def vAsyncAF equ $ff80
def vAsyncHL EQU $ff82 
def vAsyncDE EQU $ff84
def vAsyncBC equ $ff86
def vAsyncPC equ $ff88
def vAsyncNext equ $ff8a
def vAsyncAfter equ $ff8c

def SAFE_ASYNC_START EQU 144
def SAFE_ASYNC_END EQU 151

DoInAsyncVBlank: 
  di
  push af
  
  ld a, l 
  ldh [vAsyncHL], a
  ld a, h
  ldh [vAsyncHL+1], a
  ld a, e
  ldh [vAsyncDE], a
  ld a, d
  ldh [vAsyncDE+1], a
  ld a, c 
  ldh [vAsyncBC], a
  ld a, b 
  ldh [vAsyncBC+1], a
  
  pop hl
  ld a, l 
  ldh [vAsyncAF], a 
  ld a, h 
  ldh [vAsyncAF+1], a 
  
  ; set pu the next call!
  ldh a, [vAsyncNext]
  ldh [vAsyncPC], a 
  ldh a, [vAsyncNext+1]
  ldh [vAsyncPC+1], a ; put next into pc 
  ldh a, [vAsyncAfter]
  ldh [vAsyncNext], a 
  ldh a, [vAsyncAfter+1]
  ldh [vAsyncNext+1], a  ; puut after into next
  ld a, 0 
  ldh [vAsyncAfter], a ; unless this gets overwritten later, end execution after that
  ldh [vAsyncAfter+1], a 
  
  ld a, LOW(DoInAsyncVBlank_EnterThread)
  ld [INTERRUPT_LCD], a
  ld a, HIGH(DoInAsyncVBlank_EnterThread)
  ld [INTERRUPT_LCD + 1], a; set interrupt handler to "ENTER THREAD"
  ld a, SAFE_ASYNC_START ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [rLYC], a ; set LYC
  ld hl, rIE  
  set 1, [hl] ; enable vblank
  ld hl, rSTAT
  set 6, [hl] ; set the stat interrupt to LYC mode
  ld hl, rIF 
  res 1, [hl] ; clear the interrupt so we don't immediately fire it
  ei
  
  ret 

DoInAsyncVBlank_EnterThread:
  ;stack looks like: 
  ;c113 (SMC int @ LYC 90 pc), 004b (hw int @ LYC 90 pc), outer context pc

  push hl
  push bc
  push de
  push af

  ;af, de, bc, hl, c113 (SMC interrupt pc), 004b (hardwired interrput pc), outer context pc
  
  ; check if there's anything queued up for next execution
  ldh a, [vAsyncPC]
  ld l, a 
  ldh a, [vAsyncPC+1]
  ld h, a 
  or a, l 
  jp z, DoInAsyncVBlank_EndThread ; if nothing is queued up, jump to cleanup

  ld a, LOW(DoInAsyncVBlank_ExitThread)
  ld [INTERRUPT_LCD], a 
  ld a, HIGH(DoInAsyncVBlank_ExitThread)
  ld [INTERRUPT_LCD+1], a 
  ld a, SAFE_ASYNC_END ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [rLYC], a  ; set lcd interrupt handler to EXIT SAFE MODE on line 153
  
  ; what if our async thread calls return?
  ; we need to have a PC to return to. if that happens, we will want to
  ; reti. 
  ld hl, DoInAsyncVBlank_EarlyReturn
  push hl ; address for if the thread retrns
  
  ldh a, [vAsyncPC]
  ld l, a 
  ldh a, [vAsyncPC+1]
  ld h, a 
  push hl ; put vAsyncPC on the stack! for jumping to it!
  ; stack looks like: vasyncpc, early return, af, de, bc, hll, SMC interrput pc, hardwired interrupt pc, outer context pc 
  
  ldh a, [vAsyncAF]
  ld l, a 
  ldh a, [vAsyncAF+1]
  ld h, a 
  push hl
  
  ldh a, [vAsyncHL]
  ld l, a 
  ldh a, [vAsyncHL+1]
  ld h, a 
  ldh a, [vAsyncDE]
  ld e, a 
  ldh a, [vAsyncDE+1]
  ld d, a 
  ldh a, [vAsyncBC]
  ld c, a 
  ldh a, [vAsyncBC+1]
  ld b, a 
  
  pop af; putting vAsyncAF into af requires this hoop-jumping
  
  ei
  ret ; "return" to the vAsyncPC wee put on the stack previously.
  ; this is more or less a jump, not a return.
  ; is that the source of our problems?
  ; after this instruction executes, stack looks like:
  ; early return, af, de, bc, hl, PC (smc int), PC (hardwired int), PC (outer context)
  ; and we'll be in the async thread, executing
  
