[summary of problem thinking] Why is the register reset performed in user mode?

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When I saw this question in Wang daoshu , I first thought that there are many registers that are opaque , Therefore, the user mode cannot be operated , Can't even operate in the tube state , Based on this idea , I did some research .

reflection

After searching, we found , In fact, if you think of assembly On the level of （ But it's really hard for me, a non - major student, not to learn , Regrets that he is not proficient in learning ） It's a lot easier . Let's take the simplest example ：int i=0, This code actually sets the value in a register to 0, This is obviously a very common operation , It is also completely unnecessary to enter the tube state , Therefore, it is in the user mode （ Eye state ） Conduct .

Extended thinking

C Language details

After thinking about it, I thought that all the setting variables are in registers ？ But registers are limited , So the search found that C In this language, users can define register variables by themselves, such as ：register int i=0, This format , And even if not , The compiler may also store variables in registers, because obviously for some variables, it can greatly improve the efficiency of the system （ For example, frequently used variables , Variables in the loop , Based on the principle of locality , This bracket is a personal guess ）.

Register details

in addition , To verify that registers can store variables , I also think of the knowledge point of programmer transparency and programmer opacity , After consulting, it is found that , General purpose registers are opaque to assemblers , This also confirms the authenticity of the previous statement of assembly layer calling registers

Reference material

【1】http://www.cskaoyan.com/thread-7075-1-10.html
【2】https://blog.csdn.net/HumanpieceY/article/details/85917291