; variables for safe transfer async function
DEF vSafeCopySource EQU $ff80 
DEF vSafeCopyDest EQU vSafeCopySource + 2
DEF vSafeCopyCount EQU vSafeCopyDest + 2 ; check this for safe transfer being complete
DEF vSafeCopyOriginalCount EQU vSafeCopyCount + 2 
; stash previous interrupt state before using the interrupts
DEF vSafeCopyLYC EQU vSafeCopyOriginalCount + 2 ; stashes $FF45, the LYC register
DEF vSafeCopySTAT EQU vSafeCopyLYC + 1 ; stashes $FF41, the STAT register
DEF vSafeCopyInterrupt EQU vSafeCopySTAT + 1 ; stashes the previous LCD interrupt
DEF vSafeCopyInterruptEnable EQU vSafeCopyInterrupt + 2 ; stashes $FFFF, which interrupts are enabled


CopyRangeSafe: 
  ; hl is source
  ; de is destination
  ; bc is length to copy
; copy an array of bytes to a destination in memory.
; this is completely different from CopyRangeUnsafe.
; this initiates an asynchronous, interrupt-driven copy of BC bytes of memory
; from HL to DE.
; it may return a memory address to look at for progress of the transfer,
; or else it's just going to do it with a hard-coded address.
; check the transfer status address for zero; when it's zero, the transfer is done!
; this works by using the STAT interrupt in LYC mode to interrupt its own execution at 
; two scanlines inside the vblank interval to know when to start and stop.
  ; stash arguments in memory
  di
  ld a, l 
  ldh [vSafeCopySource], a
  ld a, h 
  ldh [vSafeCopySource+1], a 
  ld a, e 
  ldh [vSafeCopyDest], a
  ld a, d 
  ldh [vSafeCopyDest+1], a 
  ld a, c 
  ldh [vSafeCopyCount], a
  ld a, b 
  ldh [vSafeCopyCount+1], a 
  
  ; stash interrupt state in memory
  ldh a, [$ff41]
  ldh [vSafeCopySTAT], a ; stashes $FF41, the STAT register
  ldh a, [$ff45]
  ldh [vSafeCopyLYC], a ; stashes $FF45, the LYC register
  ld a, [INTERRUPT_LCD]
  ld [vSafeCopyInterrupt], a
  ld a, [INTERRUPT_LCD+1]
  ld [vSafeCopyInterrupt+1], a ; stashes the current STAT interrupt handler
  ld a, [$ffff]
  and a, %0000_0010
  ld [vSafeCopyInterruptEnable], a ; stashes whether LCD interrupt are enabled

  ld hl, CopyRangeSafe_EnterSafeMode
  ld a, l 
  ld [INTERRUPT_LCD], a
  ld a, h
  ld [INTERRUPT_LCD + 1], a; set interrupt handler to "ENTER SAFE MODE"
  ld a, 143 ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [$ff45], a 
  ld hl, $ffff 
  set 1, [hl]
  ld a, %0100_0000
  ld [$ff41], a 
  
  ld hl, vSafeCopyCount
  
  ld a, 0
  ld [$ff0f], a 
  nop
  nop
  nop
  
  
  ei
  nop
  nop
  nop
  ret ; return address of bytes remaining to copy

CopyRangeSafe_EnterSafeMode:
  push hl
  push bc
  push de
  push af
  ld hl, CopyRangeSafe_ExitSafeMode
  ld a, l 
  ld [INTERRUPT_LCD], a 
  ld a, h 
  ld [INTERRUPT_LCD+1], a 
  ld a, 153 ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [$ff45], a  ; set lcd interrupt handler to EXIT SAFE MODE on line 153
  ldh a, [vSafeCopySource]
  ld l, a 
  ldh a, [vSafeCopySource+1]
  ld h, a ; fetch the source
  ldh a, [vSafeCopyDest]
  ld e, a 
  ldh a, [vSafeCopyDest+1]
  ld d, a ; fetch the dest
  ldh a, [vSafeCopyCount]
  ld c, a 
  ldh a, [vSafeCopyCount+1]
  ld b, a ; fetch the count
  
