
    SVGA_CMD_RECT_ROP_BITMAP_FILL	SVGA_CAP_RECT_PAT_FILL +
					    SVGA_CAP_RASTER_OP
    SVGA_CMD_RECT_ROP_PIXMAP_FILL	SVGA_CAP_RECT_PAT_FILL +
					    SVGA_CAP_RASTER_OP
    SVGA_CMD_RECT_ROP_BITMAP_COPY	SVGA_CAP_RECT_PAT_FILL +
					    SVGA_CAP_RASTER_OP
    SVGA_CMD_RECT_ROP_PIXMAP_COPY	SVGA_CAP_RECT_PAT_FILL +
					    SVGA_CAP_RASTER_OP
    SVGA_CMD_DEFINE_CURSOR		SVGA_CAP_CURSOR
    SVGA_CMD_DISPLAY_CURSOR		SVGA_CAP_CURSOR
    SVGA_CMD_MOVE_CURSOR		SVGA_CAP_CURSOR
    SVGA_CMD_DEFINE_ALPHA_CURSOR	SVGA_CAP_ALPHA_CURSOR
    SVGA_CMD_DRAW_GLYPH                 SVGA_CAP_GLYPH
    SVGA_CMD_DRAW_GLYPH_CLIPPED         SVGA_CAP_GLYPH_CLIPPING

Note:  SVGA_CMD_DISPLAY_CURSOR and SVGA_CMD_MOVE_CURSOR should not be used.
Drivers wishing hardware cursor support should use cursor bypass (see below).

Other capabilities indicate other functionality as described below:

    SVGA_CAP_CURSOR_BYPASS
	The hardware cursor can be drawn via SVGA Registers (without requiring
	the FIFO be synchronized and will be drawn potentially before any
	outstanding unprocessed FIFO commands).

	Note:  Without SVGA_CAP_CURSOR_BYPASS_2, cursors drawn this way still
	appear in the guest's framebuffer and need to be turned off before any
	save under / overlapping drawing and turned back on after.  This can
	cause very noticeable cursor flicker.

    SVGA_CAP_CURSOR_BYPASS_2
    	Instead of turning the cursor off and back on around any overlapping
	drawing, the driver can write SVGA_CURSOR_ON_REMOVE_FROM_FB and
	SVGA_CURSOR_ON_RESTORE_TO_FB to SVGA_REG_CURSOR_ON.  In almost all
	cases these are NOPs and the cursor will be remain visible without
	appearing in the guest framebuffer.  In 'direct graphics' modes like
	Linux host fullscreen local displays, however, the cursor will still
	be drawn in the framebuffer, still flicker, and be drawn incorrectly
	if a driver does not use SVGA_CURSOR_ON_REMOVE_FROM_FB / RESTORE_TO_FB.

    SVGA_CAP_8BIT_EMULATION
    	SVGA_REG_BITS_PER_PIXEL is writable and can be set to either 8 or
	SVGA_REG_HOST_BITS_PER_PIXEL.  Otherwise the only SVGA modes available
	inside a virtual machine must match the host's bits per pixel.
	
	Note: Some versions which lack SVGA_CAP_8BIT_EMULATION also lack the
	SVGA_REG_HOST_BITS_PER_PIXEL and a driver should assume
	SVGA_REG_BITS_PER_PIXEL is both read-only and initialized to the only
	available value if SVGA_CAP_8BIT_EMULATION is not set.
        
    SVGA_CAP_OFFSCREEN_1
        SVGA_CMD_RECT_FILL, SVGA_CMD_RECT_COPY, SVGA_CMD_RECT_ROP_FILL,
        SVGA_CMD_RECT_ROP_COPY can operate with a source or destination (or
        both) in offscreen memory. 
        
        Usable offscreen memory is a rectangle located below the last scanline
        of the visible memory:
        x1 = 0
        y1 = (SVGA_REG_FB_SIZE + SVGA_REG_BYTES_PER_LINE - 1) / 
             SVGA_REG_BYTES_PER_LINE
        x2 = SVGA_REG_BYTES_PER_LINE / SVGA_REG_DEPTH
        y2 = SVGA_REG_VRAM_SIZE / SVGA_REG_BYTES_PER_LINE


Cursor Handling
---------------

Starting with GSX Server Beta 3 (after 11/15/2000), hardware cursor support
was added.  Actually, both a hardware cursor via the FIFO (SVGA_CAP_CURSOR)
and a hardware cursor via the SVGA registers (SVGA_CAP_CURSOR_BYPASS) were
added.  SVGA_CAP_CURSOR was never available without SVGA_CAP_CURSOR_BYPASS and
the FIFO hardware cursor should never be used and may be removed without
warning in the future.

Cursor bypass is programmed using the two FIFO commands SVGA_CMD_DEFINE_CURSOR
and SVGA_CMD_DEFINE_ALPHA_CURSOR in conjunction with the SVGA registers
SVGA_REG_CURSOR_ID, SVGA_REG_CURSOR_X, SVGA_REG_CURSOR_Y, and
SVGA_REG_CURSOR_ON.

