Section 1: From IBM`s X11R4 contribution towards XFree86.

This section describes what I did to obtain a 16 colour vga server.

I started with the SYSV, ppc, vga and common directories from IBM's X11R4
contribution. Those directories have the following function:
1. SYSV implements the mouse and keyboard.
2. ppc is a generic layer implementing ddx in terms of drawing operations
   through rectangles. (With spans as a boundary case: height = 1.)
3. vga implements a layer of operations drawing through rectangles.
4. common implements ddx initialization and quitting, and screen saving.

I eliminated SYSV and common since the functionality provided therein is
already provided in XFree86. Since XFree86 is actively being ported to
new operating systems, while X11R4 is out of date, I preferred the XFree86
code here above the SYSV and common code.
Whatever functionality was still needed from common (default colormap
initialization -- not much code) was moved into ppc.

Since XFree86 uses the mi-provided software cursor code, IBM's software
cursor code was deleted from ppc and vga. This is a pity, since it is
expected that it is more efficient than mi's code, but the XFree86 mouse and
keyboard code directly call mi, and I do not want to maintain the XFree86
mouse and keyboard code.

Since we cannot support a monolithic multi-screen server using all of the
x11r4 contributed code yet, all multi screen code was deleted. This includes
one header file containing a nasty copyright statement.

Since glyph handling has changed between X11R4 and X11R5, the code handling
glyphs was replaced by appropriate calls to mi.
I hope it can be modified and put back one day.

Provisionally, some code was added (viz. file vga/offscreen.c) to intercept
calls to the rectangle drawing code when we are switched out of the VT.
I hope this can be replaced by some window tree invalidation and GC validation
scheme.


Section 2: BUGS

This section describes fixed and still unfixed bugs in this code.
All bugs not labeled otherwise also occur in IBM's X11R4 code, and may be of
interest to anybody using that code.

1. (Fixed.)
I found (and provisionally fixed) a bug in the IBM bitblit code:
In file ddx/ibm/vga/vgaImages.c a function vgaReadColorImage is defined.
When this function is used to read less than 8 pixels starting on a byte
boundary it will always read precisely 8 pixels.
Thus, when space is allocated for 4 or less pixels, it will write beyond
the allocated space.
Since the code is rather convoluted, this may not be apparent at first sight,
but going through the code with an example shows the error.

2. (Fixed.)
In ppcPixmapFS.c, function ppcStipplePixmapFS there was a bug
regarding the stipple origin: The horizontal origin is added while
the vertical one is subtracted. The horizontal origin should be subtracted
instead of added here.
This bug gets visible when backing-store is enabled and one uses twm:
the submenu icons get truncated on their left hand side.
(In case you wonder why this bug appears: under those circumstances
twm prepares its menus by drawing into an unmapped window. 
The miCopyPlane function uses the ppcStipplePixmap (and many others)
to get the plane copied.)
I also fixed this bug in the other routines (ppcOpStipplePixmapFS,
ppcTilePixmapFS) in this file, although I had no visible clues for this.
I hope this is appropriate.

3. (Unfixed, since it is not visible in practice.)
I find it suspect that ppcSetSpans gives different output when one
claims that an actually sorted list of spans is unsorted.
The unsorted code is wrong, and should be made to look more like the sorted
code. I.e., use ( xStart - ppt->x ) instead of ( xStart - pbox->x1 )

4. (Fixed.)
There used to be another bug that became visible when using twm and backing
store: popping up a submenu, and moving the cursor upwards til it leaves the
submenu, the submenu would disappears, as it should.
But the submenu icon would not get restored, while it should be.
10b. By replacing the clip-computing code in ppcValidateGC by that in
cfbValidateGC, I *finally* fixed the disappearing twm menu icon problem.

5. Added mfbRegisterCopyplaneProc call. [Its omission was an error on my
   part.]
   Fixing a server core dump in XTest.

