Improve GPU state machine.

This fixes the glitch in dma_demo.gba where the rendering of the circle
was off by a scanline.


Former-commit-id: 907fefd548b6557ce46e06a99d0bc6ab83a8f332

src/core/gpu/mod.rs

@@ -16,6 +16,7 @@ pub use regs::*;
 pub const VRAM_ADDR: Addr = 0x0600_0000;
 pub const DISPLAY_WIDTH: usize = 240;
 pub const DISPLAY_HEIGHT: usize = 160;
+pub const VBLANK_LINES: usize = 68;
 
 const CYCLES_PIXEL: usize = 4;
 const CYCLES_HDRAW: usize = 960;
@@ -445,6 +446,17 @@ impl Gpu {
         &self.frame_buffer.0
     }
 
+    fn update_vcount(&mut self, value: usize, irqs: &mut IrqBitmask) {
+        self.current_scanline = value;
+        let vcount_setting = self.dispstat.vcount_setting();
+        self.dispstat
+            .set_vcount_flag(vcount_setting == self.current_scanline as u16);
+
+        if self.dispstat.vcount_irq_enable() && self.dispstat.get_vcount_flag() {
+            irqs.set_LCD_VCounterMatch(true);
+        }
+    }
+
     // Returns the new gpu state
     pub fn step(
         &mut self,
@@ -454,57 +466,54 @@ impl Gpu {
     ) -> Option<GpuState> {
         self.cycles += cycles;
 
-        if self.dispstat.vcount_setting() != 0 {
-            self.dispstat
-                .set_vcount(self.dispstat.vcount_setting() == self.current_scanline as u16);
-        }
-        if self.dispstat.vcount_irq_enable() && self.dispstat.get_vcount() {
-            irqs.set_LCD_VCounterMatch(true);
-        }
-
         match self.state {
             HDraw => {
                 if self.cycles > CYCLES_HDRAW {
-                    self.current_scanline += 1;
                     self.cycles -= CYCLES_HDRAW;
-
-                    if self.current_scanline < DISPLAY_HEIGHT {
-                        self.render_scanline(sb);
                     // HBlank
-                        self.dispstat.set_hblank(true);
+                    self.dispstat.set_hblank_flag(true);
                     if self.dispstat.hblank_irq_enable() {
                         irqs.set_LCD_HBlank(true);
                     };
                     self.state = HBlank;
                     return Some(HBlank);
-                    } else {
-                        self.dispstat.set_vblank(true);
-                        if self.dispstat.vblank_irq_enable() {
-                            irqs.set_LCD_VBlank(true);
-                        };
-                        self.state = VBlank;
-                        return Some(VBlank);
-                    };
                 }
             }
             HBlank => {
                 if self.cycles > CYCLES_HBLANK {
                     self.cycles -= CYCLES_HBLANK;
+
+                    self.dispstat.set_hblank_flag(false);
+                    self.update_vcount(self.current_scanline + 1, irqs);
+
+                    if self.current_scanline < DISPLAY_HEIGHT {
+                        self.render_scanline(sb);
                         self.state = HDraw;
-                    self.dispstat.set_hblank(false);
-                    self.dispstat.set_vblank(false);
                         return Some(HDraw);
+                    } else {
+                        self.state = VBlank;
+                        self.dispstat.set_vblank_flag(true);
+                        if self.dispstat.vblank_irq_enable() {
+                            irqs.set_LCD_VBlank(true);
+                        };
+                        return Some(VBlank);
+                    }
                 }
             }
             VBlank => {
-                if self.cycles > CYCLES_VBLANK {
+                if self.cycles > CYCLES_SCANLINE {
-                    self.cycles -= CYCLES_VBLANK;
+                    self.cycles -= CYCLES_SCANLINE;
-                    self.state = HDraw;
+
-                    self.dispstat.set_hblank(false);
+                    if self.current_scanline < DISPLAY_HEIGHT + VBLANK_LINES {
-                    self.dispstat.set_vblank(false);
+                        self.update_vcount(self.current_scanline + 1, irqs);
-                    self.current_scanline = 0;
+                        return None;
+                    } else {
+                        self.update_vcount(0, irqs);
+                        self.dispstat.set_vblank_flag(false);
                         self.render_scanline(sb);
-                    return Some(HDraw);
+                        self.state = HDraw;
+                        return Some(self.state);
+                    }
                 }
             }
         }

src/core/gpu/regs.rs

@@ -87,9 +87,9 @@ bitfield! {
     pub struct DisplayStatus(u16);
     impl Debug;
     u16;
-    pub get_vblank, set_vblank: 0;
+    pub get_vblank_flag, set_vblank_flag: 0;
-    pub get_hblank, set_hblank: 1;
+    pub get_hblank_flag, set_hblank_flag: 1;
-    pub get_vcount, set_vcount: 2;
+    pub get_vcount_flag, set_vcount_flag: 2;
     pub vblank_irq_enable, _ : 3;
     pub hblank_irq_enable, _ : 4;
     pub vcount_irq_enable, _ : 5;