os/kernel/cpu/i386/tasking_helpers.asm

section .text
global switch_to_thread_asm
extern currentThread
extern tss
;WARNING: Caller is expected to disable IRQs before calling, and enable IRQs again after function returns

switch_to_thread_asm:

    ;Save previous thread'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 thread 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,[currentThread]    ;edi = address of the previous thread's data structure
    mov [edi],esp         ;Save ESP for the thread's kernel stack in the thread's data structure

    ;Load next thread's state

    mov esi,[esp+(4+1)*4]         ;esi = address of the next thread's data structure
    mov [currentThread],esi    ;Set the current thread to the thread we are switching to

    mov esp,[esi]         ;Load ESP for next thread's kernel stack from the thread's data structure
    mov eax,[esi+8]         ;eax = address of page directory for next thread
    mov ebx,[esi+4]        ;ebx = address for the top of the next thread's kernel stack
    mov [tss+4],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 thread'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 thread's virtual address space
.doneVAS:

    pop ebp
    pop edi
    pop esi
    pop ebx

    ret                           ;Load next thread's EIP from its kernel stack

global task_init

task_init:
  pop ecx
  pop ebx
  cli
  mov ax, 0x23
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  mov eax, ecx
  push 0x23
  push eax
  pushf
  pop eax
  or eax, 0x200
  push eax
  push 0x1B
  push ebx
  iret

global wait_for_unblocked_thread_asm

wait_for_unblocked_thread_asm:
  sti ;As interrupts are stopped in tasking code, re-enable them
  hlt ;Wait for an interrupt handler to run and return.
  cli ;Clear interrupts, as tasking code must not be run with interrupts on.