Update xv6

_posts/2017-07-08-intro-xv6.md

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+---
+layout: post
+title:  "Xv6 introduction"
+date:   2017-07-28 14:56:55 -0400
+categories: xv6
+---
+
+I hate xv6, a stupid, useless education-oriented system. In this article, I will generally talk about how to implement system call to this operating system.
+
+-------
+
+## Xv6 Systemcall
+To invoke a system call, we have to first define a user mode function to be the interface of the kernel instruction  in file *user.h*.
+
+~~~~c
+void function (void);
+~~~~
+
+This interface-like function will then pass the function name, in this case function, to *usys.S*. When using user mode function in programs, *usys.S* will generate a reference to SYS_function and push system call number of this function into %eax. After that, system can know from *syscall.c* and determining whether this system call is available. We must define same name system function and add it into *syscall.h* and *syscall.c*.
+
+~~~~c
+#define SYS_function ##		// ## is the system call number
+
+[SYS_function]  sys_function	// real system function name
+
+extern int sys_function(void);	// real system function declaration
+~~~~
+
+After adding these sentences to syscall files, we can implement real function in specific place where you want to make the function works well.
+
+Sometimes, we need to pass variables among system calls. In this case, variables’ values are not necessary and even can’t be pass directly into system_function. When invoke a system call function, all variables of this system call will be pushed into current process’ stack. In file *syscall.c*, multiple functions are provided to get these variables from the process. I won’t waste time on explaining how to use these functions especially when elegant and detailed comments were written in source codes. However, I will explain concepts and how process organized and works in xv6 in future articles.