# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1466715720 -7200
# Node ID e3c785375378cee8bf135f734d8a26e3c8800593
# Parent  528acbe53d796de8179f98766f6f289b65a79787
Attempted to introduce hardware description language limitations, restructuring
to put all state-changing operations in one place, deferring updates of the
horizontal position variable.

diff -r 528acbe53d79 -r e3c785375378 ula.py
--- a/ula.py	Wed Jun 22 15:49:54 2016 +0200
+++ b/ula.py	Thu Jun 23 23:02:00 2016 +0200
@@ -211,6 +211,7 @@
         self.pdata = 0          # decoded RAM data for pixel output
         self.cycle = 1          # 8-state counter within each 2MHz period
         self.pcycle = 0         # 8/4/2-state pixel output counter
+        self.x = 0
 
         self.next_frame()
 
@@ -273,7 +274,7 @@
         self.line_start = self.pixel_address = self.screen_start
         self.line = self.line_start % LINES_PER_ROW
         self.y = 0
-        self.x = 0
+        self.x_next = 0
 
     def next_horizontal(self):
 
@@ -281,13 +282,14 @@
 
         self.pixel_address += LINES_PER_ROW
         self.wrap_address()
+        self.x_next = self.x + 1
 
     def next_vertical(self):
 
         "Reset horizontal state within the active region of the frame."
 
         self.y += 1
-        self.x = 0
+        self.x_next = 0
 
         if self.inside_frame():
             self.line += 1
@@ -420,21 +422,12 @@
         self.negedge_ram()
         self.negedge_state()
         self.negedge_pixel()
+        self.x = self.x_next
 
     def negedge_video(self):
 
         "Video signalling."
 
-        # Detect the end of the scanline.
-
-        if self.x == MAX_SCANPOS:
-            self.next_vertical()
-
-        # Detect the end of the frame.
-
-        if self.y == MAX_SCANLINE:
-            self.next_frame()
-
         # Detect any sync conditions.
 
         if self.x == 0:
@@ -510,11 +503,6 @@
 
         elif self.cycle == 128:
 
-            # Advance to the next column even if an NMI is asserted.
-
-            if self.would_access_ram():
-                self.next_horizontal()
-
             # If the ULA accessed RAM, indicate that a read needs completing.
 
             if self.access_ram():
@@ -522,10 +510,28 @@
 
     def negedge_state(self):
 
-        "Start a new cycle."
+        "Start a new cycle and update the pixel counters."
+
+        # Detect the end of the scanline.
+
+        if self.x + 1 == MAX_SCANPOS:
+
+            # Detect the end of the frame.
+
+            if self.y + 1 == MAX_SCANLINE:
+                self.next_frame()
+            else:
+                self.next_vertical()
+
+        # Advance to the next column even if an NMI is asserted.
+
+        elif self.cycle == 128 and self.would_access_ram():
+            self.next_horizontal()
+
+        else:
+            self.x_next = self.x + 1
 
         self.cycle = rotate(self.cycle, 1)
-        self.x += 1
 
     def negedge_pixel(self):