位操作示例：最低为的1 判断一个十六进制数是否为字母

assets/深入理解计算机系统（原书第3版）可编辑_1693568429838_0.edn

@@ -1942,4 +1942,4 @@
                           :page 478},
                :content {:text "[:span]", :image 1694001388059},
                :properties {:color "purple"}}],
- :extra {:page 499}}
+ :extra {:page 500}}

code/CSAPP/Chapter02/LowBit.c

+/*
+位操作练习题
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+* LowBit - return value of low-order 1 bit
+*   Examples: LowBit(0x1F) = 0x1
+*             LowBit(0x300) = 0x100
+*             LowBit(0x0) = 0x0
+*/
+unsigned int LowBit(unsigned int x) {
+    return x & (~x + 1);
+}
+
+/*
+* hexAllLetter - return 1 when all 4 hex digits are letters
+*   Examples: hexAllLetter(0x1234) = 0
+*             hexAllLetter(0x1ABF) = 0
+*             hexAllLetter(0xABCD) = 1
+A hexadecimal number orresponding to the four binary number x3x2x1x0
+Let's check whether the hexadecimal number is a letter
+
+            x3x2=00   x3x2=01   x3x2=10   x3x2=11
+x1x0= 00       0        0         0         1
+x1x0= 01       0        0         0         1
+x1x0= 10       0        0         1         1
+x1x0= 11       0        1         1         1
+*/
+unsigned hexAllLetter(unsigned x) {
+    // x3 x2 x1 x0  ->  hex
+    // 0  0  1  0   ->  hex constent 0x2
+    unsigned x1 = x & 0x22222222;           // ...xxxx0010
+    // 0  1  0  0   ->  hex constent 0x4    // ...xxxx0100
+    unsigned x2 = x & 0x44444444;           // ...xxxx1000
+    // 1  0  0  0   ->  hex constent 0x8
+    unsigned x3 = x & 0x88888888;           // ...xxx10000
+    // x3 * (x1 + x2)  ==> x3 & (x1 + x2)
+    unsigned a = (x3 >> 3) & ((x2 >> 2) | (x1 >> 1));
+    return a;
+}
+
+
+int main() {
+    printf("%#x\n", LowBit(0xf)); // 0xf = 1111 -> 0x1
+    printf("%#x, is letter %#x\n", 0xabcdefab, hexAllLetter(0xabcdefab));
+    printf("%#x, is letter %#x\n", 0x11111111, hexAllLetter(0x11111111));
+    printf("%#x, is letter %#x\n", 0xa0a0a0a0, hexAllLetter(0xa0a0a0a0));
+    return 0;
+}
+

pages/CC-Computer Composition.md

@@ -19,4 +19,5 @@
 			- `[符号位(0, 1), 阶码(移码表示), 尾数]`
 	- 我们知道数据在计算机是如何表示的了，下面我们介绍指令和控制如何存储的 [[CC-程序的机器级表示]]
 	- 到目前为止，我们依赖于一个简单的计算机系统模型，CPU 执行指令，而存储期系统为 CPU 存放质量和数据。[[$green]]==在简单模型中，存储器系统是一个线性的字节数组==，而 CPU 能够在一个常数时间内访问每个存储期位置。[[$green]]==实际上，存储期系统是一个具有不同容量、成本和访问时间的存储设备的层次结构==。下面我们描述 [[CC-存储器层次结构]]
-	-
+	-
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+-
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