fiber.asm

default rel
bits 64

; stuff we need to export to C
global _fiber_new
global _fiber_yield
global _fiber_start
global _fiber_resume
global _main

; stuff we need from C
extern _malloc
extern _printf

section .data
str:            db 'Hello, world',10,0

section .bss
current_fiber:  resq 1
saved_rbp:      resq 1
saved_rsp:      resq 1

section .text
_fiber_new:
  mov r12, rdi
  mov rdi, 4096                   ; allocate space for a fiber
  call _malloc
  mov qword [rax], r12            ; set the initial rip of the fiber to the
                                  ; function pointer passed in as the first
                                  ; argument
  mov rbx, rax
  add rbx, 4088
  mov qword [rax + 16], rbx
  mov qword [rax + 24], rbx       ; set up initial stack
  ret

_fiber_yield:
  pop qword [current_fiber]       ; the fiber should resume at the return
                                  ; address of this function
  jmp _fiber_cleanup

_fiber_resume:
  mov [current_fiber], rdi        ; save the address of the fiber
  pop qword [rdi + 8]             ; pop the return address into memory

  mov qword [saved_rsp], rsp
  mov qword [saved_rbp], rbp
  mov rbp, qword [rdi + 16]
  mov rsp, qword [rdi + 24]

  call [rdi]
  jmp _fiber_cleanup

_fiber_start:
  jmp _fiber_resume

_fiber_cleanup:
  mov rax, [current_fiber]
  add rax, 16
  mov qword [rax], rbp
  add rax, 8
  mov qword [rax], rsp
  mov rsp, qword [saved_rsp]
  mov rbp, qword [saved_rbp]
  sub rax, 16
  jmp [rax]

_main:
  mov rdi, _func
  call _fiber_new

  mov rax, 0
  ret

_func:
  push rbp
  mov rbp, rsp
  sub rsp, 8

  mov rdi, str
  call _printf

  add rsp, 8
  mov rsp, rbp
  pop rbp
  ret