DoInAsyncVBlank_EarlyReturn:
  ; stack: 
  ; af, de, bc, hl, PC (smc int @ LYC90), PC (hw int @ LYC 90), PC (outer context)
  PRINTln "early return handle is ", DoInAsyncVBlank_EarlyReturn
  ; save state of registers
  di
  push af 
  
  ld a, l
  ldh [vAsyncHL], a
  ld a, h
  ldh [vAsyncHL+1], a
  ld a, e
  ldh [vAsyncDE], a 
  ld a, d 
  ldh [vAsyncDE+1], a 
  ld a, c 
  ldh [vAsyncBC], a
  ld a, b 
  ldh [vAsyncBC+1], a 
  
  pop hl
  ld a, l 
  ldh [vAsyncAF], a 
  ld a, h
  ldh [vAsyncAF+1], a 
  
  
  
  ; set pu the next call!
  ldh a, [vAsyncNext]
  ldh [vAsyncPC], a 
  ldh a, [vAsyncNext+1]
  ldh [vAsyncPC+1], a ; put next into pc 
  ldh a, [vAsyncAfter]
  ldh [vAsyncNext], a 
  ldh a, [vAsyncAfter+1]
  ldh [vAsyncNext+1], a  ; puut after into next
  ld a, 0 
  ldh [vAsyncAfter], a ; unless this gets overwritten later, end execution after that
  ldh [vAsyncAfter+1], a 
  
  ld a, LOW(DoInAsyncVBlank_EnterThread) ; set up next call
  ld [INTERRUPT_LCD], a 
  ld a, HIGH(DoInAsyncVBlank_EnterThread)
  ld [INTERRUPT_LCD+1], a 
  ld a, SAFE_ASYNC_START
  ld [rLYC], a  
  
   ; restore the pre-interrupt registers, return and enable interrupts
  pop af 
  pop de
  pop bc
  pop hl
  reti

DoInAsyncVBlank_EndThread:
  ; end execution of the thread by disabling the interrupt
  ; and restore the state for the outer thread
  ld hl, rIE  
  res 1, [hl] ; disable STAT/vblank interrupt
  pop af
  pop de
  pop bc
  pop hl
  reti

DoInAsyncVBlank_ExitThread:
  ; at this point, it's an interrpt being called from inside the interrupt
  ; our stack probably looks like this:
  ; PC (smc int), PC (hardwired int), PC (inside thread), early return, af, de, bc, hl, PC (smc int), PC (hardwired int), PC (outer context)
  ; first, save the interrpt thread registers.
  push af 
  
  ld a, l
  ldh [vAsyncHL], a
  ld a, h
  ldh [vAsyncHL+1], a
  ld a, e
  ldh [vAsyncDE], a 
  ld a, d 
  ldh [vAsyncDE+1], a 
  ld a, c 
  ldh [vAsyncBC], a
  ld a, b 
  ldh [vAsyncBC+1], a
  
  pop hl
  ld a, l 
  ldh [vAsyncAF], a 
  ld a, h
  ldh [vAsyncAF+1], a 
  
  ;god i'm such a genius
  ; this was called as an interrupt inside the interrupt
  ;so our stack looks like this:
  ; PC (smc int), PC (hardwired int), PC (inside thread), early return, af, de, bc, hl, PC (smc int), PC (hardwired int), PC (outer context)
  ; we've got the two PCs from the inner interrupt call stack.
  ; get rid of them! we're running without handrails! i know what i'm doing! I hope!
  pop hl
  pop hl
  ; now, save the interrupt thread's pc., which is on the stack from this 
  ; interrupt being called.
  pop hl
  ld a, l 
  ldh [vAsyncPC], a 
  ld a, h 
  ldh [vAsyncPC+1], a
  ; now we are done with this entire execution of thread. now we need to set up
  ; the next execution of this thread.
  ld a, LOW(DoInAsyncVBlank_EnterThread)
  ld [INTERRUPT_LCD], a 
  ld a, HIGH(DoInAsyncVBlank_EnterThread)
  ld [INTERRUPT_LCD+1], a 
  ld a, SAFE_ASYNC_START ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [rLYC], a  ; set lcd interrupt handler to EXIT SAFE MODE on line 153

  ;at present the stack looks like:
  ; early return, af, de, bc, hl, pc (smc int @ LYC 90), pc (hardwired int at LYC 90), PC (outer context)
  ; pop the early return and discard it.
  pop hl
  ; and finally, we can restore the state of the registers to what they were
  ; before this whole handler got called.
  pop af
  pop de 
  pop bc 
  pop hl
  reti ; this is a proper return