Page 172 - Programming Microcontrollers in C

Microcontroller Memory 157

operations. The program can arbitrarily change the stack pointer value
so that room for automatic variables can be easily provided or elimi­
nated. In the small microcontrollers, automatic variables are stored
in RAM and their scope is not limited to the block in which they are
defined. Their access is limited to their block, however. Consider the
following code segment:
main()
{
int i;
.
.
.
}

void able(void)
{
int i;
.
.
.
}
The two occurrences of the variable i in this case will cause no
trouble because each i will be given a unique location in RAM and
the scoping arrangement will insure that any reference to iin main()
will not be confused with the i in able() and vice versa.
An important implication of this change in storage: recursion is
no longer available! Only one memory location is available for each
variable in the program. When a stack is used to store automatic
variables, a function can call itself and a new block is created each
time the function is entered. Thus, each time a function calls itself, a
new stack frame that contains space for all automatic storage in the
function is created. The function can call itself repeatedly as long as
there is space on the stack to create new stack frames for the succes­
sive calls. Without stack space for variable storage, recursion is
impossible.
A second limitation that occurs is in the available arguments for
function calls. The compiler C6805 for the M68HC05 family de­
fines an int as an 8-bit number and a long as a 16-bit number.