Page 306 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

9.6 Examples from Character String Procedures 283

3: char *strchr(char *str, int chr){
0000088A 3B PSHD
4: while (*str) {
QOG0088B 200B BRA *+13 ;abs = 0898
5: if(*str == chr) return (str);
0000088D B715 SEX B,X
0000088F AE80 CPX 0,SP
00000891 2711 BEQ *+19 ;abs = 08A4
6: -M-str;
00000893 EE84 LDX 4,SP
00000895 08 INX
00000896 6E84 STX 4,SP
4: while (*str) {
00000898 EE84 LDX 4,SP
0000089A E600 LDAB 0,X
0000089C 26EF BNE *-15 ?abs = 088D
8: if(*str == chr) return str;
0000089E B715 SEX B,X
000008AO AE80 CPX 0,SP
000008A2 2603 BNE *+5 ;abs = 08A7
000008A4 EC84 LDD 4,SP
000008A6 8FC787 CPS #51079
10: }
000008A9 30 PULX
000008AA 3D RTS
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 null-
terminated 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.