Adding New System Calls to Xv6
+ ++ +
What is a system call?
+As you all know, an operating system supports two modes; the kernel mode and the user mode. + When a program in user mode requires access to RAM or a hardware resource, it must ask the kernel to + provide access to that particular resource. This is done via a system call. When a program makes a + system call, the mode is switched from user mode to kernel mode. + There are many system calls in an operating system which executes different types of tasks when they + are called. +
+ +Adding a custom system call in Xv6
+In order to define your own system call in Xv6, you need to make changes to 5 files. Namely, these + files are as follows.
+1. syscall.h
2. syscall.c
3. sysproc.c
4. usys.S
5. user.hLet’s write a system call to return the year Unix version 6 was released.
+We would start the procedure by editing syscall.h in which a number is given to every system call.
+ This file already contains 21 system calls. In order to add the custom system call, the following
+ line needs to be added to this file.
#define SYS_getyear 22
+ Next, we need to add a pointer to the system call in the syscall.c file. This file contains an array
+ of function pointers which uses the above-defined numbers (indexes) as pointers to system calls
+ which are defined in a different location. In order to add our custom system call, add the following
+ line to this file.
+
[SYS_getyear] sys_getyear,+ What exactly happens in here? +
+The underlying meaning of the above two changes is as follows.
+-
+
-
+ When the system call with number 22 is called by a user program, the function pointer
+
sys_getyearwhich has the indexSYS_getyearor 22 will call the system call function + +
+
+ Therefore, we need to implement the system call function. However, we do not implement the system
+ call function in the syscall.c file. Instead, we only add the function prototype in here and we
+ define the function implementation in a different file. The function prototype which needs to be
+ added to the syscall.c file is as follows.
+
extern int sys_getyear(void);
+ Next, we will implement the system call function. In order to do this, open the sysproc.c file where
+ system call functions are defined.
+
// return the year of which the Unix version 6 was released
int
sys_getyear(void)
{
return 1975;
}
+ The basic implementation of the system call is now complete. However, there are 2 more minor steps + remaining. +
+ +Add the interface for the system call
+In order for a user program to call the system call, an interface needs to be added. Therefore, we
+ need to edit the usys.S file where we should add the following line.
+
SYSCALL(getyear)
+ Next, the user.h file needs to be edited.
+
int getyear(void);
+ This would be the function which the user program calls. This function will be mapped to the system
+ call with the number 22 which is defined as SYS_getyear preprocessor directive.
+
+ If you have completed all of the above, you have successfully added a new system call to Xv6. + However, in order to test the functionality of this, you would need to add a user program which + calls this system call. +
++ The user program could be as follows. +
+#include “types.h”
#include “stat.h”
#include “user.h”
int
main(void)
{
printf(1, “Unix V6 was released in the year %d\n”, getyear());
exit();
}