vdsp提供了两个函数用以实现fract16与float之间的相互转换：
fract16 float_to_fr16 (float _x);
float fr16_to_float (fract16 _x);
看看这两个转换函数到底做了什么。
1.1 float_to_fr16
这个函数的原始代码在Blackfin\lib\src\libc\runtime\fl2fr.asm中，先看看它的注释：
/***************************************************************************
*
* Function: FLOAT_TO_FR16 -- Convert a floating-point value to a fract16
*
* Synopsis:
*
* #include <fract2float_conv.h>
* fract16 float_to_fr16(float x);
*
* Description:
*
* The float_to_fr16 function converts a single precision, 32-bit IEEE
* value into a fract16 number in 1.15 notation. Floating-point values
* that cannot be converted to fract15 notation are handled as follows:
*
* Return 0x7fffffff if x >= 1.0 or NaN or +Inf
* Return 0x80000000 if x < -1.0 or -NaN or -Inf
* Return 0 if fabs(x) < 3.0517578125e-5
*
* (Note that the IEEE single precision, 32-bit, representation
* contains 24 bits of precision, made up of a hidden bit and 23
* bits of mantissa, and thus some precision may be lost by converting
* a float to a fract16).
*
* Algorithm:
*
* The traditional algorithm to convert a floating-point value to 1.15
* fractional notation is:
*
* (fract16) (x * 32768.0)
*
* However on Blackfin, floating-point multiplication is relatively
* slow is emulated in software, and this basic algorithm does not
* handle out of range results.
*
* This implementation is based on the support routine that converts
* a float to fract32, and then converts the fract32 into a fract16
* by performing an arithmetic right shift by 16 bits. (It is possible
* to avoid the shift by coding the function to "multiply" the input
* input argument by 2^15 (rather than 2^31) but this approach can lead
* to the loss of 1-bit precision when handling negative inputs).
*
* The following is a C implementation of this function and is about
* a third slower:
#include <fract2float_conv.h>
extern fract16
float_to_fr16(float x)
{
int temp;
fract32 result;
temp = *(int *)(&x);
if ((temp & 0x7f800000) >= 0x3f800000) {
result = 0x7fffffff;
if (temp < 0)
result = 0x80000000;
} else {
temp = temp + 0x0f800000;
result = *(float *)(&temp);
}
return (result >> 16);
}
*
* WARNING: This algorithm assumes that the floating-point number
* representation is conformant with IEEE.
*
* Cycle Counts:
*
* 31 cycles when the result is within range
* 30 cycles when the result is out of range
* 28 cycles when the input is 0.0
*
* These cycle counts were measured using the BF532 cycle accurate
* simulator and include the overheads involved in calling the function
* as well as the costs associated with argument passing.
*
* Code Size:
*
* 76 bytes
*
* Registers Used:
*
* R0 - the input argument
* R1 - various constants
* R2 - the exponent of the input argument or a shift amount
* R3 - the mantissa of the input argument
*
* (c) Copyright 2006 Analog Devices, Inc. All rights reserved.
* $Revision: 1.3 $
*
***************************************************************************/