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Booting Raspberry Pi 3: Difference between revisions
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→Booting the Kernel
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Line 100:
_start:
mrs
and
cbz
0: wfe
b 0b
</source>
The mrs instruction loads data from specialty registers into a standard register. This special register in question is called the MPIDR or the Multiprocessor Affinity Register. Caring only about the first two bits only we use a bitwise and to weed out any core ID's that aren't zero. Then we apply the cbz instruction, which is a shorthand instruction for comparing the
If the core ID is 0, we branch to another section that will initialize the environment and will hand control over to the kernel. For now, the C function for our kernel will simply by kern_main.
<source lang="asm">
_init: ldr x4, =_start
mov sp, x4
b kern_main
</source>
Note that with the latest firmware, this is not necessary, as only the primary core runs. It is enough to set up stack and branch to kern_main.
<source lang="asm">
// for latest firmware, after 2020-01-01
.section .boot
.global _start
_start: ldr x4, =_start
mov sp, x4
b kern_main
</source>
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