//https://wiki.osdev.org/Multitasking_Systems

//C declaration:
//   void switch_tasks(thread_control_block *next_thread)//
//
//WARNING: Caller is expected to disable IRQs before calling, and enable IRQs again after function returns

.type switch_tasks, @function
.global switch_tasks

switch_tasks:
    push %ebp
    mov %esp, %ebp
    mov +12(%esp), %eax
    mov %esp, (%eax) //save old esp
    mov +8(%esp), %eax
    mov %eax, %esp // move the forged stack to esp
    pop %ebp // the top of the forged stack contains ebp
    ret //the top of the forged stack contains eip to go to

    //Save previous task's state
    //Notes:
    //  For cdecl// EAX, ECX, and EDX are already saved by the caller and don't need to be saved again
    //  EIP is already saved on the stack by the caller's "CALL" instruction
    //  The task isn't able to change CR3 so it doesn't need to be saved
    //  Segment registers are constants (while running kernel code) so they don't need to be saved

    // push %ebx
    // push %esi
    // push %edi
    // push %ebp

    // mov %edi,[current_task]    //edi = address of the previous task's "thread control block"
    // mov [edi+TCB.ESP], %esp         //Save ESP for previous task's kernel stack in the thread's TCB

    // Load next task's state

    // mov %esi,[esp+(4+1)*4]         //esi = address of the next task's "thread control block" (parameter passed on stack)
    // mov [current_task], %esi    //Current task's TCB is the next task TCB

    // mov %esp,[esi+TCB.ESP]         //Load ESP for next task's kernel stack from the thread's TCB
    // mov eax,[esi+TCB.CR3]         //eax = address of page directory for next task
    // mov %ebx,[esi+TCB.ESP0]        //ebx = address for the top of the next task's kernel stack
    // mov [TSS.ESP0],ebx            //Adjust the ESP0 field in the TSS (used by CPU for for CPL=3 -> CPL=0 privilege level changes)
    // mov ecx,cr3                   //ecx = previous task's virtual address space

    // cmp eax,ecx                   //Does the virtual address space need to being changed?
    // je .doneVAS                   // no, virtual address space is the same, so don't reload it and cause TLB flushes
    // mov cr3,eax                   // yes, load the next task's virtual address space
// .doneVAS:

    // pop %ebp
    // pop %edi
    // pop %esi
    // pop %ebx