第一次互评作业：MIPS汇编程序设计

第一题：用系统功能调用实现简单输入输出

利用系统功能调用从键盘输入，转换后在屏幕上显示，具体要求如下：

(1) 如果输入的是字母（A~Z，区分大小写）或数字（0~9），则将其转换成对应的英文单词后在屏幕上显示，对应关系见下表

(2) 若输入的不是字母或数字，则在屏幕上输出字符“*”，

(3) 每输入一个字符，即时转换并在屏幕上显示，

(4) 支持反复输入，直到按“?”键结束程序。

      .data
u_word:     .asciiz
            "Alpha ","Bravo ","China ","Delta ","Echo ","Foxtrot ",
            "Golf ","Hotel ","India ","Juliet ","Kilo ","Lima ",
            "Mary ","November ","Oscar ","Paper ","Quebec ","Research ",
            "Sierra ","Tango ","Uniform ","Victor ","Whisky ","X-ray ",
            "Yankee ","Zulu "
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