C Language Call Assembly

The entry for the program is main, which calls the assembly function in main.

The first thing to do is to solve the problem of defining functions

In the C language, to extern a function declaration, and then this function in the assembly is implemented.

In the Assembly, use export to introduce the function name defined in the C language, and then start writing the segment where the function name begins.

An example is a function that adds six numbers.

C Language:

 #include <stdio.h>extern  int  sum (int  a,int  b,int  c,int  d,int  e,int   f);  int   Main () { int  result = SUM (1 , 2 , 3 , 4 , 5 , 6         );  return  0     

Assembly Code:

Area     example,code,readonly        EXPORT    sum        ENTRY        sum        add r0,r0,r1        add R2,R2,R3        ADD r0,r0,r2                LDR    r4,[sp]        ldr r5,[sp,#4]                add r4,r4,r5        add r0,r4,r0                BX LR                END 

In fact, the key problem is the parameter and the return value.

We can see that C calls this function, and C gives it a parameter.

How does a compilation accept these parameters?

4 or less parameters, stored directly in the R0~R3 4 registers inside.

4 later parameters are placed in the stack.

If the function has a return value, then the return value is placed in the R0.

Debug See how the compiler handles it, and we'll take the parameters out.

It's easier to understand.

First,

R0 =6

R1=5

R2=3

R3=4

Then put the value of the R1 on the stack, r13=5, and then r1=2

Then the value of the R0 is placed in the next position of the stack, r13+4=6, and then r0=1.

After that:

R0=1,r1=2,r2=3,r3=4

Inside the stack:

So the time to take the parameters is:

LDR    R4,[SP]; r4=5LDR r5,[sp,#4]; r5=6

Put the added result in R0, then BX LR returns

can see:

You can verify that the R0 is the one that holds the return value.

C Language Call Assembly