274 Chapter 9 Implementationof C Procedures

9.4 Loop Statements, Arrays, and Structs

In this section, we show how statements within a loop can be repeated until a condition is met, governed by an expression much like the expressions of the conditional statements. First, we consider an accumulation of variables incremented or decremented in the loop, in different loop constructs. Then we discuss array elements accessed using indexes. Struct elements are accessed using AND, OR, and shift operations. We will access a two-dimensional array using indexes in for loops and a struct in a do while loop.

The for and while statements test the condition before the loop is executed and are useful if, for example, a loop may have to be done 0 times. The do while statement performs the statement once before it tests the condition. See Figure 9.10.

int i;

b. Assembly Language developed from Part (a)

Figure 9.10.For, While, and Do While Loops

In Figure 9.10 a for loop has an initialization expression, shown first in the for list of expressions; a test expression, shown in the middle; and a loop termination expression, shown last:

A while loop has a test expression that is executed before the loop is executed once:

A do while loop has a test expression that is executed after the loop is executed:

Figure 9.11 illustrates nested for loops and the two-dimesional array index addressing mechanism. This example shows how loop statements can themselves be loops, in a nested loop construction, and how optimizing compilers make loops more efficient. The outer for loop, for (i = sum = 0 ; i < 10; i++) is encoded asan initialization:

and by the outer loop termination:

struct spiDevice{

unsigned int spie:l,spe:l,swom:l,mstr:lfcpol:lrcpha:l,ssoeil, lsbf:l;

} *spiPtr = (struct spiDevice *)OxdO; #define spi (*spiPtr)

b. Assembly Language developed from Part (a)

Figure 9.13. A Program Setting and Testing a Bit

The do while statement in Figure 9.13a tests the condition after the loop is executed at least once, but it tests the result of the loop's activities. This is very useful in I/O software because it lets you get data within the statement and test the result in the conditional expression, which is not executed until the statement is executed at least once. See Figure 9.13. The program sets a bit of a struct and tests it repeatedly until it is cleared (by hardware). It is compiled into assembly language shown in Figure 9.13b. The struct definition shown below merely defines accesses using the pointer spiPtr. The elements can be accessed using "arrow" notation. For instance the bit spe can be

The statement do ; while (spi. spe); is implemented with

BRSET 0, X,#6 4 , * - 6 ; wait while bit 6 of location OxDO is 1

Generally, if the bitfield is more than one bit, data to be inserted will have to be shifted to the correct bit position, and masked parts of it are ORed with masked parts of bits in other fields, to be written into the memory. This code looks like the code for statement lui = (lui « 3) + (lui « 1) + Isc - ' 0 ' ; that westudied at theend of Section 9.2. Data read from such a bitfield will have to be shifted and masked in like manner.

4:while (*str) {

5:if(*str == chr) return (str);

6:-M-str;

4:while (*str) {

8:if(*str == chr) return str;

Figure 9.18. The Strchr Procedure

The procedure str chr searches for a character in a null-terminated string. See Figure 9.18. The first argument specifies the string. The second argument is a character. The procedure searches the string for a matching character; if it finds the character, it returns the address of the character in the string, otherwise it retuns a null (0).

We now show strncpy which is used to copy characters from and to a nullterminated string. See Figure 9.19. We show the calling routine for this example to illustrate the passing of more than three arguments. The main procedure calls the strncpy procedure with three arguments. Notice how arguments are pushed in order or their appearance from left to right, so the leftmost string, pushed first, is at 8,SP inside strncpy. You should step through the while loop to see how each C statement is compiled into assembly language. Note, however, that the pointers keep getting reloaded into X and Y registers from their local variable storage locations. You can do a lot better by writing the program in assembler language. But you can use this code, produced by the Hiware C++ compiler, as a starting point for a tightly coded assembler language program.

The procedure strncmp compares characters in two null-terminated strings, specified by the first two arguments, up to a number of characters specified in the third argument. See Figure 9.20. Observe the condition used to execute the while loop. If any of the three conditions are false, the subroutine terminates.