  
  ; before starting transfer, make sure the zero flag is false.
  ld a, 1 
  cp a, 0
CopyRangeSafe_TransferLoop:
  ei
  nop ; ei only sets the flag one instruction later apparently. safety nop!
  di ; process interrupts
  jp z, CopyRangeSafe_CleanUp ; zero flag will only be set if the exitsafemode handler fired
  ld a, [hl+]
  ld [de], a ; 
  inc de
  dec bc
  ld a, b 
  or a, c
  jp nz, CopyRangeSafe_TransferLoop
  jp CopyRangeSafe_Done

CopyRangeSafe_ExitSafeMode:
  ld a, 0 
  cp a, 0 ; set the zero flag, which we're using as a signal to stop transferring
  reti ; set a to zero and set the zero flag true. now the transfer loop will end
  
CopyRangeSafe_CleanUp:
  ld a, l 
  ldh [vSafeCopySource], a
  ld a, h 
  ldh [vSafeCopySource+1], a  ; store new source
  ld a, e 
  ldh [vSafeCopyDest], a
  ld a, d 
  ldh [vSafeCopyDest+1], a ; store new dest
  ld a, c 
  ldh [vSafeCopyCount], a
  ld a, b 
  ldh [vSafeCopyCount+1], a ; store new count
  
  ld hl, CopyRangeSafe_EnterSafeMode
  ld a, l 
  ld [INTERRUPT_LCD], a 
  ld a, h 
  ld [INTERRUPT_LCD+1], a 
  ld a, 143 ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [$ff45], a  ; set lcd interrupt handler to ENTER SAFE MODE on line 148
  pop af
  pop de
  pop bc
  pop hl
  reti ; we're done with this memcpy cycle so we return from interrupt.

CopyRangeSafe_Done: ; called when the complete transfer is finished, 
; this restores interrupts to how they were.
  ; stash interrupt state in memory
  ld a, l 
  ldh [vSafeCopySource], a
  ld a, h 
  ldh [vSafeCopySource+1], a  ; store new source
  ld a, e 
  ldh [vSafeCopyDest], a
  ld a, d 
  ldh [vSafeCopyDest+1], a ; store new dest
  ld a, c 
  ldh [vSafeCopyCount], a
  ld a, b 
  ldh [vSafeCopyCount+1], a ; store new count
  
  ld hl, CopyRangeSafe_EnterSafeMode
  ld a, l 
  ld [INTERRUPT_LCD], a 
  ld a, h 
  ld [INTERRUPT_LCD+1], a 
  ld a, 143 ; CHANGE ME TO ADJUST SAFE TRANSFER TIMING
  ld [$ff45], a  ; set lcd interrupt handler to ENTER SAFE MODE on line 148
  pop af
  pop de
  pop bc
  pop hl
  
  ldh a, [vSafeCopySTAT]
  ldh [$ff41], a 
  ldh a, [vSafeCopyLYC]
  ldh [$ff45], a 
  ldh a, [vSafeCopyInterrupt]
  ld [INTERRUPT_LCD], a 
  ldh a, [vSafeCopyInterrupt+1]
  ld [INTERRUPT_LCD+1], a 
  
  ld hl, $ffff
  ld a, [hl] 
  cpl 
  set 1, a 
  cpl ; turn off the lcd interrupt enable
  ld [hl], a 
  
  ld a, [vSafeCopyInterruptEnable]
  cp a, 0 ; if the stashed enable was 0 return. if the stashed enable was 1 then turn it on 
  jp z, CopyRangeSafe_Return
  set 1, [hl] ; turn on the lcd interrupt
  
CopyRangeSafe_Return:
  reti

CopyRangeUnsafe:
  ; hl is source
  ; de is destination
  ; bc is length to copy
  ld a, [hl+]
  ld [de], a
  inc de
  dec bc
  ld a, b
  or a, c ; check if bc is zero
  jp nz, CopyRangeUnsafe
  ret
  
CopyRangeUnsafeBy8s:
  ; hl is source
  ; de is destination
  ; bc is length to copy
  
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  ld a, [hl+]
  ld [de], a
  inc de
  dec bc
  ld a, b
  or a, c ; check if bc is zero
  jp nz, CopyRangeUnsafeBy8s
  ret