feat(demo): exercise Mode 4 and 5 page switching

src/bitmap_demo.rs

@@ -0,0 +1,75 @@
+//! Original Mode 4/5 page-flipping demos. ARM code copies both images through
+//! BIOS services, samples input, and switches the displayed page during VBlank.
+
+use crate::{
+    affine_demo,
+    graphics_demo::{GraphicsDemo, GraphicsError},
+    input::Buttons,
+    machine::Machine,
+    memory::MemoryError,
+    video::Framebuffer,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u16)]
+pub enum BitmapMode {
+    Mode4 = 4,
+    Mode5 = 5,
+}
+
+/// Debug words: completed updates, signed pan X, signed pan Y.
+pub const BITMAP_STATE: u32 = 0x0200_0000;
+
+pub struct BitmapDemo {
+    runner: GraphicsDemo,
+    mode: BitmapMode,
+}
+impl BitmapDemo {
+    pub fn new(mode: BitmapMode) -> Result<Self, MemoryError> {
+        Ok(Self {
+            runner: GraphicsDemo::with_program(affine_demo::bitmap_program(mode as u16))?,
+            mode,
+        })
+    }
+    pub fn machine(&self) -> &Machine {
+        self.runner.machine()
+    }
+    pub fn frame(
+        &mut self,
+        buttons: Buttons,
+        output: &mut Framebuffer,
+    ) -> Result<usize, GraphicsError> {
+        self.runner
+            .frame_with_control(buttons, output, 0x400 | self.mode as u16)
+    }
+}
+
+pub(crate) fn assets(mode: u16) -> Vec<u8> {
+    let mut palette = [0_u16; 256];
+    palette[0] = 0x4000;
+    for (i, color) in palette.iter_mut().enumerate().skip(1) {
+        *color = ((i & 31) | (((i >> 3) & 31) << 5) | (((i * 3) & 31) << 10)) as u16;
+    }
+    let mut data: Vec<u8> = palette.into_iter().flat_map(u16::to_le_bytes).collect();
+    let mut vram = vec![0_u8; 0x14000];
+    let (width, height) = if mode == 4 { (240, 160) } else { (160, 128) };
+    for page in 0..2 {
+        for y in 0..height {
+            for x in 0..width {
+                let offset = y * width + x;
+                if mode == 4 {
+                    vram[page * 0xa000 + offset] = ((x / 8 + 3 * (y / 8) + page * 64) & 255) as u8;
+                } else {
+                    let color = (((x / 5 + page * 11) & 31)
+                        | (((y / 4 + page * 7) & 31) << 5)
+                        | (((x / 8 + y / 8 + page * 5) & 31) << 10))
+                        as u16;
+                    let address = page * 0xa000 + offset * 2;
+                    vram[address..address + 2].copy_from_slice(&color.to_le_bytes());
+                }
+            }
+        }
+    }
+    data.extend(vram);
+    data
+}