# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1356139367 -3600
# Node ID 575d8cc03fcee91e86470c7af058030e0a51866f
# Parent  53cd92fd0b9670d7cb3d0b77cc22aa2bab4f8fe8
Fixed the description of flashing colours since all logical colours can be made
to flash by mapping any of them to physical colours 8 through 15, and only these
physical colours are able to support the flashing effect.

diff -r 53cd92fd0b96 -r 575d8cc03fce ULA.txt
--- a/ULA.txt	Sat Dec 22 01:02:20 2012 +0100
+++ b/ULA.txt	Sat Dec 22 02:22:47 2012 +0100
@@ -305,11 +305,46 @@
 that the palette registers must be updated upon the flash counter - read and
 written by OSBYTE &C1 (193) - reaching zero and that some way of changing the
 colour pairs to be any combination of colours might be possible, instead of
-having colour complements as pairs. It is conceivable that the interrupt code
-responsible does the simple thing and merely inverts the current values for
-colours 8 through 15, and this might be verified by looking for an operation
-that involves the bit pattern 11001100, since this value would be EORed with
-the existing palette register values in order to perform this value inversion.
+having colour complements as pairs.
+
+It is conceivable that the interrupt code responsible does the simple thing
+and merely inverts the current values for any logical colours for which the
+associated physical colour (as supplied as the second parameter to the VDU 19
+call) has the top bit of its four bit value set. These top bits are not
+recorded in the palette registers but are presumably recorded separately and
+used to build bitmaps as follows:
+
+      2 colour  4 colour  16 colour
+      --------  --------  ---------
+   0  00010001  00010001  00010001
+   1  01000100  00100010  00010001
+   2            01000100  00100010
+   3            10001000  00100010
+   4                      00010001
+   5                      00010001
+   6                      00100010
+   7                      00100010
+   8                      01000100
+   9                      01000100
+  10                      10001000
+  11                      10001000
+  12                      01000100
+  13                      01000100
+  14                      10001000
+  15                      10001000
+
+An operation in the interrupt code would then combine the bitmaps for all
+logical colours in 2 and 4 colour modes, with the 16 colour bitmaps being
+combined for groups of logical colours as follows:
+
+   0,  2,  8, 10
+   4,  6, 12, 14
+   5,  7, 13, 15
+   1,  3,  9, 11
+
+These combined bitmaps would be EORed with the existing palette register
+values in order to perform the value inversion necessary to produce the
+flashing effect.
 
 Palette Definition Lists
 ------------------------