[VGADDI]

Explain how VGA write mode 2 works. No code changes!
CORE-8164


svn path=/trunk/; revision=72607

reactos/win32ss/drivers/displays/vga/vgavideo/vgavideo.c

@@ -681,6 +681,19 @@ vgaWriteScan ( int x, int y, int w, void *b )
             i += 8, off++;
         while (i < w)
         {
+            /*
+             * In write mode 2, the incoming data is 4-bit and represents the
+             * value of entire bytes on each of the 4 memory planes. First, VGA
+             * performs a logical operation on these bytes and the value of the
+             * latch register, but in this case there is none. Then, only the
+             * bits that are set in the bit mask are used from the resulting
+             * bytes, and the other bits are taken from the latch register.
+             *
+             * The latch register always contains the value previously read from
+             * VGA memory, and therefore, we must first read from vp[off] to
+             * load the latch register, and then write bp[i] to vp[off], which
+             * will be converted to 4 bytes of VGA memory as described.
+             */
             dummy = vp[off];
             dummy = bp[i];
             vp[off] = dummy;