将python中的IEEE浮点数转换为TI TMS320C30 32位浮点数 [英] Convert IEEE float to TI TMS320C30 32bits float in python
问题描述
我需要按照以下约定将python float转换为TI DSP TMS320C30 float表示形式:
I need to convert a python float to a TI DSP TMS320C30 float representation, following this convention:
http://www.ti.com/lit/an/spra400/spra400.pdf#page=13
我已经尝试了一些方法,但是似乎无法将所提出的算法笼罩在脑海中.我还发现了
I've tried a few things, but I can't seem to wrap my head around the proposed algorithm. I also found a C version of the algorithm, but it looks like it is a version that runs in the TI DSP, so there are operations that I can't figure out (reverse the sign, for instance).
我在下面有一个非常幼稚的实现,但是它不起作用...
I have a very naive implementation below, but it doesn't work...
# see http://www.ti.com/lit/an/spra400/spra400.pdf
import math
def twos_comp(val, bits):
"""compute the 2's complement of int value val"""
if( (val&(1<<(bits-1))) != 0 ):
val = val - (1<<bits)
return val
def float_to_ti( f ):
m,e = math.frexp( f )
# print m, e,
mantissa = int(str(m)[2:])
exponent = twos_comp((e - 127), 8) if e != 0 else (-128)
sign = 1 if f < 0 else 0
# print sign, mantissa, exponent
return ((sign << 30) + (exponent << 24) + mantissa) & 0xffffffff
一些期望值的例子:
# Float TI Decimal value of the resulting 32bits
#################################################
# 0.0 2147483648
# 1.0 0
# 100 105381888
# 0.000021 4029688104
# 10 52428800
# -1.0 4286578688
# -10.0 65011712
# -0.0021 4160118745
# -84.5487 114747153
我认为它可以归结为python返回尾数/有效位数的方式,但我不确定.
您如何从这里开始?
I think it boils down to the way python returns the mantissa/significand, but I'm not sure.
How would you start things here?
注意:我发现了此问题可能相关,我将研究一下struct pack并解压缩.
Note: I found this question that might be related, I'll look into the struct pack and unpack..
FYI:我用C程序检索了理论值,该程序是这样加载到DSP中的:
FYI: I retrieved the theoretical values with a C program that I load in the DSP like this:
{
float f = -1.0; printf("F: %f -> %u", f, *(unsigned int*)&f);
f = -10.0; printf("F: %f -> %u", f, *(unsigned int*)&f);
f = -0.0021; printf("F: %f -> %u", f, *(unsigned int*)&f);
f = -84.5487; printf("F: %f -> %u", f, *(unsigned int*)&f);
}
有效的实施方式
按照阿明的回答,我知道它适用于负数:
Working implementation
Following Armin's answer, I got it working for negative numbers:
def float_to_ti(f):
m, e = math.frexp(f)
ma = m
m = abs(m * 2)
e -= 1
sign = (math.copysign(1.0, f) < 0)
man = int((m - 1.0) * (1 << 23))
if sign:
man *= -1
if e == 0 and sign == 1:
e = 255
if f == 0.0:
return (128 << 24) | (sign << 23)
return ((e & 0xff) << 24) | (sign << 23) | man & 0x7fffff
推荐答案
以下代码通过了您的测试:
The following code passes your tests:
def float_to_ti( f ):
m, e = math.frexp(f)
m *= 2
e -= 1
sign = (math.copysign(1.0, f) < 0)
if f == 0.0:
return (128 << 24) | (sign << 23)
if sign:
m += 2.0
if m == 1.0:
m = 0.0
e -= 1
else:
m -= 1.0
assert 0.0 <= m < 1.0
return ((e & 0xff) << 24) | (sign << 23) | int(m * (1 << 23) + 0.5)
请注意不同的顺序(指数,符号,尾数).还要注意math.frexp()不会以IEEE格式返回任何内容,因此该代码不必担心任何IEEE细节:(e& 0xff)
转换指数(作为签名字符)转换为未签名的数字.最后,请注意,C30格式不支持反规范,这意味着隐含了尾数的最高位(因此 m-1.0
).
Note the different order (exponent, sign, mantissa). Note also that math.frexp() doesn't return anything in the IEEE format, so this code doesn't worry about any IEEE details: the (e & 0xff)
converts the exponent (as a signed char) to an unsigned number. Finally, note that the C30 format doesn't support denormals, which means that its mantissa's top bit is implied (hence the m - 1.0
).
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