arm汇编函数返回值(arm汇编函数调用过程)

Introduction

Arm assembly language is widely used for coding applications that require faster execution times. In programming, it is common to pass arguments to a function, but sometimes we also need to return some value from the function to the calling code. In Arm assembly language, returning a value from a function can be done in several ways. In this article, I will discuss the common methods for returning values from Arm assembly functions.

Returning a value in a register

One way to return a value from a function is to put the returned value in a register. The most common register for this purpose is R0. After executing the function, the value in R0 is automatically passed back to the calling code. For instance, consider the following function to return the sum of two integers:

```
.globl sum
sum:
add r0, r0, r1
bx lr
```

In this example, the R0 register holds the sum of r0 and r1. The function then finishes by using the 'bx lr' instruction to return control to the calling code. When the function returns to the calling code, the value in R0 is passed back to the caller.

Returning a value in memory location

Another way to return a value from a function is to store it in a memory location. This method is recommended when returning large or complex data types, such as structures or arrays. To do this, the memory location must be passed as an argument to the function using one of the registers R0-R3. For instance, consider the following example:

```
.globl returnArraySum
returnArraySum:
mov r3, r0
ldr r0, [r1]
ldr r1, [r2]
add r0, r0, r1
str r0, [r3]
bx lr
```

In this example, the function takes three arguments- a pointer to the memory location where the result will be stored, and two pointers to the array elements to be added. The function then performs the addition and stores the result in the memory location pointed to by the first argument.

Conclusion

Returning a value from a function is an essential concept in programming. In Arm assembly language, there are mainly two ways of returning a value- by returning it in a register or storing it in a memory location. It is essential to choose the right method for returning values based on the data type returned, the function call overheads, and the complexity of the function. By considering these factors, one can optimize their Arm assembly code for better performance.