Architecture and Instruction Set of the C6x Processor - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 105

// Factorial.c Finds factorial of n. Calls function factfunc

#include <stdio.h> //for print statement

void main()
{
short n = 7; //set value
short result; //result from asm function
result = factfunc(n); //call ASM function factfunc
printf("factorial = %d", result);//print result from asm function
}

FIGURE 3.10. C program that calls an ASM function to find the factorial of a number
( factorial.c ).

; Factfunc.asm Assembly function called from C to find factorial

.def _factfunc ;ASM function called from C

_factfunc: MV A4,A1 ;setup loop count in A1

SUB A1,1,A1 ;decrement loop count

LOOP: MPY A4,A1,A4 ;accumulate in A4

NOP ;for 1 delay slot with MPY

SUB A1,1,A1 ;decrement for next multiply

[A1] B LOOP ;branch to LOOP if A1 # 0

NOP 5 ;five NOPs for delay slots

B B3 ;return to calling routine

NOP 5 ;five NOPs for delay slots

.end
FIGURE 3.11. ASM function called from C that finds the factorial of a number
( factfunc.asm ).

are not specified in this program. The resulting factorial is returned to the calling C
program through A4.
Build and run this project as factorial . Verify that factorial and its value
of 5040 (7!) are printed. Note that the maximum value of n is 7, since result is
casted as a short and 8! is greater than 2 15 .

Example 3.4: 32-bit Pseudorandom Noise Generation Using C Calling an
Assembly Function (Noisegen_casm)

The C source program Noisegen_casm.c in Figure 3.12 calls the function noisefunc
located in the file Noisegen_casmfunc.asm (Figure 3.13) to generate a 32-bit pseudo-
random noise sequence using the following scheme:

1. A 32-bit seed value such as 0 7E521603 is chosen.

2. A modulo 2 summation is performed between bits 17, 28, 30, and 31.

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