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src/main.rs

@@ -14,8 +14,12 @@ mod uart;
 unsafe extern "C" fn _enter() -> ! {
     use core::arch::asm;
     asm!(
+        // load hartid into t0; if t0 =/= 0, then jump to busy loop
+        "csrr t0, mhartid",
+        "bnez t0, 3f",
+
         // before we use the `la` pseudo-instruction for the first time,
-        // we need to set `gp` (google linker relaxation)
+        // we need to set `gp` (look up linker relaxation)
         ".option push", // pushes the current option stack
         ".option norelax", // disable relaxation so we can properly set `gp`
         // this instruction compiles to:
@@ -38,7 +42,14 @@ unsafe extern "C" fn _enter() -> ! {
         "bne t0, t1, 1b",
     "2:",
         // BSS is clear!
-
+        "j 4f",
+    
+    // busy loop if hartid =/= 0
+    "3:",
+        "wfi", // wait for interrupt (or nop)
+        "j 3b",
+    
+    "4:",
         // "tail-call" to {start} (call without saving a return address)
         "tail {start}",
         start = sym start, // {start} refers to the function [start] below
@@ -62,8 +73,10 @@ extern "C" fn start() -> ! {
     unsafe { uart::init_console(0x1000_0000) };
     println!("Hello, world!");
 
+    // print current hartid
     println!("hartid: {}", riscv::register::mhartid::read());
 
+    // poll console for input and print characters back
     loop {
         let c = uart::CONSOLE.lock().as_mut().and_then(uart::Device::get);
         if let Some(c) = c {
@@ -74,6 +87,7 @@ extern "C" fn start() -> ! {
 
 #[panic_handler]
 fn on_panic(info: &PanicInfo) -> ! {
+    // print panic info and hang
     println!("{}", info);
 
     loop {}