Merge #213

213: add comment on where to look to view 'Hello World' message r=jrvanwhy a=JOE1994

This PR adds a one helping comment to the example program **examples/hello_world.rs**.
The comment tells where to look to see the "Hello World" message printed out.

I thought adding this line would be helpful to newcomers like me :smile_cat: 

p.s. 
Is running `tockloader listen` the canonical way to view messages printed to the console?

Thank you for reviewing this PR :) :cake: 

Co-authored-by: JOE1994 <joseph942010@gmail.com>

core/src/entry_point/mod.rs

diff --git a/core/src/entry_point/mod.rs b/core/src/entry_point/mod.rs
index 2ab2351..850c213 100644
--- a/core/src/entry_point/mod.rs
+++ b/core/src/entry_point/mod.rs
@@ -104,10 +104,8 @@
     );
 
     // Zero .bss (specified by the linker script).
-    let bss_end = layout_header.bss_start + layout_header.bss_size; // 1 past the end of .bss
-    for i in layout_header.bss_start..bss_end {
-        core::ptr::write(i as *mut u8, 0);
-    }
+    let bss_start = layout_header.bss_start as *mut u8;
+    core::ptr::write_bytes(bss_start, 0u8, layout_header.bss_size);
 
     // TODO: Wait for rustc to have working ROPI-RWPI relocation support, then
     // implement dynamic relocations here. At the moment, rustc does not have

layout_generic.ld

diff --git a/layout_generic.ld b/layout_generic.ld
index 35b10f4..dd101ab 100644
--- a/layout_generic.ld
+++ b/layout_generic.ld
@@ -88,6 +88,15 @@
     /* Application stack */
     .stack (NOLOAD) :
     {
+        /* elf2tab requires that the `_sram_origin` symbol be present to
+         * mark the first address in the SRAM memory. Since ELF files do
+         * not really need to specify this address as they only care about
+         * loading into flash, we need to manually mark this address for
+         * elf2tab. elf2tab will use it to add a fixed address header in the
+         * TBF header if needed.
+         */
+        _sram_origin = .;
+
         . = . + STACK_SIZE;
 
 	_stack_top_unaligned = .;