Page 99 - Programming Microcontrollers in C

84 Chapter 2 Advanced C Topics

3. Write a function that will calculate the vector product of two 1 by
4 matrices. The vector product of two arrays is the sum of the
products of the corresponding elements of the two arrays.

4. Write a function that will calculate the matrix product of two 4 by
4 matrices. The matrix product is a matrix whose i,j element is
the vector product of the ith row of the first matrix and the jth
column of the second matrix. As such, the product of two 4 by 4
matrices is a 4 by 4 matrix.

C has pointers to functions. A common use for pointers for func­
tions is in creating vector tables for microcontrollers. Most
microcontroller applications involve the use of interrupts. When an
interrupt occurs, the machine status is saved, and program control is
transferred to an interrupt service routine. At the close of the inter­
rupt service routine, the machine status is returned to the earlier
condition, and control is returned to the interrupted program. An
address table in memory called the vector table contains the addresses
of each interrupt service routine. It is the programmer’s responsibil­
ity to fill the vector table with the proper addresses for the various
interrupt service routines. Pointers to functions allow the program­
mer access to the addresses of the interrupt service routines. We will
see several different methods to create the vector tables in the chap­
ters on the individual microcontrollers that follow.
A pointer to a function is identified by
int (*function_ptr)();

The above declaration says that function_ptr is a pointer to a
function that returns a type int. The arguments are not declared here.
The parentheses surrounding *function_ptr are required. If they
were not included, the declaration would declare function_ptr as
a function that returns a pointer to the type int. If function_ptr
is a valid pointer to a function, the function can be accessed by

(*function_ptr)(args);
The above declaration form can be used for arrays as well as
functions. The declaration
char (*array_ptr)[];