.syntax unified
.thumb
.cpu cortex-m4
.arch armv7e-m
.fpu fpv4-sp-d16
/* Changes from unprivileged to privileged mode. */
.thumb_func
.section .kernel
.global raise_privilege
.type raise_privilege, %function
raise_privilege:
mrs r0, control
bic r0, r0, #1
msr control, r0
dsb
isb
bx lr
this is part of arm assembly code. I can check chip manual to figure out the meaning of the instructions. But I don't know how to figure out the behavior of assembler directives like .thumb_func. What's more, I also don't know how to use this part code, it doesn't' look like regular function. So I don't know how to "call" it.
. are really assembler directives. You can look them up in GAS: ARM machine directives.syntax unified signals the use of unified ARM / Thumb assembly syntax. The concept is explained here and here..thumb_func signals the start of a Thumb mode function for ARM-Thumb interwork. The concept is explained here and here.raise_privilege looks exactly like a void raise_privilege(void) leaf function (i.e. it doesn't call other functions) in C to me. Call it with:bl raise_privilege