Store vga code and font data in RAM, as flash is too slow

It looks like the pixel generating loop is nearly exhausting the time
budget, and there is no margin left for flash cache misses.

I had to increase the RAM reserved for the BIOS from 16k to 24k.
This might be excessive. Perhaps it is possibe to extract a smaller
selection of really timing critical code and move only that to RAM?

Author: jannic

memory.x

@@ -25,11 +25,11 @@ MEMORY {
     /*
      * This is the bottom of the four striped banks of SRAM in the RP2040.
      */
-    RAM_OS : ORIGIN = 0x20000000, LENGTH = 0x3C000
+    RAM_OS : ORIGIN = 0x20000000, LENGTH = 0x3A000
     /*
      * This is the top of the four striped banks of SRAM in the RP2040.
      */
-    RAM : ORIGIN = 0x2003C000, LENGTH = 16K
+    RAM : ORIGIN = 0x2003A000, LENGTH = 24K
     /*
      * This is the fifth bank, a 4KB block. We use this for Core 0 Stack.
      */

src/main.rs

@@ -1545,6 +1545,7 @@ extern "C" fn time_ticks_per_second() -> common::Ticks {
 /// Called when DMA raises IRQ0; i.e. when a DMA transfer to the pixel FIFO or
 /// the timing FIFO has completed.
 #[interrupt]
+#[link_section = ".data"]
 fn DMA_IRQ_0() {
 	unsafe {
 		vga::irq();

src/vga/font16.rs

@@ -36,6 +36,7 @@ pub static FONT: super::Font = super::Font {
 };
 
 /// Our font data - arranged as 256 glyphs of 1 byte/row x 16 row/glyph.
+#[link_section = ".data"]
 static DATA: [u8; 256 * 16] = [
 	// Char::Null
 	0b0000_0000,

src/vga/font8.rs

@@ -38,6 +38,7 @@ pub static FONT: super::Font = super::Font {
 };
 
 /// Our font data - arranged as 256 glyphs of 1 byte/row x 8 row/glyph.
+#[link_section = ".data"]
 static DATA: [u8; 256 * 8] = [
 	// Char::Null
 	0b0000_0000,

src/vga/mod.rs

@@ -693,6 +693,7 @@ pub fn get_num_scan_lines() -> u16 {
 /// # Safety
 ///
 /// Only run this function on Core 1.
+#[link_section = ".data"]
 unsafe extern "C" fn core1_main() -> u32 {
 	CORE1_START_FLAG.store(true, Ordering::Relaxed);
 
@@ -721,6 +722,7 @@ unsafe extern "C" fn core1_main() -> u32 {
 /// # Safety
 ///
 /// Only call this from the DMA IRQ handler.
+#[link_section = ".data"]
 pub unsafe fn irq() {
 	let dma: &mut super::pac::DMA = match DMA_PERIPH.as_mut() {
 		Some(dma) => dma,
@@ -813,6 +815,7 @@ impl RenderEngine {
 		}
 	}
 
+	#[link_section = ".data"]
 	pub fn poll(&mut self) {
 		if DMA_READY.load(Ordering::Relaxed) {
 			DMA_READY.store(false, Ordering::Relaxed);