Ch1.计算机系统漫游

C编译(ccl)与链接(ld)
Switch是否总时比if-else高效？
while循环总比for循环高效么？
指针引用比数组高效么?
函数的本地临时变量为什么比入参的引用更高效？
算数表达式的括号也能影响运算速度?

Ch2.信息的表示和处理

Integer – 补码与符号位

事实上，有符号数（two’s complement，补码）的符号位，是具有权重的，只不过需要取反，如-2表示为 $1111,1110=\red {-2^7}+\sum_{\red {w=1}}^{w=6}2^w+0*2^{\red {w=0}}=-2$

$T 2 U (x) = B 2 U (T 2 B (x)) = u x$
$U 2 T (u x) = B 2 T (U 2 B (x)) = x$
$T_{MIN}=-T_{MAX}-1$
$U_{MAX}=2\times T_{MAX}+1=01111<<1+1$

" $u x$ and $x$ have same bit pattern，just a bunch of bits to computer itself."

//sizeof return unsigned int, cast a into unsigned, you got stuck forever
for(int a=1;a-sizeof(a)>=0;a--)
//so be care of unsigned "i" used for array in case a[i]
//i=0; i--=UMAX; a[i] may cause out of bounds
int main()
{
    unsigned int a=numeric_limits<unsigned int>::max();
    int b=-1;
    unsigned int c=-3;
    cout<<(int)a<<" "<<a<<endl;  //-1 4294967295
    cout<<(b==a?"True":"Flase")<<endl; //True
    cout<<(b>a?"True":"Flase")<<endl; //Flase
    cout<<std::hex<<c<<" "<<-c<<" "<<c+(-c)<<endl; //fffffffd 3 0
    cout<<std::hex<<b<<"\n"<<numeric_limits<int>::max()<<endl; //ffffffff 7fffffff 
    cout<<b+numeric_limits<int>::max()<<endl; //7ffffffe
    return 0;
}

符号位扩展/截断

sign bit shifts left, others remain
$-0\times2^{3}+1\times2^{2}+1\times2^{1}+0\times2^{0}=6$
$-1\times2^{3}+1\times2^{2}+1\times2^{1}+0\times2^{0}=-2$
$\red{-1\times2^{4}+1\times2^{3}}+1\times2^{2}+1\times2^{1}+0\times2^{0}=\red{-1\times2^{3}}+1\times2^{2}+1\times2^{1}+0\times2^{0}=-2$