Architecture and Instruction Set of the C6x Processor - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 113

// Dotp4clasm.c Multiplies two arrays using C calling linear ASM func

short dotp4clasmfunc(short *a,short *b,short ncount); //prototype
#include <stdio.h> //for printing statement
#include "dotp4.h" //arrays of data values
#define count 4 //number of data values
short x[count] = {x_array}; //declare 1st array

short y[count] = {y_array}; //declare 2nd array

volatile int result = 0; //result

main()
{
result = dotp4clasmfunc(x,y,count); //call linear ASM func
printf("result = %d decimal \n", result); //print result
}

FIGURE 3.21. C program calling a linear ASM function to find the sum of products
( dotp4clasm.c ).

; Dotp4clasmfunc.sa Linear assembly function to multiply two arrays
.ref _dotp4clasmfunc ;ASM func called from C
_dotp4clasmfunc: .cproc ap,bp,count ;start section linear ASM
.reg a,b,prod,sum ;asm optimizer directive
zero sum ;init sum of products
loop: ldh *ap++,a ;pointer to 1st array->a
ldh *bp++,b ;pointer to 2nd array->b
mpy a,b,prod ;product = a*b
add prod,sum,sum ;sum of products -->sum
sub count,1,count ;decrement counter
[count] b loop ;loop back if count # 0

.return sum ;return sum as result

.endproc ;end linear ASM function

FIGURE 3.22. Linear ASM function called from C to find the sum of products
( dotp4clasmfunc.sa ).

Functional units are optional as in an assembly-coded program. Registers a , b ,
prod , and sum are defined by the linear assembler directive . reg . The addresses
of the two arrays x and y and the size of the array (count) are passed to the linear
assembly function through the registers ap , bp , and count . Both ap and bp are
registers used as pointers, as in C code. The instruction field is seen to be as in an
assembly-coded program, and the subsequent field uses a syntax as in C program-
ming. For example, the instruction

loop: ldh *ap++,a

(the first time through the loop section of code) loads the content in memory, whose
address is specified by register ap , into register a . Then the pointer register ap is
postincremented to point to the next higher memory address, pointing at the