;;;;;;;;ISR for PIC:

extern timerTick

irq0:
    cli
    mov al, 0x20
    out 0x20, al
    mov eax, timerTick
    mov ecx, [esp]          ;grab eip off stack
    mov edx, [esp+8]        ;and grab eflags
    mov ebx, esp
    add ebx, 12   ;eip, cs, eflags were pushed on when this ISR was called, so the old esp is currentESP+12
    push ecx
    push ebx
    push edx
    call eax
    pop ecx
    pop ecx
    pop ecx
    sti
    iret

////////////////timerTick, called by ISR:
int tick=0;
void timerTick (unsigned int eflags, unsigned int esp, unsigned int eip) {
    tick++;
    if (tick == 20) {
        tick = 0;
        switchTasks (eflags, esp, eip);
    }
}

//////////////switchTasks, does the actual task-switching
typedef struct task {
    unsigned int esp, ebp, eip, eax, ebx, ecx, edx, edi, esi, eflags, id, *pageDir;
    struct task *nextTask;
} task_t;

task_t *firstTask, *currentTask;

unsigned int switchTasks (unsigned int eflags, unsigned int esp, unsigned int eip) {
    if (firstTask->nextTask == firstTask)   {           //if we only have one task, just return
        return 0;
    }

    currentTask->eip = eip;                     //eip is taken from the stack when the ISR is called, and passed into this function
    currentTask->esp = esp;                     //(through the previous function, tick, in timer.c), esp is passed in from ISR too
    currentTask->eflags = (eflags & 0xFFFFFDFF);            //clear the IF bit, so we aren't interrupted between when we restore the flags register and jmp to next task
    asm volatile ("mov %0, ebp" : "=r" (currentTask->ebp));     //store all the general registers and base pointer in the task struct
    asm volatile ("mov %0, eax" : "=r" (currentTask->eax));
    asm volatile ("mov %0, ebx" : "=r" (currentTask->ebx));
    asm volatile ("mov %0, ecx" : "=r" (currentTask->ecx));
    asm volatile ("mov %0, edx" : "=r" (currentTask->edx));
    asm volatile ("mov %0, esi" : "=r" (currentTask->esi));
    asm volatile ("mov %0, edi" : "=r" (currentTask->edi));

    currentTask = currentTask->nextTask;                //load next task struct

    asm volatile ("mov cr3, %0" :: "r" (currentTask->pageDir)); //switch to new page directory
    asm volatile ("mov esp, %0" :: "r" (currentTask->esp));     //and update all the registers with the new task's values
    asm volatile ("mov ebp, %0" :: "r" (currentTask->ebp));
    asm volatile ("mov eax, %0" :: "r" (currentTask->eax));
    asm volatile ("mov ebx, %0" :: "r" (currentTask->ebx));
    asm volatile ("mov ecx, %0" :: "r" (currentTask->ecx));
    asm volatile ("mov edi, %0" :: "r" (currentTask->edi));
    asm volatile ("mov edx, %0" :: "r" (currentTask->edx));     //below: put eflags into esi, push it, popfd pops the top value on the stack into the eflags register, then store esi's value in esi
    asm volatile ("mov esi, %0; push esi; popfd; mov esi, %1" :: "r" (currentTask->eflags), "r" (currentTask->esi));
    asm volatile ("sti; jmp %0" :: "r" (currentTask->eip));     //jmp to next task's eip

    return 0;
}