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The bootloader is the program that runs in an embedded processor immediately after a reset of the processor and is responsible for initializing the connected devices an..."

2017-05-03T23:06:24Z

Created page with " The bootloader is the program that runs in an embedded processor immediately after a reset of the processor and is responsible for initializing the connected devices an..."

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The bootloader is the program that runs in an embedded processor immediately after a reset of the processor and is responsible for initializing the connected devices and starting the main software (for example, the kernel of an OS). The ARM bootloader defines the '''vector table''' of the computing environment.
 
 
Assume that the main program is coded in C and the entry function is called ''entry'' as shown below:
<pre class="code">
int entry(void)
{
return 0;
}
</pre>
which is compiled for the ARM Cortex-M0 (ARMv6-M architecture) target like this:
<pre class="terminal">
arm-none-eabi-gcc -c -mcpu=cortex-m0 hello_world.c -o hw-entry.o
</pre>
 
We also create an assembly file (hw-startup.s) that specifies the vector table, including the reset handler:
<pre class="code">
/* Vector table for bootloader. Assemble via
arm-none-eabi-as -mcpu=cortex-m0 hw-startup.s -o hw-startup.o
*/
.section INTERRUPT_VECTOR, "x"
.global _Reset
_Reset:
b Reset_Handler /* Reset */
b . /* Undefined */
b . /* SWI */
b . /* Prefetch Abort */
b . /* Data Abort */
b . /* Reserved */
b . /* IRQ */
b . /* FIQ */

Reset_Handler:
/* In Thumb mode we can not load into SP directly.
Only into the eight low registers */
ldr r1, =stack_top
mov sp, r1
/* In ARM mode we can simply write
ldr sp, =stack_top
*/
bl entry
b .
</pre>
and assemble like this:
<pre class="terminal">
arm-none-eabi-as -mcpu=cortex-m0 hw-startup.s -o hw-startup.o
</pre>
 
The object files are linked into an executable Linux binary in ELF format like this:
<pre class="terminal">
arm-none-eabi-ld -T hw-boot.ld hw-entry.o hw-startup.o -o hello_world.elf
</pre>
where ''hw-boot.ld'' is the linker file that defines where the code from the object files will be placed into the executable:
<pre class="code">
ENTRY(_Reset)
SECTIONS
{
. = 0x0;
.text : {
hw-startup.o (INTERRUPT_VECTOR)
*(.text)
}
.data : { *(.data) }
.bss : { *(.bss COMMON) }
. = ALIGN(8);
. = . + 0x100; /* allocating 4 KB for stack memory */
stack_top = .;
}
</pre>
The ELF file can be inspected with the ''readelf'' tool like this:
<pre class="terminal">
arm-none-eabi-readelf -A hello_world.elf
Attribute Section: aeabi
File Attributes
Tag_CPU_name: "Cortex-M0"
Tag_CPU_arch: v6S-M
Tag_CPU_arch_profile: Microcontroller
Tag_THUMB_ISA_use: Thumb-1
Tag_ABI_PCS_wchar_t: 4
Tag_ABI_FP_denormal: Needed
Tag_ABI_FP_exceptions: Needed
Tag_ABI_FP_number_model: IEEE 754
Tag_ABI_align_needed: 8-byte
Tag_ABI_enum_size: small
</pre>
and disassembled via ''objdump'' like this:
<pre class="terminal">
arm-none-eabi-objdump -d hello_world.elf

hello_world.elf: file format elf32-littlearm

Disassembly of section .text:

00000000 <_Reset>:
0: e006 b.n 10 <Reset_Handler>
2: e7fe b.n 2 <_Reset+0x2>
4: e7fe b.n 4 <_Reset+0x4>
6: e7fe b.n 6 <_Reset+0x6>
8: e7fe b.n 8 <_Reset+0x8>
a: e7fe b.n a <_Reset+0xa>
c: e7fe b.n c <_Reset+0xc>
e: e7fe b.n e <_Reset+0xe>

00000010 <Reset_Handler>:
10: 4902 ldr r1, [pc, #8] ; (1c <Reset_Handler+0xc>)
12: 468d mov sp, r1
14: f000 f804 bl 20 <entry>
18: e7fe b.n 18 <Reset_Handler+0x8>
1a: 0000 .short 0x0000
1c: 00000130 .word 0x00000130

00000020 <entry>:
20: b580 push {r7, lr}
22: af00 add r7, sp, #0
24: 2300 movs r3, #0
26: 0018 movs r0, r3
28: 46bd mov sp, r7
2a: bd80 pop {r7, pc}
</pre>
 
Finally, ''objcopy'' is used to remove the ELF header and convert the file into an executable that can be run as-is on the Cortex-M0.
<pre class="terminal">
arm-none-eabi-objcopy -O binary hello_world.elf hello_world.bin
</pre>

ARM Bootloader - Revision history

Kai: Created page with " The bootloader is the program that runs in an embedded processor immediately after a reset of the processor and is responsible for initializing the connected devices an..."

Kai: Created page with "
The bootloader is the program that runs in an embedded processor immediately after a reset of the processor and is responsible for initializing the connected devices an..."