The only thing we have to do to free memory is to take the pointer we're given, back up sizeof (struct mem_control_block) bytes, and mark it as available. Here is the code for that.

为了释放内存，我们必须要做的惟一一件事情就是，获得我们给出的指针，回退size of (struct mem_control_block)个字节，并将其标记为可用的。

youdao

System counters within this category track memory data such as the number of bytes available and the number of bytes cached.

此类别内的系统计数器跟踪内存数据，如可用字节数和缓存的字节数。

youdao

I have no idea what's here at the moment so I'm just going to draw a question mark because that memory might have been used previously for some other purpose, but we know it's currently available to us so we have four bytes of memory.

我不知道在此刻这里是什么,所以我画一个问号，因为那块内存可能在之前,为其它的目的而使用过，但是我们知道它可以为我们所使用,这样我们就有4个字节的内存。

哈佛公开课 - 计算机科学课程节选