6. Fixed not-very-high tile bug in function ppcTileRect, file emulTile.c
   (I.e., if the tile was higher than the area to be tiled, far too much
   was drawn, causing server core dumps in XTest.)
   Actually, "savey" ought to be used to determine the height of the tiles in
   the top line to be tiled, instead of "pTile->drawable.height".
7. A use of height where width was intended was fixed in ppcTileRect:
   "savehcount = w / pTile->drawable.height;" should use "... .width" and
   "savehcount = ( x + w - htarget ) / pTile->drawable.height;" too.

8. Deleted overly clever code in ppcCReduce.c
   (All code that tried to pre-compute how alu's could be replaced by
    other alu's with inverted colors, etc. was deleted. I think it is
   at least wrong for FillSolid.)

9. Looked at suspicious code in ppcSetSp.c
   "tmpx = *pdst;" was never updated during the loop. We took it out of the
   initialization position of the for and moved it into the loop.

10. Add xSrc := GC->patOrg.x + pDrawable.x and ySrc := ... in ppcPixmapFS.c,
   functions ppcStipplePixmapFS, ppcOpStipplePixmapFS, and ppcTilePixmapFS.
   This because stipple and tile origins are taken relative to the drawable.
   Also use a "modulo" function that gets the cases of a negative stipple
   or tile offset right. (When the stipple origin is to the right of or
   below the origin of the drawable.)

11. File vgaSolid.c, function vgaFillSolid:
   Inverting is XORing with all ones. Not with the color we want to AND/OR
   later. So we'll have to set the color to VGA_ALLPLANES
   whenever we want to invert existing data, and reset it before the
   AND/OR is done.
   Also we replaced an outb( 0x3CF, tmp2 ) by the SetVideoGraphicsData( tmp2 )
   it is representing. (Just a cosmetic replacement.)

12. File vgaImages, function vgaDrawColorImage.
   Moved a line "invert_existing_data = TRUE;" two lines down, past a case
   label. Now it is also part of the code executed for GXorReverse, as it
   should be.

13. The pixmap FillSpans routines (file ppcPixmapFS.c) got somewhat better
   after importing some code from ddx/ibm/vga. (A getbits function that does
   wrapping.)
   They were wrong for the FillStippled and FillOpaqueStippled modes.
   I don't understand the old code. How could it handle stipples of a size not
   an exact multiple of 32? (or 8, for that matter.)

14. In function vgaBitBlt file vgaBitBlt.c, in the shortcuts for
    GXSet, GXClear and GXInvert, the source (x0,y0) is accidentally operated
    upon by vgaFillSolid, instead of the destination (x1,y1).

15. Notice that in DoMonoSingle and DoMonoMany in file vgaStipple.c, the left
   edge of the square to be stippled is treated wrong.
   Correct would be to get the bits with getbits using offset xshift, and
   to shift them right (x & 07) places. 
   [One might wish to use the variable tmp1 at this place, since it had been
    set to (x & 07) at this place; but that is already re-used at this point.]
   Also note that NeedValX is set wrong: The implicit assumption was that
   stipples are more than 8 wide.
   This only fixes the problem when miPushPixel is used instead of ppcPushPixel.
   I think I should look some more into this.
   
16. I took out some code of the CopyArea function, in which a no-op function
   was called while a real one was needed. My fix does not completely work,
   although it improved the behaviour of GetImage somewhat.

17.  After finding three kinds of errors in this single function,
   -- the new kinds being the right side not being always written due to
   an incorrect if scope, and the lower end not always being written due
   to variables being updated at the wrong place --
   (requiring modifications to be made at at least 10 places,
   I decided to REWRITE the body of the ppcTileRect function from scratch.
   This version simply computes all relevant margins in advance, and does
   not try to reuse temporary variables. I leave that to the compiler.
   (This was a maintenance and robustness nightmare anyway.)

MORE NOTES:
   It is funny that there are two files in mi that require compilation
   with the proper #defines ( -DXF86VGA16  in my case ):
   Besides the obvious mibitblt.c, there is also mipushpxl.c.



