Adding a System Call and RTC driver to xv6 RISC-V OS

This repository explains how to run the xv6 RISC-V operating system on Linux, add a new system call and RTC driver to it. xv6 is a simple Unix-like educational operating system developed at MIT. Download full report here.

Prerequisites

• Basic understanding of C programming
• Familiarity with operating system concepts
• A working xv6 development environment on RISC-V

Ensure you have the following installed on your Linux distribution:

• git
• qemu
• gcc (with RISC-V support)
• make

Setting Up xv6 RISC-V on Linux

xv6 Installation Steps

1. Clone the Repository

git clone https://github.com/mit-pdos/xv6-riscv.git
cd xv6-riscv

2. Install RISC-V Toolchain

Follow the instructions here to install the RISC-V GNU toolchain. If you don't want to download a lot of files, installing the following is enough:

Debian or Ubuntu:

sudo apt-get install git build-essential gdb-multiarch qemu-system-misc gcc-riscv64-linux-gnu binutils-riscv64 linux-gnu 

Arch:

sudo pacman -S riscv64-linux-gnu-binutils riscv64-linux-gnu-gcc riscv64-linux-gnu-gdb qemu-emulators-full

3. Build the xv6 Kernel

make

4. Run xv6 using QEMU

make
make qemu

Steps to Add a System Call

1. Define the System Call

First, decide on the name and functionality of the new system call. For this example, we add a simple system call named sys_helloworld that prints "Hello, world!" to the console.

2. Modify the System Call Number

Edit the kernel/syscall.h file to add a new system call number. Append the new system call number to the list of existing system calls.

//
...

#define SYS_helloworld  22 // Assign an unused number

...
//

3. Declare the System Call Function

Declare the new system call function in the kernel/syscall.c file.

extern uint64 sys_helloworld(void);

4. Implement the System Call Function

Implement the system call in a new function in kernel/sysproc.c.

uint64
sys_helloworld(void)
{
    printf("Hello, world!\n");
    return 0; // Return value to user program
}

5. Add System Call to syscalls Array

Add the system call to the syscalls array in kernel/syscall.c.

static uint64 (*syscalls[])(void) = {
[SYS_fork]        sys_fork,
[SYS_exit]        sys_exit,
[SYS_wait]        sys_wait,
[SYS_pipe]        sys_pipe,
[SYS_read]        sys_read,
[SYS_kill]        sys_kill,
[SYS_exec]        sys_exec,
[SYS_fstat]       sys_fstat,
[SYS_chdir]       sys_chdir,
[SYS_dup]         sys_dup,
[SYS_getpid]      sys_getpid,
[SYS_sbrk]        sys_sbrk,
[SYS_sleep]       sys_sleep,
[SYS_uptime]      sys_uptime,
[SYS_open]        sys_open,
[SYS_write]       sys_write,
[SYS_mknod]       sys_mknod,
[SYS_unlink]      sys_unlink,
[SYS_link]        sys_link,
[SYS_mkdir]       sys_mkdir,
[SYS_close]       sys_close,
[SYS_helloworld]  sys_helloworld, // Add this line
};

6. Update the User Space Interface

Add a user space interface for the system call in user/user.h.

int helloworld(void);

Implement the user space interface in user/usys.pl.

entry("helloworld")

7. Test the System Call

Create a user program to test the new system call. For example, add a new file user/helloworld.c.

#include "kernel/types.h"
#include "user/user.h"

int
main(void)
{
    helloworld();
    exit(0);
}

Add the new test program to the Makefile to compile it.

# In Makefile

...
U_PROGS=\
    $U_cat\
    $U_echo\
    $U_forktest\
    $U_grep\
    $U_helloworld\ # Add this line
...

8. Compile and Run xv6

Compile the xv6 kernel and run it.

make clean
make
make qemu

In the xv6 shell, run the helloworld program to see the output of the new system call.

$ helloworld
Hello, world!

Adding a Real-Time Clock (RTC) Driver

The Real-Time Clock (RTC) is a crucial component in many operating systems, providing accurate timekeeping and periodic interrupts for various time-related tasks. In xv6, integrating an RTC driver involves several steps, including interacting with hardware registers, handling interrupts, and providing a user interface for reading the current time.

The RTC driver in xv6 interacts with the hardware clock to retrieve the current time and date. This involves reading from specific hardware registers that maintain time information. Additionally, the RTC can generate interrupts at regular intervals, which can be used for tasks such as system timekeeping or scheduling.

Steps to Implement the RTC Driver

1. Identify RTC Registers and Hardware Interface

The RTC hardware typically provides registers for seconds, minutes, hours, day, month, and year. These registers are mapped to specific memory addresses, which the driver will read to get the current time.

2. Define RTC Data Structures

Define a structure to hold the time and date information.

struct rtcdate {
    int second;
    int minute;
    int hour;
    int day;
    int month;
    int year;
};

3. Initialize the RTC Driver

Set up the necessary hardware initialization, which might involve configuring the RTC to generate interrupts or setting initial values.

void rtcinit(void) {
    // Initialize RTC hardware settings if needed
}

4. Read Time from RTC

Implement a function to read the current time from the RTC registers.

void rtc_read(struct rtcdate *r) {
    // Read from the RTC hardware registers
    r->second = // read from hardware register;
    r->minute = // read from hardware register;
    r->hour =   // read from hardware register;
    r->day =    // read from hardware register;
    r->month =  // read from hardware register;
    r->year =   // read from hardware register;
}

5. Handle RTC Interrupts

Set up an interrupt handler for the RTC to handle periodic updates or alarms. This involves configuring the CPU to handle interrupts generated by the RTC.

void rtc_interrupt_handler(void) {
    // Handle the RTC interrupt
}

void init_interrupts(void) {
    // Register the RTC interrupt handler
}

6. Expose RTC Functionality to User Programs

Add a system call to allow user programs to read the current time.

extern uint64 sys_rtcdate(void);

uint64 sys_rtcdate(void) {
    struct rtcdate r;
    rtc_read(&r);
    // Copy the rtcdate structure to user space
    // ...
    return 0; // Return appropriate value
}

Modify syscall.h to add a new system call number.

#define SYS_rtcdate  23

Add the system call to the syscall dispatch table in syscall.c.

static uint64 (*syscalls[])(void) = {
    // ... existing system calls ...
    [SYS_rtcdate]   sys_rtcdate,
};

Add a user space interface for the system call.

int rtcdate(struct rtcdate *r) {
    return syscall(SYS_rtcdate, r);
}

Implement the user space interface in usys.pl.

entry("rtcdate")

7. Test the RTC Driver

Create a user program to test the RTC functionality.

#include "kernel/types.h"
#include "user/user.h"
#include "kernel/date.h"

int main(void) {
    struct rtcdate r;
    rtcdate(&r);
    printf("Date: %d-%d-%d Time: %d:%d:%d\n",
           r.year, r.month, r.day,
           r.hour, r.minute, r.second);
    exit(0);
}

Add the new test program to the Makefile to compile it.

# In Makefile...
U_PROGS=\
    ... \
    $U_rtc_test  # Add this line

Compile and run xv6, and execute the test program in the xv6 shell to see the output of the RTC.

$ rtc_test
Date: 2024-5-27 Time: 15:30:45

License

MIT