`0xBAAAAAAD` address

Not all the peripheral memory can be accessed. Look at this program (examples/bad.rs).

#![no_main]
#![no_std]

use core::ptr;

#[allow(unused_imports)]
use registers::entry;

#[entry]
fn main() -> ! {
    registers::init();

    unsafe {
        ptr::read_volatile(0x5000_A784 as *const u32);
    }

    loop {}
}

This address is close to the OUT address we used before but this address is invalid, in the sense that there's no register at this address.

Now, let's try it.

$ cargo run
(..)
Resetting and halting target
Target halted
(gdb) continue
Continuing.

Breakpoint 1, registers::__cortex_m_rt_main_trampoline () at src/07-registers/src/main.rs:9
9	#[entry]
(gdb) continue
Continuing.

Program received signal SIGINT, Interrupt.
registers::__cortex_m_rt_main () at src/07-registers/src/main.rs:10
10	fn main() -> ! {
(gdb) continue
Continuing.

Breakpoint 3, cortex_m_rt::HardFault_ (ef=0x2001ffb8) at src/lib.rs:1046
1046	    loop {}
(gdb)

We tried to do an invalid operation, reading memory that doesn't exist, so the processor raised an exception: a hardware exception.

In most cases, exceptions are raised when the processor attempts to perform an invalid operation. Exceptions break the normal flow of a program and force the processor to execute an exception handler, which is just a function/subroutine.

There are different kind of exceptions. Each kind of exception is raised by different conditions and each one is handled by a different exception handler.

The registers crate depends on the cortex-m-rt crate which defines a default hard fault handler, named HardFault_, that handles the "invalid memory address" exception. embed.gdb placed a breakpoint on HardFault; that's why the debugger halted your program while it was executing the exception handler. We can get more information about the exception from the debugger. Let's see:

(gdb) list
1040  #[allow(unused_variables)]
1041	#[doc(hidden)]
1042	#[cfg_attr(cortex_m, link_section = ".HardFault.default")]
1043	#[no_mangle]
1044	pub unsafe extern "C" fn HardFault_(ef: &ExceptionFrame) -> ! {
1045	    #[allow(clippy::empty_loop)]
1046	    loop {}
1047	}
1048	
1049	#[doc(hidden)]
1050	#[no_mangle]

ef is a snapshot of the program state right before the exception occurred. Let's inspect it:

(gdb) print/x *ef
$1 = cortex_m_rt::ExceptionFrame {
  r0: 0x5000a784,
  r1: 0x3,
  r2: 0x2001ff24,
  r3: 0x0,
  r12: 0x1,
  lr: 0x4403,
  pc: 0x43ea,
  xpsr: 0x1000000
}

There are several fields here but the most important one is pc, the Program Counter register. The address in this register points to the instruction that generated the exception. Let's disassemble the program around the bad instruction.

(gdb) disassemble /m ef.pc
Dump of assembler code for function core::ptr::read_volatile<u32>:
1654	pub unsafe fn read_volatile<T>(src: *const T) -> T {
   0x000043d2 <+0>:	push	{r7, lr}
   0x000043d4 <+2>:	mov	r7, sp
   0x000043d6 <+4>:	sub	sp, #16
   0x000043d8 <+6>:	str	r0, [sp, #4]
   0x000043da <+8>:	str	r0, [sp, #8]

1655	    // SAFETY: the caller must uphold the safety contract for `volatile_load`.
1656	    unsafe {
1657	        assert_unsafe_precondition!(
   0x000043dc <+10>:	b.n	0x43de <core::ptr::read_volatile<u32>+12>
   0x000043de <+12>:	ldr	r0, [sp, #4]
   0x000043e0 <+14>:	movs	r1, #4
   0x000043e2 <+16>:	bl	0x43f4 <core::ptr::read_volatile::precondition_check>
   0x000043e6 <+20>:	b.n	0x43e8 <core::ptr::read_volatile<u32>+22>

1658	            check_language_ub,
1659	            "ptr::read_volatile requires that the pointer argument is aligned and non-null",
1660	            (
1661	                addr: *const () = src as *const (),
1662	                align: usize = align_of::<T>(),
1663	            ) => is_aligned_and_not_null(addr, align)
1664	        );
1665	        intrinsics::volatile_load(src)
   0x000043e8 <+22>:	ldr	r0, [sp, #4]
   0x000043ea <+24>:	ldr	r0, [r0, #0]          ; <-- That's the one!
   0x000043ec <+26>:	str	r0, [sp, #12]
   0x000043ee <+28>:	ldr	r0, [sp, #12]

1666	    }
1667	}
   0x000043f0 <+30>:	add	sp, #16
   0x000043f2 <+32>:	pop	{r7, pc}

End of assembler dump.

The exception was caused by the ldr r0, [r0, #0] instruction, a read instruction. The instruction tried to read the memory at the address indicated by the r0 CPU register. By the way, a CPU (processor) register not a memory mapped register; it doesn't have an associated address like, say, OUT.

Wouldn't it be nice if we could check what the value of the r0 register was right at the instant when the exception was raised? Well, we already did! The r0 field in the ef value we printed before is the value of r0 register had when the exception was raised. Here it is again:

(gdb) print/x *ef
$1 = cortex_m_rt::ExceptionFrame {
  r0: 0x5000a784,
  r1: 0x3,
  r2: 0x2001ff24,
  r3: 0x0,
  r12: 0x1,
  lr: 0x4403,
  pc: 0x43ea,
  xpsr: 0x1000000
}

r0 contains the value 0x5000_A784 which is the invalid address we called the read_volatile function with.

Discovery

0xBAAAAAAD address

`0xBAAAAAAD` address