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Section 1 / Calling and Returning From Functions
Calling functions, passing parameters to them and receiving back return values is basic to using C and and C++. Calling methods (which are functions connected to classes) is similar but with enough differences to warrant its own discussion to be provided later in the chapter on structs.
Bottom Line Concept
The name of a (non-inline) function is a label to which a branch with link ('bl') can be made.
The bl instruction is stands for Branch with Link. The link concept is what enables a function (or method) to return to the instruction after the function call.
Note: this chapter is only a first pass at functions and parameter passing. To fully explore functions and methods, additional knowledge is required.
A Trivial Function
In C, here is a trivial function:
void func() {
}
The function func() takes no parameters, does nothing and returns nothing.
Here it is in assembly language:
func: ret
Notice that func is a label. The only instruction in the function is ret. Strictly speaking, the assembly language function might more explicitly look like this in C:
void func() {
return;
}
To call this function in C you would do this:
func();
This would be done this way in assembly language:
bl func
Notice that calling a function is a branch. But it is a special branch instruction - branch with link. It is the link that allows the function to return.
Returning Values
LEFT OFF HERE