Add zero-init-ram feature

Add the 'zero-init-ram' feature that initializes the RAM with zeros during startup. This is normally
not necessary but might be required on custom hardware. If this step is skipped on such hardware,
reading from memory that was never written to will cause a hard-fault.

Author: Norbert Fabritius

cortex-m-rt/Cargo.toml

@@ -45,6 +45,7 @@ required-features = ["device"]
 device = []
 set-sp = []
 set-vtor = []
+zero-init-ram = []
 
 [package.metadata.docs.rs]
 features = ["device"]

cortex-m-rt/link.x.in

@@ -60,7 +60,9 @@ PROVIDE(__pre_init = DefaultPreInit);
 /* # Sections */
 SECTIONS
 {
-  PROVIDE(_stack_start = ORIGIN(RAM) + LENGTH(RAM));
+  PROVIDE(_ram_start = ORIGIN(RAM) + LENGTH(RAM));
+  PROVIDE(_ram_end = ORIGIN(RAM));
+  PROVIDE(_stack_start = _ram_start);
 
   /* ## Sections in FLASH */
   /* ### Vector table */

cortex-m-rt/src/lib.rs

@@ -514,6 +514,19 @@ cfg_global_asm! {
     "ldr r0, =_stack_start
      msr msp, r0",
 
+    // If enabled, initialize RAM with zeros. This is normally not necessary but might be required
+    // on custom hardware.
+    #[cfg(feature = "zero-init-ram")]
+    "ldr r0, =_ram_end
+     ldr r1, =_ram_start
+     movs r2, #0
+     0:
+     cmp r1, r0
+     beq 1f
+     stm r0!, {{r2}}
+     b 0b
+     1:",
+
     // If enabled, initialise VTOR to the start of the vector table. This is normally initialised
     // by a bootloader when the non-reset value is required, but some bootloaders do not set it,
     // leading to frustrating issues where everything seems to work but interrupts are never
@@ -533,24 +546,24 @@ cfg_global_asm! {
     "ldr r0, =__sbss
      ldr r1, =__ebss
      movs r2, #0
-     0:
+     2:
      cmp r1, r0
-     beq 1f
+     beq 3f
      stm r0!, {{r2}}
-     b 0b
-     1:",
+     b 2b
+     3:",
 
     // Initialise .data memory. `__sdata`, `__sidata`, and `__edata` come from the linker script.
     "ldr r0, =__sdata
      ldr r1, =__edata
      ldr r2, =__sidata
-     2:
+     4:
      cmp r1, r0
-     beq 3f
+     beq 5f
      ldm r2!, {{r3}}
      stm r0!, {{r3}}
-     b 2b
-     3:",
+     b 4b
+     5:",
 
     // Potentially enable an FPU.
     // SCB.CPACR is 0xE000_ED88.