From bcdf8745571675069c6eb29cd740cbb63480dea7 Mon Sep 17 00:00:00 2001
From: agg23 <adam@appcannonsoftware.com>
Date: Wed, 24 Feb 2021 16:59:55 -0800
Subject: [PATCH] Fixed #13: Properly buffer screen transitions to prevent
 memory corruption

---
 main.mfk      | 46 +++++++++++++++++++++++++++-------------------
 ui/screen.mfk | 15 +++++++++++----
 2 files changed, 38 insertions(+), 23 deletions(-)

diff --git a/main.mfk b/main.mfk
index 08c2dea..8e7996e 100644
--- a/main.mfk
+++ b/main.mfk
@@ -54,12 +54,15 @@ void main() {
 
   while(true) {
     // Wait for NMI to complete rendering frame
-    while (new_frame) {}
-
-    sprite_tick()
-    input_and_screen_tick()
-    clear_remaining_oam()
-
+    while (new_frame && not(buffer_screen_draw)) {}
+
+    if (buffer_screen_draw) {
+      screen_draw_tick()
+    } else {
+      sprite_tick()
+      input_and_screen_tick()
+      clear_remaining_oam()
+    }
     // wait_for_sprite0_clear()
 
     new_frame = true
@@ -68,17 +71,6 @@ void main() {
 }
 
 void nmi() {
-  if (buffer_screen_draw) {
-    screen_transfer_tick()
-    nmi_always_run()
-    return
-  }
-
-  if (not(drawing_enabled)) {
-    nmi_always_run()
-    return
-  }
-
   if in_nmi != 0 {
     // Overran NMI
     nmi_fail = 1
@@ -89,6 +81,24 @@ void nmi() {
 
   in_nmi = 1
 
+  nmi_work()
+
+  in_nmi = 0
+}
+
+inline void nmi_work() {
+  if (buffer_screen_blank) {
+    screen_transfer_tick()
+    nmi_always_run()
+    new_frame = false
+    return
+  }
+
+  if (not(drawing_enabled)) {
+    nmi_always_run()
+    return
+  }
+
   if (new_frame) {
     nmi_frame()
 
@@ -96,8 +106,6 @@ void nmi() {
   }
 
   nmi_always_run()
-
-  in_nmi = 0
 }
 
 void nmi_frame() {
diff --git a/ui/screen.mfk b/ui/screen.mfk
index 2f3a7df..8513163 100644
--- a/ui/screen.mfk
+++ b/ui/screen.mfk
@@ -19,6 +19,7 @@ enum Screen {
 }
 
 bool buffer_screen_draw = false
+bool buffer_screen_blank = false
 Screen current_screen
 
 bool screen_tick() {
@@ -47,7 +48,7 @@ void draw_screen() {
 
 void set_screen_forcibly(Screen new_screen) {
   current_screen = new_screen
-  buffer_screen_draw = true
+  buffer_screen_blank = true
 }
 
 void set_screen(Screen new_screen) {
@@ -56,18 +57,24 @@ void set_screen(Screen new_screen) {
   }
 
   current_screen = new_screen
-  buffer_screen_draw = true
+  buffer_screen_blank = true
 }
 
 void screen_transfer_tick() {
   // Wait for NMI to disable rendering and NMIs
-  // Draw new screen
   drawing_enabled = false
-  buffer_screen_draw = false
+  buffer_screen_blank = false
   read_ppu_status()
   ppu_ctrl = standard_ppu_ctrl
   ppu_mask = 0
 
+  buffer_screen_draw = true
+}
+
+void screen_draw_tick() {
+  // NMI has cleared screen, now write it
+  // Draw new screen
+  buffer_screen_draw = false
   ppu_wait_vblank()
 
   clear_buffer()