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Fake double layered horizontal scroll in screen 4
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I guess this technique is known and perhaps has even been used in some project.  
Anyway, i wish to share the source code of this little demo.

Using the hardware horizontal scroll capabilites and some trickery, it is 
possible to do a double layered scroll pixel by pixel in screen 4. Actually, 
the front layer moves by 2 pixels, while the background layer moves by 1 pixel.

The scroll is done every 1/15 seconds aproximately. In theory is possible to 
update the screen every 1/60 seconds, although the results are not very 
pleasing to look at (flickering). Perhaps if an alternative technique is used 
(see the Extensions section), the scroll will also look good at 1/30 seconds.

WARNING
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I have ran this on openmsx only. Don't know if there are too fast vdp access 
problems.

Requirements
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This code is designed for the V9958.
16 kB of ram are assumed to be present on page 3 ($c000-$ffff).

The "game" area is 32x20 tiles (Maze of Galious size). This leaves 4 tiles for 
information.  Some sort of scroll split will be required (it is not implemented 
in this example: the first four lines are black).

Increase game area and you get screen flickering. Perhaps this side effect could 
be reduced optimizing the source code.

Tile trickery
-------------

The idea is simple.

For this example i use 16 tiles arranged in a 4x4 square. I use only 2 colors 
for the square to avoid color clash. However, two colors per horizontal 32 pixel 
line are possible in this arrangement (i mean, background tiles having different 
colors vertically).

Please take into account that, in this case, each horizontal
arrangement of tiles should have the same exact color: that is, for instance,
$fe,$fe,$fe,$fe. If you use bmp2msx converter, some tiles may get the
"wrong" $ef color. Beware of that! (for me, this was a cause of some headaches!)

There is nothing particular about this 4x4 arrangement. Of course you can
use any arrangement you like (4x8, 8x4, 6x6...), but take into account
that the more tiles, the more memory will you need to store all the tiles.

I generate 8 background tilesets (8x16=128 tiles), each one of them rotated one 
pixel to the left of the other:  that's the reason of the two colors per 
horizontal line restriction. In this example, i am restricted to two colors per 
32 horizontal pixels. These tilesets are stored in RAM, addresses $c100 to $c4ff 
(128 tiles x 8 bytes /tile = $400 bytes). Only the patters are rotated, the 
colors need not to be rotated in this example.

Let's consider scrolling to the left. The screen is horizontally scrolled by 
setting the appropriate registers of V9958 that displace the screen by 2 pixels.  
Each time the screen is scrolled, the background tileset gets substituted by the 
the one corresponding to one pixel *right* rotated version of the actual 
background tileset. By doing this, the background tiles will appear to be moved 
one pixel to the left (2pixels to the left from hardware scroll + 1 pixel to the 
right from "tile scroll" = 1 pixel to the left, while the foreground layer has 
been moved 2 pixels to the left).

Extensions
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Note that this can be used with V9938 if the hardware scroll is changed to a 
"set adjust" scroll.

If this technique is used for vertical scrolling, then the V9938 can handle it 
as well, and furthermore the two color per line restriction does not apply: 
background and foreground tiles share only the 2 color per 8 pixel constraint. 

Note that for the vertical scolling case, the color data needs to be rotated and 
stored also (doubling the memory requirements).

On the other hand, given that the minimum VRAM of the V9938/V9958 is 128kB, and
provided that you are ok with just 8 tilesets (and for a repetitive background
configuration such as a repetition of a n times m tile block i think you are),
you could store all the tilesets in the vram, and the "tile scroll" is just a
change on the base address of the pattern generator and color tables.  This
change involves a few out instructions i think. Furthermore, no RAM memory will
be required to store the tilesets!

On the current implementation, the "tile scroll" involves a transfer of
128x3=384 bytes each time the screen is scrolled: 16 tiles x 8 bytes
/tile=128 bytes, and multiplied by 3 because three tilesets compose a
screen.

Conclusions
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I don't know if the operations that need to be performed are too heavy. I also 
don't know if a game (say, a shoot'em'up) is possible using this technique. 

If you want pixel perfect, double layer horizontal/vertical scrolling on MSX, 
use the V9990!

Perhaps, using this method, you can get triple or quadruple layer pixel scroll 
with V9990.

Contact
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username qilipalmer at the mail service of the searching company.