Kernel Multitasking

From OSDev Wiki
Jump to: navigation, search
Difficulty level
Difficulty 2.png

This page is under construction! This page or section is a work in progress and may thus be incomplete. Its content may be changed in the near future.

This page or section refers to its readers or editors using I, my, we or us. It should be edited to be in an encyclopedic tone.

In this tutorial, we'll cover the creation of a co-operative (not pre-emptive) kernel-level multitasking system. That is, kernel threads and processes. The code here is specific to the IA-32 architecture.




This header defines the function and types used in here. Its code is

#ifndef __TASK_H__
#define __TASK_H__
#include <stdint.h>
extern void initTasking();
typedef struct {
    uint32_t eax, ebx, ecx, edx, esi, edi, esp, ebp, eip, eflags, cr3;
} Registers;
typedef struct Task {
    Registers regs;
    struct Task *next;
} Task;
extern void initTasking();
extern void createTask(Task*, void(*)(), uint32_t, uint32_t*);
extern void yield(); // Switch task frontend
extern void switchTask(Registers *old, Registers *new); // The function which actually switches
#endif /* __TASK_H__ */


This file defines the actual wrappers that switchTask() uses, createTask() and yield():

#include "task.h"
static Task *runningTask;
static Task mainTask;
static Task otherTask;
static void otherMain() {
    printk("Hello multitasking world!"); // Not implemented here...
void initTasking() {
    // Get EFLAGS and CR3
    asm volatile("movl %%cr3, %%eax; movl %%eax, %0;":"=m"(mainTask.regs.cr3)::"%eax");
    asm volatile("pushfl; movl (%%esp), %%eax; movl %%eax, %0; popfl;":"=m"(mainTask.regs.eflags)::"%eax");
    createTask(&otherTask, otherMain, mainTask.regs.eflags, (uint32_t*)mainTask.regs.cr3); = &otherTask; = &mainTask;
    runningTask = &mainTask;
void createTask(Task *task, void (*main)(), uint32_t flags, uint32_t *pagedir) {
    task->regs.eax = 0;
    task->regs.ebx = 0;
    task->regs.ecx = 0;
    task->regs.edx = 0;
    task->regs.esi = 0;
    task->regs.edi = 0;
    task->regs.eflags = flags;
    task->regs.eip = (uint32_t) main;
    task->regs.cr3 = (uint32_t) pagedir;
    task->regs.esp = (uint32_t) allocPage() + 0x1000; // Not implemented here
    task->next = 0;
void yield() {
    Task *last = runningTask;
    runningTask = runningTask->next;
    switchTask(&last->regs, &runningTask->regs);


This is the file that actually changes between tasks. It defines a function, switchTask(), which does all the magic. It saves all registers to from and loads them from to. It is trickier than you might think. Its function prototype is

void switchTask(Registers *from, Registers *to);

Its code is

.section .text
.global switchTask
    mov %cr3, %eax #Push CR3
    push %eax
    mov 44(%esp), %eax #The first argument, where to save
    mov %ebx, 4(%eax)
    mov %ecx, 8(%eax)
    mov %edx, 12(%eax)
    mov %esi, 16(%eax)
    mov %edi, 20(%eax)
    mov 36(%esp), %ebx #EAX
    mov 40(%esp), %ecx #IP
    mov 20(%esp), %edx #ESP
    add $4, %edx #Remove the return value ;)
    mov 16(%esp), %esi #EBP
    mov 4(%esp), %edi #EFLAGS
    mov %ebx, (%eax)
    mov %edx, 24(%eax)
    mov %esi, 28(%eax)
    mov %ecx, 32(%eax)
    mov %edi, 36(%eax)
    pop %ebx #CR3
    mov %ebx, 40(%eax)
    push %ebx #Goodbye again ;)
    mov 48(%esp), %eax #Now it is the new object
    mov 4(%eax), %ebx #EBX
    mov 8(%eax), %ecx #ECX
    mov 12(%eax), %edx #EDX
    mov 16(%eax), %esi #ESI
    mov 20(%eax), %edi #EDI
    mov 28(%eax), %ebp #EBP
    push %eax
    mov 36(%eax), %eax #EFLAGS
    push %eax
    pop %eax
    mov 24(%eax), %esp #ESP
    push %eax
    mov 40(%eax), %eax #CR3
    mov %eax, %cr3
    pop %eax
    push %eax
    mov 32(%eax), %eax #EIP
    xchg (%esp), %eax #We do not have any more registers to use as tmp storage
    mov (%eax), %eax #EAX
    ret #This ends all!

Doing it

Put this on some source file

#include "task.h"
void doIt() {
    printk("Switching to otherTask... \n");
    printk("Returned to mainTask!\n");

Now, from your kernel_main() call doIt()!

Congratulations! You've just implement kernel multitasking!

(Note: Article on proper kernel multi-tasking coming soon)

Personal tools