Page 344 - Programming Microcontrollers in C

Digital Signal Processor Operations 329

void dot_product(char length, int* xdata, int* ydata)
{
int i,*xp, *yp;
xp=xdata;
yp=ydata;
i=length;
}
Listing 6-8: Sample Parameter Handling Function
The compiled version of the above function is shown below. The
first important instruction is the .even assembly directive that causes
the code to follow to start on an even boundary. Code must be started
at an even address. Note in the program above that three parameters
are passed to the function. After the contents of the X and the D
registers are saved, the stack pointer is decremented by 6 to provide
space for the variables i, *xp, and *yp. The two pointer param­
eters require 16 bits each, and the character parameter needs only 8
bits. The program, in favor of greater speed, will store the variable i
in a 16-bit location even though i is only 8 bits wide. At this time,
the value contained in the stack pointer is transferred into the X reg­
ister. During this transfer, the value will be automatically incremented
by two so that the contents of the X register will be pointed to the last
value stored on the stack rather than to the next empty location on
the stack as is found in the stack pointer.
1 ; Compilateur C pour MC68HC16 (COSMIC-France)
2 .include “macro.h16”
3 .list +
4 .psect _text
5 ; 1 void dot_product(char length, int* xdata, int* ydata)
6 ; 2 {
7 .even
8 _dot_product:
9 pshm x,d
10 ais #-6
11 tsx
12 .set OFST=6
13 ; 3 int i,*xp, *yp;
14 ; 4
15 ; 5 xp=xdata;
16 ldd OFST+8,x
17 std OFST-4,x
18 ; 6 yp=ydata;
19 ldd OFST+10,x