# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1330294304 -3600
# Node ID 726152e626d558fd3336c7ecdc2f0a46018cc8d6
# Parent  99b0ecdba781b21bdf32c25df7f1a77c33b26b96
Simplified the logic slightly around reading and displaying pixels.

diff -r 99b0ecdba781 -r 726152e626d5 ula.py
--- a/ula.py	Mon Feb 13 22:00:09 2012 +0100
+++ b/ula.py	Sun Feb 26 23:11:44 2012 +0100
@@ -18,7 +18,7 @@
 MAX_PIXELLINE = MIN_PIXELLINE + MAX_HEIGHT
 
 MAX_HSYNC = 75          # the number of cycles in each hsync period
-MIN_PIXELPOS = 264      # the first cycle involving pixel generation
+MIN_PIXELPOS = 256      # the first cycle involving pixel generation
 MAX_SCANPOS = 1024      # the number of cycles in each scanline
 
 MAX_PIXELPOS = MIN_PIXELPOS + MAX_WIDTH
@@ -73,7 +73,7 @@
 
     def update(self):
         if MIN_PIXELLINE <= self.y < MAX_PIXELLINE:
-            if MIN_PIXELPOS <= self.x < MAX_PIXELPOS:
+            if MIN_PIXELPOS + 8 <= self.x < MAX_PIXELPOS + 8:
                 self.screen[self.pos] = self.colour[0]; self.pos += 1
                 self.screen[self.pos] = self.colour[1]; self.pos += 1
                 self.screen[self.pos] = self.colour[2]; self.pos += 1
@@ -158,6 +158,8 @@
         self.access = 0         # counter used to determine whether a byte needs reading
         self.ram_address = 0    # address given to the RAM
         self.data = 0           # data read from the RAM
+        self.have_pixels = 0    # whether pixel data has been read
+        self.writing_pixels = 0 # whether pixel data can be written
         self.buffer = [0] * 8   # pixel buffer for decoded RAM data
 
         self.reset_vertical()
@@ -277,8 +279,7 @@
 
     def in_frame(self): return MIN_PIXELLINE <= self.y < MAX_PIXELLINE
     def inside_frame(self): return MIN_PIXELLINE < self.y < MAX_PIXELLINE
-    def read_pixels(self): return MIN_PIXELPOS - 8 <= self.x < MAX_PIXELPOS - 8 and self.in_frame()
-    def make_pixels(self): return MIN_PIXELPOS <= self.x < MAX_PIXELPOS and self.in_frame()
+    def read_pixels(self): return MIN_PIXELPOS <= self.x < MAX_PIXELPOS and self.in_frame()
 
     def update(self):
 
@@ -312,6 +313,14 @@
             if access_ram:
                 self.ram_address = (self.address & 0xff80) >> 7
 
+            # Initialise the pixel buffer if appropriate.
+
+            if not self.writing_pixels and self.have_pixels:
+                self.xcounter = self.xscale
+                self.buffer_index = 0
+                self.fill_pixel_buffer()
+                self.writing_pixels = 1
+
         # Latch row address, set column address (for ULA access only).
 
         elif self.cycle == 1:
@@ -373,6 +382,7 @@
 
             if access_ram:
                 self.data = self.data | self.ram.data
+                self.have_pixels = 1
 
                 # Advance to the next column.
 
@@ -418,34 +428,27 @@
 
         # Detect spacing between character rows.
 
-        if not self.make_pixels() or self.ssub:
+        if not self.writing_pixels or self.ssub:
             self.video.colour = BLANK
 
         # For pixels within the frame, obtain and output the value.
 
         else:
-            # Detect the start of the pixel generation.
 
-            if self.x == MIN_PIXELPOS:
-                self.xcounter = self.xscale
-                self.buffer_index = 0
-                self.fill_pixel_buffer()
+            self.xcounter -= 1
+            self.video.colour = self.buffer[self.buffer_index]
 
             # Scale pixels horizontally, only accessing the next pixel value
             # after the required number of scan positions.
 
-            elif self.xcounter == 0:
+            if self.xcounter == 0:
                 self.xcounter = self.xscale
                 self.buffer_index += 1
 
-                # Fill the pixel buffer, assuming that data is available.
+                # Handle the buffer empty condition.
 
                 if self.buffer_index >= self.buffer_limit:
-                    self.buffer_index = 0
-                    self.fill_pixel_buffer()
-
-            self.xcounter -= 1
-            self.video.colour = self.buffer[self.buffer_index]
+                    self.writing_pixels = 0
 
         self.x += 1