A driver defines an AND/XOR hardware cursor using SVGA_CMD_DEFINE_CURSOR to
assign an ID and establish the AND and XOR masks with the hardware.  A driver
uses SVGA_CMD_DEFINE_ALPHA_CURSOR to define a 32 bit mask whose top 8 bits are
used to blend the cursor image with the pixels it covers.  Alpha cursor
support is only available when SVGA_CAP_ALPHA_CURSOR is set.

Once a cursor is defined, a driver can draw it to the screen at any time by
writing the SVGA_REG_CURSOR_ID register with the ID used when the cursor was
defined, writing SVGA_REG_CURSOR_X and SVGA_REG_CURSOR_Y with the location of
the cursor, and SVGA_CURSOR_ON_SHOW to SVGA_REG_CURSOR_ON.  The drawing occurs
when SVGA_REG_CURSOR_ON is written.

Writing SVGA_CURSOR_ON_HIDE to SVGA_REG_CURSOR_ON will turn the cursor off and
make it vanish from the display and, if present, from the framebuffer.
SVGA_CURSOR_ON_REMOVE_FROM_FB will ensure the cursor is not in the
framebuffer, but will only turn it off if there's no other way to remove it.
SVGA_CURSOR_ON_RESTORE_TO_FB is the complement to
SVGA_CURSOR_ON_REMOVE_FROM_FB.  Whenever possible, the device will not put the
cursor in the framebuffer and Remove From / Restore To will be NOPs.

Note: The cursor must be out of the frame buffer before the driver (or any
agent in the virtual machine) touches an overlapping portion of the frame
buffer, because it is actually drawn into the frame buffer memory in the
case of direct graphics mode (e.g. full screen mode on Linux).  The cursor
does not have to be touched before issuing an accelerated command via the
command FIFO, this case is handled by the SVGA device.

Note: If SVGA_CAP_CURSOR_BYPASS2 is not present, the driver must use
SVGA_CURSOR_ON_HIDE and SVGA_CURSOR_ON_HIDE to be certain the cursor is out of
the framebuffer.


Driver Version Numbers
----------------------

The SVGA drivers use the following convention for their version numbers:

Version 10.0 - The first version that uses the FIFO
Version 10.1 - The version that uses the hardware cursor emulation via the FIFO
Version 10.2 - The version that uses the cursor that bypasses the FIFO
Version 10.3 - The version that can also support the 0405 chipset
Version 10.4 - The version that knows about SVGA_CAP_CURSOR_BYPASS2
Version 10.5 - [Never released or well defined]
Version 10.6 - The version that knows about SVGA_CAP_8BIT_EMULATION
Version 10.7 - The version that knows about SVGA_CAP_ALPHA_CURSOR
Version 10.8 - The version that knows about SVGA_CAP_GLYPH
Version 10.9 - The version that knows about SVGA_CAP_OFFSCREEN_1

Note that this is merely the convention used by SVGA drivers written and
maintained by VMware, Inc. and describes the capabilities of the driver, not
the virtual hardware.  An SVGA driver can only use the intersection of the
functionality it supports and the functionality available in the virtual SVGA
hardware.


Frequently Asked Questions
--------------------------

1.  My driver doesn't display anything, what's going on?

First check if you are issuing an SVGA_CMD_UPDATE after drawing to
the screen.  Another check you can do is to run your driver in full
screen mode on a Linux host.  In this case you are drawing directly
on the frame buffer, so what you draw to the screen will be immediately
visible.  If nothing is visible in this case, then most likely your
driver hasn't mapped the frame buffer correctly.

A discrepancy between what you get in full screen mode and what you
get in window mode indicates that you have a missing or incorrect
update command.


2.  What's the difference between bitmaps and pixmaps?

Pixmaps have the same depth as the screen, while bitmaps have depth one.
When a bitmap is drawn, the command also takes two colors, foreground and
background.  The set bits in the bitmap are replaced with the foreground
color, and the unset bits are replaced with the background color.

Pixmaps, on the other hand, can be directly copied to the screen.


3.  What's the significance of the ROP in the commands SVGA_CMD_RECT_ROP_FILL,
SVGA_CMD_RECT_ROP_BITMAP_COPY, etc. ?

The ROP in the ...ROP... commands is a raster operation.  It has the same
significance (and encoding) as it does in X.  The ROP value SVGA_ROP_COPY
means the source is copied to the destination, which makes these commands the
same as their non-ROP counterparts.  The most commonly used raster operation
other than copy is probably SVGA_ROP_XOR, which combines the source and
destination using exclusive-or.


4.  Tell me more about bitmaps and pixmaps.  For example, the macro
SVGA_CMD_DEFINE_BITMAP has a field <scanlines>.  What should this be
set to?  Likewise with SVGA_CMD_DEFINE_PIXMAP.  And when should the
SCANLINE macros be used?

