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XV6 Device Driver

Why we have a Driver?

A driver is the code in an operating system that manages a particular device: 1. It tells the device hardware to perform operations,. 2. Configures the device to generate interrupts when done. 3. Handles the resulting interrupts,. 4. Interacts with processes that may be waiting for I/O from the device.


Config Hardware

Xv6’s main calls consoleinit (kernel/console.c:189) to initialize the UART hardware, and to configure the UART hardware to generate input interrupts (kernel/uart.c:34).
initlock(&cons.lock, “cons”);
// connect read and write system calls
// to consoleread and consolewrite.
devsw[CONSOLE].read = consoleread;
devsw[CONSOLE].write = consolewrite;

Create console device

In init.c, create a console device:
mknod(“console”, 1, 1);
open(“console”, O_RDWR);
The file path is console, type is T_DEVICE.
mknod implementation.
struct inode *ip;
char path[MAXPATH];
int major, minor;
if((argstr(0, path, MAXPATH)) < 0 ||
argint(1, &major) < 0 ||
argint(2, &minor) < 0 ||
(ip = create(path, T_DEVICE, major, minor)) == 0){
return -1;
At this point, we have a console device. A file descriptor 0 points to this device.

How to read or write from device

Let’s look at fileread
fileread(struct file *f, uint64 addr, int n)
int r = 0;
if(f->readable == 0)
return -1;
if(f->type == FD_PIPE){
r = piperead(f->pipe, addr, n);
} else if(f->type == FD_DEVICE){
if(f->major < 0 || f->major >= NDEV || !devsw[f->major].read)
return -1;
r = devsw[f->major].read(f, 1, addr, n);
If the file type is DEVICE, we call the special read function to perform read operation. Note: The special read is set up in consoleinit.
consoleread sleeps and wait for a line of inputs, then copy to user space. Details see flow section.


Every input char generates an interrupt

When the user types a character, the UART hardware asks the RISC-V to raise an interrupt.

Trap handler determines interrupt source

xv6’s trap handling code calls devintr (kernel/trap.c:177). devintr looks at the RISC-V scause register to discover that the interrupt is from an external device. Then it asks a hardware unit called the PLIC [1] to tell it which device interrupted (kernel/trap.c:186). If it was the UART, devintrcalls uartintr.

Accumulate inputs and Wake up

Read one char and hand over

uartintr (kernel/uart.c:84) reads any waiting input characters from the UART hardware and hands them to consoleintr.
// trap.c calls here when the uart interrupts.
int c = uartgetc();
if(c == -1)

Accumulate inputs

The job of consoleintr is to accumulate input characters in cons.buf until a whole line arrives.

Wake up when get a whole line

When a newline arrives, consoleintr wakes up a waiting consoleread (if there is one).

Copy to user space

Once woken, consoleread will observe a full line in cons.buf, copy it to user space and return to user space.


How does open(“console", O_RDWR) opens a file with DEVICE type?

Research: It seems this console string is defined somewhere. I tried to change to other string, and it does not work. Using this console string to open file, the file inode returns a file with type is DEVICE.
OS create a console device using system call mknod. Any read or write with console is interacting with the UART hardware.