print syscall

abi/src/lib.rs

@@ -3,6 +3,16 @@
 use abi_sys::{Syscall, call_abi};
 use shared::keyboard::{KeyCode, KeyEvent, KeyState, Modifiers};
 
+pub fn print(msg: &str) {
+    let syscall = Syscall::Print {
+        msg: msg.as_ptr(),
+        len: msg.len(),
+    };
+    unsafe {
+        call_abi(&syscall);
+    }
+}
+
 pub mod display {
     use crate::{Syscall, call_abi};
     use embedded_graphics::{

abi_sys/src/lib.rs

@@ -25,4 +25,8 @@ pub enum Syscall {
         pixels: *const Pixel<Rgb565>,
         len: usize,
     },
+    Print {
+        msg: *const u8,
+        len: usize,
+    },
 }

kernel/src/abi.rs

@@ -28,5 +28,15 @@ pub extern "C" fn call_abi(call: *const Syscall) {
                 .draw_iter(slice.iter().copied())
                 .unwrap();
         }
+        Syscall::Print { msg, len } => {
+            // SAFETY: we're trusting the user program here
+            let slice = unsafe { core::slice::from_raw_parts(*msg, *len) };
+
+            if let Ok(str) = str::from_utf8(slice) {
+                defmt::info!("{:?}", str);
+            } else {
+                defmt::error!("Failed to parse user print str")
+            }
+        }
     }
 }

user-apps/calculator/src/main.rs

@@ -1,7 +1,7 @@
 #![no_std]
 #![no_main]
 
-use abi::display::Display;
+use abi::{display::Display, print};
 use core::panic::PanicInfo;
 use embedded_graphics::{
     Drawable,
@@ -19,6 +19,7 @@ fn panic(_info: &PanicInfo) -> ! {
 
 #[unsafe(no_mangle)]
 pub extern "C" fn _start() {
+    print("Starting Calculator app");
     let mut display = Display;
 
     let character_style = MonoTextStyle::new(&FONT_6X10, Rgb565::RED);