# Forum: ARM programming with GCC/GNU tools ARM fixed point vs. floating point

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Hi,
I'm creating application on ARM STM32F103VE. I have to multiply several
matrix in very short time. I'm thinking about using Fixed Point, to
speed up my calculations. So that I wrote union:

typedef union fxpntu{
int full;
struct fxpnts{
int_fast32_t fraction : 31;
int_fast32_t integer : 1;
} part;
} fxpnt_t;

My Fixed Point representation is Q1.31.

Than, I created multiplying function:

/* Return: c = a * b */
fxpnt_t fxmul(fxpnt_t* a, fxpnt_t* b) {
fxpnt_t c;

_fxpnt_accu64.full = a->full*b->full;

fxreg |= (_fxpnt_accu64.part.H == 0 && (a->full != 0 || b->full !=
0))<<0x05;
fxreg |= FX_UFLOW >> 3;

c.full = _fxpnt_accu64.part.H;

c.full = FX_UFLOW?0x01:_fxpnt_accu64.part.H;
FX_UFLOW_CLEAR;

return c;
}

where _fxpnt_accu64 is global object, to hold result.
I made few tests in CrossWorks and results are weird:

float a = -150.534433;
float b = 0.05323;
c = a*b;            // 47 Instructions
c = a+b;            // 55 Instructions
c = a-b;            // 49 Instructions
c = a/b;            // 139 Instructions
ans = fxmul(&fx2, &fx1);   // 165 Instructions

I checked my own multiplying function, and
fxreg |= (_fxpnt_accu64.part.H == 0 && (a->full != 0 || b->full !=
0))<<0x05;  <-- this line takes 43 Instructions.

on ARM Cortex M-3, or example code? Maybe I'm doing something wrong,
maybe I don't get point of fixed multiplying..

Thanks for Your feedback.

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danrok wrote:

Could you provide an example of what you are actually compiling? The
fragments above do not make sense. For instance there is no structure
with member H, fxreg is not defined, FX_UFLOW and FX_UFLOW_CLEAR are
unknown macros.

Please test the example before posting otherwise you are wasting all of

--
Marcus
http://www.doulos.com/arm/

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