ARM bare metal program development (10): C language environment initialization

Part 1 Stack Initialization

1. Full/Empty Stack

1.1 Full stack: When the stack pointer SP always points to the last data pushed into the stack (used by ARM)

1.2 Empty stack: When the stack pointer SP always points to the next empty location where data will be placed

2. Stack Up/Down

According to the moving direction of the SP pointer when pushing into the stack: if it is from low to high, it is stacking up, and from high to low, it is stacking down (used by ARM)

3. Stack Frame

Definition: The part of the stack used by a function is called a stack frame

Two boundaries of the stack frame: fp(r11) and sp(r13)

3. Stack Function

3.1 Saving local variables

3.2 Passing parameters: When the number of parameters to be passed is greater than 4, use the stack to pass them, otherwise, use general registers to pass them

3.3 Passing Register Values

Part 2 BSS segment 1 initialization

Part 3 Jump from assembly to C

Jump mode: absolute jump

Part 4 C and assembly mixed programming

1. Where to use assembly: 1. Where efficiency is high; 2. Where direct hardware operation is required (such as coprocessor operation)

2. Calling C in assembly: directly assign the entry pointer of C to PC

(C code is in a separate C file)

3. Calling assembly from C: Declare the label as .global in the assembly file, and then call it directly as a function in C.

(Assembly code is in a separate file)

IV. Embedded Assembly in C

4.1 Format

__asm__(

Assembly statement part

: Output section

: Input part

: Destruction description section

)；

//Example 1

void write_p15_c1(unsigned long value)

{

    __asm__(

“mcr p15, 0, %0, c1, c0, 0n” @%0 is a placeholder parameter

:

:"r" (value) @r indicates a general register

);

}

//Example 2

usigned long read_p15_c1 (void)

{

  usigned long value

__asm__(

  "mrc p15, 0, %0, c1, c0, 0n" The value read from @c1 is sent to Value

    ："=r" (value) @'=' indicates a write-only operand, used for output

    ：

    ：”memory”

);

return value;

}

4.2 Optimization

In programming, volatile is used to tell the compiler not to optimize the following part of the code (especially when operating on hardware)