firmware: simulable demo

Run:
qemu-system-riscv32 -nographic -machine sifive_u -kernel target/riscv32imc-unknown-none-elf/release/helloworld

firmware/memory.x

@@ -1,5 +1,5 @@
 MEMORY
 {
-  FLASH : ORIGIN = 0x20000000, LENGTH = 16M
-  RAM : ORIGIN = 0x80000000, LENGTH = 16K
+  FLASH : ORIGIN = 0x80000000, LENGTH = 16M
+  RAM : ORIGIN = 0x81000000, LENGTH = 16K
 }

firmware/src/main.rs

@@ -8,5 +8,11 @@ use riscv_rt::entry;
 
 #[entry]
 fn main() -> ! {
-    loop {}
+    let foo = "hello world\n";
+    loop {
+        for c in foo.chars() {
+            let mem = 0x1001_3000 as *mut u8;
+            unsafe { *mem = c as u8 }
+        }
+    }
 }