OK, I'll use pixmaps as an example.  First you have to define the pixmap:

#define  SVGA_CMD_DEFINE_PIXMAP		6
	 /* FIFO layout:
	    Pixmap ID, Width, Height, Depth, <scanlines> */

The ID is something you choose, which you subsequently use to refer to
this pixmap.  It must be an integer between 0 and SVGA_MAX_ID.

The width and height and depth are the dimensions of the pixmap.  For now,
the depth of the pixmap has to match the depth of the screen.

The scanlines are the pixels that make up the pixmap, arranged one row
at a time.  Each row is required to be 32-bit aligned.  The macros
SVGA_PIXMAP_SCANLINE_SIZE and SVGA_PIXMAP_SIZE give the size of a
single scanline, and the size of the entire pixmap, respectively, in
32-bit words.

The second step is to use it:

#define  SVGA_CMD_RECT_PIXMAP_FILL	9
	 /* FIFO layout:
	    Pixmap ID, X, Y, Width, Height */

The ID here is the one you chose when defining the pixmap.  X, Y,
Width, and Height define a rectangle on the screen that is to be filled
with the pixmap.  The pixmap is screen aligned, which means that the
coordinates in the pixmap are defined by the screen coordinates modulo
the pixmap dimensions.

If you want a different alignment between the screen and the pixmap,
then you can use this command, which allows the pixmap coordinates to
be defined:

#define  SVGA_CMD_RECT_PIXMAP_COPY	11
	 /* FIFO layout:
	    Pixmap ID, Source X, Source Y, Dest X, Dest Y, Width,
	    Height */

The Source X and Source Y are pixmap coordinates, and the Dest X and
Dest Y are screen coordinates.


5.  OK, now it works briefly, then stops displaying anything.  Also,
my log file is filled with lines like:
  Unknown Command 0xff in SVGA command FIFO
What's happening?

The most common problem at this point is that the FIFO gets out
of sync.  This can happen if the amount of data in the FIFO doesn't
match what the VMware SVGA device expects.  To track this down, try
to isolate the particular command which causes the problem.

Another way this can happen is if the wraparound in the FIFO isn't
done correctly.  Here is some example code for writing to the FIFO
(mem is an array of 32-bit integers that points to the FIFO memory
region):

while (TRUE) {
    fifo_min = mem[SVGA_FIFO_MIN] / 4;
    fifo_max = mem[SVGA_FIFO_MAX] / 4;
    fifo_next = mem[SVGA_FIFO_NEXT_CMD] / 4;
    fifo_stop = mem[SVGA_FIFO_STOP] / 4;

    tmp_next = fifo_next+1;
    if (tmp_next == fifo_max)
	tmp_next = fifo_min;    // Wraparound

    if (tmp_next == fifo_stop) {
	sync_fifo();		// FIFO full
	continue;		// retry
    }

    mem[fifo_next] = item;
    mem[SVGA_FIFO_NEXT_CMD] = tmp_next * 4;
    break;
}

This isn't the most efficient code, but it should work.  It's important
to do the increment with wraparound before the FIFO full check, and to
check FIFO full before updating the next command pointer.


6. My driver tries to switch modes and either nothing happens or the
display becomes completely garbled.  What's going on?

When you change modes, make very sure you reread all of the registers listed
above under SetMode.  Getting the pitch (SVGA_REG_BYTES_PER_LINE) incorrect
will cause a heavily garbled display.  Also, if you change
SVGA_REG_BITS_PER_PIXEL, make certain that SVGA_CAP_8BIT_EMULATION is present
in the SVGA_REG_CAPABILITIES register.  Also, even with 8 bit emulation, the
driver must still use either 8 bpp or SVGA_REG_HOST_BITS_PER_PIXEL bpp,
nothing else.


7. Why does my driver's hardware cursor work when my virtual machine is in
window mode, but draw/erase incorrectly or in garbled locations in fullscreen
mode?

You need to make sure you use SVGA_CURSOR_ON_REMOVE_FROM_FB and
SVGA_CURSOR_ON_RESTORE_TO_FB _every_ time your driver or the virtual machine
touches a region of the framebuffer that overlaps the cursor.  If you forget
to remove it then it can show up when doing save-under operations or get mixed
in with other drawing.  If you forget to restore it then can disappear.  You
also need to make sure SVGA_CAP_CURSOR_BYPASS2 is available, or else you will
have to use SVGA_CURSOR_ON_SHOW and SVGA_CURSOR_ON_HIDE (which will flicker,
even in window mode), or else a software cursor.  Newer version of the virtual
SVGA hardware will never put the hardware cursor in the framebuffer while in
window mode, so everything will appear to work correctly there.


8. Why do my accelerated glyphs look funny?  OR  Why does the fifo complain
about invalid commands when I draw accelerated glyphs?

The bitmap data passed to SVGA_CMD_DRAW_GLYPH_* must not have any per-scanline
alignment.  If there are any remaining bits left in the last byte of a scanline,
the first bits of the next scanline should use them.  

The bitmap data as a whole must be 4 byte aligned.

$XFree86: xc/programs/Xserver/hw/xfree86/drivers/vmware/README,v 1.5 2002/10/16 22:12:53 alanh Exp $