Initialize RAM in assembly

Author: jonas-schievink

cortex-m-rt/Cargo.toml

@@ -17,7 +17,6 @@ autoexamples = true
 links = "cortex-m-rt" # Prevent multiple versions of cortex-m-rt being linked
 
 [dependencies]
-r0 = "1.0"
 cortex-m-rt-macros = { path = "macros", version = "=0.6.11" }
 # Note: Do not depend on `cortex-m` here. This crate is used for testing `cortex-m`, so we need to
 # avoid pulling in multiple versions of `cortex-m`.

cortex-m-rt/asm.s

@@ -62,6 +62,38 @@ PreResetTrampoline:
   # set LR to the initial value used by the ARMv7-M (0xFFFF_FFFF)
   ldr r0,=0xffffffff
   mov lr,r0
+
+  # run the pre-init code
+  bl __pre_init
+
+  # initialize .data and .bss memory
+  ldr r0,=__sbss
+  ldr r1,=__ebss
+  ldr r2,=0
+0:
+  cmp r1, r0
+  beq 1f
+  stm r0!, {r2}
+  b 0b
+1:
+
+  # copy to here
+  ldr r0,=__sdata
+  # ...up to here
+  ldr r1,=__edata
+  # copy from here
+  ldr r2,=__sidata
+2:
+  cmp r1, r0
+  beq 3f
+  # load 1 word from r2 to r3, inc r2
+  ldm r2!, {r3}
+  # store 1 word from r3 to r0, inc r0
+  stm r0!, {r3}
+  b 2b
+3:
+
+  # jump to Rust
   b Reset
   .cfi_endproc
   .size PreResetTrampoline, . - PreResetTrampoline

cortex-m-rt/bin/thumbv6m-none-eabi.a

@@ -438,7 +438,6 @@
 #![no_std]
 
 extern crate cortex_m_rt_macros as macros;
-extern crate r0;
 
 use core::fmt;
 use core::sync::atomic::{self, Ordering};
@@ -923,41 +922,12 @@ pub fn heap_start() -> *mut u32 {
 #[doc(hidden)]
 #[link_section = ".vector_table.reset_vector"]
 #[no_mangle]
-#[cfg(not(armv6m))]
-pub static __RESET_VECTOR: unsafe extern "C" fn() -> ! = Reset;
-
-#[doc(hidden)]
-#[link_section = ".vector_table.reset_vector"]
-#[no_mangle]
-#[cfg(armv6m)]
 pub static __RESET_VECTOR: unsafe extern "C" fn() -> ! = PreResetTrampoline;
 
 #[doc(hidden)]
 #[link_section = ".Reset"]
 #[no_mangle]
 pub unsafe extern "C" fn Reset() -> ! {
-    extern "C" {
-
-        // These symbols come from `link.x`
-        static mut __sbss: u32;
-        static mut __ebss: u32;
-
-        static mut __sdata: u32;
-        static mut __edata: u32;
-        static __sidata: u32;
-    }
-
-    extern "Rust" {
-        // This symbol will be provided by the user via `#[pre_init]`
-        fn __pre_init();
-    }
-
-    __pre_init();
-
-    // Initialize RAM
-    r0::zero_bss(&mut __sbss, &mut __ebss);
-    r0::init_data(&mut __sdata, &mut __edata, &__sidata);
-
     #[allow(clippy::match_single_binding)]
     match () {
         #[cfg(not(has_fpu))]
@@ -1038,7 +1008,6 @@ pub enum Exception {
 pub use self::Exception as exception;
 
 extern "C" {
-    #[cfg(armv6m)]
     fn PreResetTrampoline() -> !;
 
     fn NonMaskableInt();