📚 Year 12 CAIE Computer Science: Unit Test Mock Paper Walkthrough | Year 12 CAIE 计算机：单元测试模拟卷解析
Mock exams are essential for Year 12 students preparing for CAIE Computer Science. This walkthrough breaks down typical questions from a unit test, covering key topics such as data representation, logic, software, networking, databases, and programming. By reviewing model answers and common pitfalls, you can strengthen your understanding and exam technique.
模拟考试对准备 CAIE 计算机科学的 Year 12 学生至关重要。本文解析单元测试中的典型题目，涵盖数据表示、逻辑、软件、网络、数据库和编程等核心主题。通过回顾标准答案和常见错误，你可以巩固理解并提升应试技巧。
1. Number System Conversion | 数制转换
A classic question asks you to convert a binary number to its hexadecimal equivalent. For example, take 10110110₂.
经典题目要求将二进制数转换为十六进制。例如，取 10110110₂。
en convert each nibble: 1011₂ = 11 in decimal = B, 0110₂ = 6. Therefore, 10110110₂ = B6₁₆.
要转换，从右侧开始将二进制位分组为四位一组。10110110 变成 1011 0110。然后转换每个四位组：1011₂ = 十进制 11 = B，0110₂ = 6。因此，10110110₂ = B6₁₆。
A common mistake is padding the wrong side. Always group from the least significant bit (rightmost). If the leftmost group has fewer than 4 bits, add leading zeros. For instance, 111011₂ groups as 0011 1011, which is 3B₁₆.
常见错误是在错误的一侧补零。务必从最低有效位（最右侧）分组。如果最左侧组不足 4 位，则添加前导零。例如，111011₂ 分组为 0011 1011，即 3B₁₆。
Always check your work by converting back to binary or using decimal as an intermediate step.
始终通过转换回二进制或使用十进制作为中间步骤来检查你的工作。
2. Binary Arithmetic and Overflow | 二进制运算与溢出
Consider adding two 8-bit numbers: 01101001 (105) and 01011100 (92).
考虑两个 8 位数的加法：01101001 (105) 和 01011100 (92)。
Perform column addition:
进行列加法：
01101001 + 01011100 ----------- 11000101
within 8 bits. No overflow occurs for unsigned arithmetic. However, if we treat these as 2' e result has MSB=1, indicating a negative number. This is an overflow for signed addition.
在无符号解释中，和为 197，可以在 8 位内表示，无符号算术没有溢出。但如果我们将这些视为二进制补码有符号数，两个操作数均为正（MSB=0）。结果 MSB=1，表示负数，这就发生了有符号溢出。
Overflow = Carry₇ XOR Carry₆
arry out is 1, so the overflow flag is set. A quick check: 105 + 92 = 197, but the 8-bit 2's complement range is -128 to +127; 197 exceeds 127, confirming overflow.
进入符号位（第7位）的进位为 0，进位输出为 1，因此溢出标志置位。快速检验：105 + 92 = 197，但 8 位补码范围是 -128 到 +127；197 超出 127，确认溢出。
3. Logic Gates and Truth Tables | 逻辑门与真值表
h inputs A, B, C and output X = (A AND B) OR (NOT C). We need to complete the truth table.
典型题目给出一个逻辑电路，输入 A、B、C，输出 X = (A AND B) OR (NOT C)。需要填真值表。
here are 2³ = 8 rows. Then compute intermediate columns for A AND B, NOT C, and finally X.
首先，列出 A、B、C 的所有组合（0 和 1），共 2³ = 8 行。然后计算中间列 A AND B、NOT C，最后得到 X。
| A | B | C | A AND B | NOT C | X |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 |
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