For example, it is difficult to test whether a program is making proper use of the filesystem without actually allowing it to read and write from the filesystem.
例如，实际上如果不允许一个程序从文件系统进行读和写，很难测试它是否正确使用了该文件系统。
Instead of IO, where you could read a file and write a file, ABC's designers decided to just have global variables in the program.
ABC的设计者放弃了用来读写文件的IO的方法，他们转而在程序里使用一些全局变量。
A program can also read and write data into the user Spaces on the system.
也可以通过程序在系统的用户空间中读写数据。
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