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Xv6 Operating System -adding a new system call

Prerequisite – Xv6 Operating System -add a user program 

In last post we got to know how to add user program in Xv6 Operating System. Now here you will see how to add new system call in Xv6 Operating System. A

dding new system call to xv6: A system call is way for programs to interact with operating system. A computer program makes system call when it makes request to operating system’s kernel. System calls are used for hardware services, to create or execute processes, and for communicating with kernel services, including application and process scheduling. 

Overview: You are going to implement system call called getyear which will return 1975 from kernel always. Unix version 6 was released in that year. In order to define your own system call in xv6, you need to make changes to 5 files. Namely, these files are as follows.

 

Create system call to return year Unix version 6 was released: You could start working from syscall.h file where the number is assigned to every system call in this Xv6 system. As you can see, there are 21 system calls already defined in this file. Let’s go ahead and add the following line to the reserver system call number for your own system call.

#define SYS_getyear 22

Next, you need to add pointer to system call in syscall.c file. This file contains an array of function pointers which uses above-defined numbers (indexes) as pointers to system calls which are defined in different location. In order to add your custom system call, add following line to this file.

[SYS_getyear] sys_getyear

What changes happen here ? This means, when system call occurred with system call number 22, function pointed by function pointer sys_getyear will be called. So, you have to implement this function. However, this file is not place you are going to implement it. You will just put function prototype here inside this file. So, find suitable place inside this file and add following line. You can see that all other 21 system call functions are defined similarly. The function prototype which needs to be added to syscall.c file is as follows.

extern int sys_getyear(void)

Next, you will implement system call function. In order to do this, open sysproc.c file where system call functions are defined.

//return the year of which
//Unix version 6 was released

int 
sys_getyear(void) 
{

return 1975;
}

Now you have just two little files to edit and these files will contain interface for your user program to access system call. Open file called usys.S and add line below at the end.

SYSCALL(getyear)

Next,open file called user.h and add following line. This is function that user program will be calling. As you know now, there’s no such function implemented in system. Instead, call to below function from user program will be simply mapped to system call number 22 which is defined as SYS_getyear preprocessor directive. The system knows what exactly is this system call and how to handle it.

int getyear(void);

If you have completed all above procedure, you have successfully added new system call to xv6. However, in order to test functionality of this, you would need to add 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, "Note: Unix V6 was released in year %d\n", getyear());
    exit();
}

In order to add this user program to xv6, you need to follow above steps for adding user program. At last, run user program in qemu window which can be obtained by running command make qemu on terminal. After executing everything successfully, you will get on terminal. Output:

"Note:Unix V6 was released in year 1975"

Xv6 is a simple Unix-like operating system used for educational purposes. Adding a new system call to Xv6 requires modifying the kernel code and adding a new entry to the system call table. Here are the high-level steps involved:

  1. Define the new system call: Define the new system call by creating a new function in the kernel code. The function should take the necessary arguments and return the required value. The function should also check for any errors and return an appropriate error code.
  2. Add the system call number: Choose a unique system call number for the new system call and add it to the system call table in the kernel code. This table is typically defined in the file syscall.h.
  3. Modify the system call dispatcher: Modify the system call dispatcher function in the kernel code to handle the new system call. The dispatcher function reads the system call number from the process’s register and calls the corresponding system call function.
  4. Add the user-level interface: Add a user-level interface to the new system call by adding a new entry to the system call library in the file usys.S. This entry should call the system call with the appropriate arguments and return the result.
  5. Modify the user-level program: Modify the user-level program to use the new system call by including the header file that defines the new system call and calling the system call function with the appropriate arguments.

Build and test: Build the Xv6 kernel and user-level programs and test the new system call to ensure that it works correctly.

Adding a new system call to Xv6 can be a challenging task, as it requires a good understanding of the operating system’s architecture and programming in C. However, it can also be a valuable learning experience for students interested in operating system design and implementation.

xv6 is a simple Unix-like operating system designed for teaching purposes. Adding a new system call to xv6 involves modifying several components of the operating system, including the system call interface, system call dispatching mechanism, and the kernel code that implements the new system call.

Here are the general steps to add a new system call to xv6:

  1. Define the system call interface: The first step is to define the interface for the new system call, including its name, arguments, and return value. This information should be added to the syscall.h header file.
  2. Assign a system call number: Next, assign a unique number to the new system call. This number should be added to the syscall.h header file, as well as to the syscall() function in the syscall.c file.
  3. Modify the system call dispatcher: The system call dispatcher is responsible for determining which system call to execute based on the system call number passed by the user program. This mechanism should be updated to handle the new system call number.
  4. Implement the new system call: The final step is to implement the new system call in the kernel code. This involves modifying the appropriate kernel function to handle the new system call and adding any necessary data structures or helper functions.
  5. Once these steps are completed, the modified xv6 operating system can be recompiled and tested to ensure that the new system call is functioning as expected.

It is worth noting that adding a new system call to xv6 is a complex task that requires a deep understanding of operating system design and implementation. It is recommended to consult the xv6 documentation and other relevant resources before attempting to add a new system call.

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Operating Systems