How do I split up a long value (32 bits) into four char variables (8bits) using C?

Question

I have a 32 bit long variable, CurrentPosition, that I want to split up into 4, 8bit characters. How would I do that most efficiently in C? I am working with an 8bit MCU, 80

Answer 1

If You are using an 8 bit MCU shifting a whole 32 bit variable is a bit of work. In this case it's better to read 4 bytes of CurrentPosition using pointer arithmetic. The cast:

unsigned char *p = (unsigned char*)&CurrentPosition;

doesn't change the CurrentPosition, but if You try to write to p[0] You will change the least significant byte of the CurrentPosition. If You want a copy do this:

unsigned char *p = (unsigned char*)&CurrentPosition;
unsigned char arr[4];
arr[0] = p[0];
arr[1] = p[1];
arr[2] = p[2];
arr[3] = p[3];

and work with arr. (If you want most significant byte first change the order in those assignments).

If You prefer 4 variables You can obviously do:

unsigned char CP1 = p[0];
unsigned char CP2 = p[1];
unsigned char CP3 = p[2];
unsigned char CP4 = p[3];