From 62966cf7418c73d68870a6f1adb87042a23c2471 Mon Sep 17 00:00:00 2001
From: Patrick Kingston <patrick@pkingston.xyz>
Date: Fri, 13 Feb 2026 20:47:44 -0500
Subject: Get background rewriting/scroll working

---
 main.fab |  60 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 nEs.cfg  |   1 +
 nEs.nes  | Bin 40976 -> 40976 bytes
 3 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/main.fab b/main.fab
index ebbb2a4..8624f9d 100644
--- a/main.fab
+++ b/main.fab
@@ -1,19 +1,73 @@
-fn load_level()
+vars
+  U[8] buf      // A buffer that can store the current and next row
+  U row_a = 0   // We swap back and forth between these two offsets for each row
+  U row_b = 4
+  UU drawing_row_offset = 0// Use this to track which row we're drawing
+  U current_rule = %00011110
+  U next_cell = 0
+
+fn init_screen()
   ppu_reset_addr($2000)
 
-  for UU i = 0; i < 1024; i += 1
+  // Set background tiles
+  for UU i = 0; i < 960; i += 1
     {PPUDATA}(1)
 
+  U i = 0
+  // Set attributes for background
+  for i = 0; i < 64; i+= 1
+    {PPUDATA}(%00011011)
+
+  // Init the buffers to 0
+  for i = 0; i < 8; i += 1
+    buf[i] = 0
+
+fn calc_and_upload_next_row()
+  // "scroll" to the row we're at
+  UU row_start = $2000
+  row_start += (drawing_row_offset * 32)
+  ppu_reset_addr(row_start)
+
+  for UU i = 0; i < 32; i += 1
+    //next_cell = 0 //get_next_gen(i)
+    {PPUDATA}(next_cell)
+
+  drawing_row_offset += 1
+
+  if drawing_row_offset == 30
+    drawing_row_offset = 0
+    if next_cell
+      next_cell = 0
+    else
+      next_cell = 1
+
+nmi main_nmi()
+  ppu_upload_oam_poll_pads(0)
+
+  calc_and_upload_next_row()
+
+  // Turn on rendering sprites and bg
+  {PPUMASK}(PPUMASK_ON | PPUMASK_NO_CLIP)
+
+  ppu_reset_scroll(0, 0)
+
+
 mode main()
+: nmi main_nmi
   // Set the palette:
   palette = example_palette
   ppu_upload_palette()
 
   // Load the background
-  {PPUMASK}(PPUMASK_BG_ON | PPUMASK_NO_CLIP)
+  init_screen()
+
+  // Turn the NMI on
+  {PPUCTRL}(PPUCTRL_NMI_ON)
 
   // Loop forever:
   while true
+    update_pads()
+    nmi
 
 // Define the tileset (commonly called CHR):
 chrrom
diff --git a/nEs.cfg b/nEs.cfg
index cf96c4e..5712d83 100644
--- a/nEs.cfg
+++ b/nEs.cfg
@@ -1,4 +1,5 @@
 mapper = NROM
+mirroring = H
 nesfab-dir = ../nesfab/
 input = main.fab
 input = lib/nes.fab
diff --git a/nEs.nes b/nEs.nes
index 500b066..f9ae3ed 100644
Binary files a/nEs.nes and b/nEs.nes differ
-- 
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