Xv6 introduction
+ +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.
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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.
+ +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.
+ +#define SYS_function ## // ## is the system call number
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+[SYS_function] sys_function // real system function name
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+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.
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