C：如何更改变量写入的寄存器？

Question

C newbie here, wondering if anyone can point me in the direction of some reading material to solve this problem. I have 2 registers and a variable:

Register->Foo;
Register->Bar;
myVar;

myVar is the output of a math function, and the result gets written repeatedly to Register->Foo for many cycles, then when a button is pressed, myVar gets written repeatedly to Register->Bar for many cycles.

Currently, I'm doing a check every cycle, like so:

if button not pressed:
    write myVar to Register->Foo
else
    write myVar to Register->Bar

But I only really need to do a check whenever the button is pressed, not every instruction cycle. How can I change where myVar points to only on a button press, and have that setting stick for all cycles until the button is pressed again? Is something like this possible?

OutputRegister = myVar // have this run all the time inside a loop

// somewhere else
OutputRegister = Register->Foo or Register->Bar 
// depending on button
// What data structure or method can I use to make this!

Answer 1

You use an extra variable, eg is_assigned . When the button is pressed it's set to zero (meaning "false"), then you change the condition and assignments like this:

if (button_not_pressed && !is_assigned)
{
    if (button_not_pressed)
        Register->Foo = myVar;
    else
        Register->Bar = myVar;

    is_assigned = 1;
}

This means that when button_not_pressed is true, and is_assigned is false, then do the statements inside. As is_assigned is then set to true, the next iteration the condition will be false.

Answer 2

You should keep a pointer to the correct location. That way you can assign to it and change the destination on some other trigger.

For example:

int foo;
int bar;
int * dest;

void on_button_press() {
    dest = &bar;
}

void on_button_release() {
    dest = &foo;
}

void main() {
    for (;;) {
        // code
        *dest = myVar;
        // code
    }
}