a small math demo
Reinhard
Posts: 489
Hi , today I make a small experiment using the propeller log table, how descibed in the propeller manual page 420 ff.
It's calculate a cubic root without math library.
Thats not the fastest ( in my C - implementation) but a good exercise for me.
It's calculate a cubic root without math library.
Thats not the fastest ( in my C - implementation) but a good exercise for me.
/*
propeller-elf-gcc -mlmm -o log2.elf log2.c
*/
#include <stdio.h>
#ifdef __PROPELLER__
#include <propeller.h>
#endif
//#define DEBUG_LOG2
int time_clocks()
{
#if defined(__propeller__)
return (CNT);
#else
return 0;
#endif
}
unsigned short findLeadingBitPosition(unsigned aNumber)
{
short pos;
if(aNumber == 0)
return 0;
for(pos = 31; pos >= 0; pos--)
{
if( aNumber & (1 << pos))
{
return (unsigned short)pos;
}
}
}
int errUnsignedToBase2Exponent( unsigned num , unsigned * exponent)
{
unsigned short bitpos,tmp2,index,exp=0;
unsigned tmp,remainder;
int shift;
if(num <= 0 )
{
// underflow
*exponent = ~2;
return -2;
}
if(num >= 0x80000000)
{
// overflow
*exponent = ~1;
return -1;
}
bitpos = findLeadingBitPosition(num);
#ifdef DEBUG_LOG2
printf("bitpos: 0x%X %d\n",bitpos,bitpos);
#endif
tmp2 = bitpos;
tmp = 1 << bitpos;
#ifdef DEBUG_LOG2
printf("tmp: 0x%X %d\n",tmp,tmp);
#endif
remainder = num - tmp;
#ifdef DEBUG_LOG2
printf("remainder: 0x%X %d\n",remainder,remainder);
#endif
if(remainder <= 0x0F)
{
index = remainder << 8;
}
else if(remainder <= 0xFF)
{
index = remainder << 4;
}
else if(remainder <= 0xFFF)
{
index = remainder;
}
else
{
for(shift = 11; shift >= 0; shift--)
{
bitpos --;
if(remainder & (1<<bitpos))
{
index |= (1 << shift);
}
}
}
index &= 0xFFF;
index += 0xC000;
#ifdef DEBUG_LOG2
printf("index: 0x%X %d\n",index,index);
#endif
#ifdef __PROPELLER__
exp = *( unsigned short *)(index);
#endif
#ifdef DEBUG_LOG2
printf("exp: 0x%X\n",exp);
#endif
*exponent = (tmp2<<16) | exp;
return 0;
}
int main(int argc, char *argv[])
{
unsigned num[] = {0x60000000,2048,2047,2049,0x1FFF,0x40,2,4,8,17,0,0x80000000};
unsigned n,n1,n2,exp,exp1,exp2;
int err,i;
int start, end;
for(i=0; i < sizeof(num)/sizeof(unsigned); i++)
{
start= time_clocks();
err = errUnsignedToBase2Exponent( num[i] , &exp);
end = time_clocks();
printf("Error=%02d\tnumber=0x%08X\tBase2Exponent=0x%08X\t\tclocks:%d\n",err,num[i],exp,end-start);
}
printf("\n\n------------------------- cube root demo ---------------------\n");
n1 = 2048;
err = errUnsignedToBase2Exponent( n1 , &exp1);
exp2 = exp1/3;
printf("Error=%02d\tnumber=0x%08X\tBase2Exponent=0x%08X\t0x%08X\n",err,n1,exp1,exp2);
printf("exact: 2048 ^ (1/3) = 12.699208\n");
printf("result: %X integer part = %X fractional part = %X\n",exp2,(exp2&0xFF0000)>>16,exp2&0xFFFF);
printf("means: 2 ^ ( 3 + 1/2 + 1/8 + 1/32 + 1/128 .... + 1/32768) = 12.699118\n");
return 0;
}
Comments
I have a some questions.
- Is there any particular reason you chose to surround #include <propeller.h> with an #ifdef ?
- Why didn't you just use the ANSI C library clock() function for time_clocks() ?
Thanks for your continuing efforts. Please PM me if you need extra support.--Steve
Thank you Steve.
1. the reason is I wrote first the program with devcpp on personal computer, for debugging the flow logic.
In this case I get errors if including <propeller.h>, and runtime error on pointer access to memory location.
If I see that programm works, so I use this C - file without changes and compile with propeller-gcc.
2. I forgot, this ANSI C library function, Thanks for remember me !
best regards,
Reinhard