//------------------------------------------------------------------------
//    This file is part of OmniPas.
//
//    You can find OmniPas here:
//    https://bitbucket.org/paul_nicholls/omnipas/src/master/
//
//    this file was translated (and modified) from:
//    https://github.com/SlithyMatt/zxs-assembly-tutorial/blob/main/Lesson12/irq.asm
//
//    OmniPas is free software: you can redistribute it and/or modify
//    it under the terms of the GNU General Public License as published by
//    the Free Software Foundation, either version 3 of the License, or
//    (at your option) any later version.
//
//    OmniPas is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY// without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU General Public License for more details.
//
//    You should have received a copy of the GNU General Public License
//    along with OmniPas.  If not, see <https://www.gnu.org/licenses/>.
//------------------------------------------------------------------------

unit spectrum_irq;

type
  TIRQ = record
    framecounter : Word;
    vbSync       : Byte;
  end;

var
  IRQ : TIRQ;

macro saveAllRegisters(); assembler;
asm
  // stash all registers to stack
  push af
  push bc
  push de
  push hl
  ex af,af'                  // exchange AF and AF'
  exx                        // exchange BE,DE,HL with BE',DE',HL'
  push af
  push bc
  push de
  push hl
  push ix
  push iy
end;

macro restoreAllRegisters(); assembler;
asm
  // restore all registers from stack
  pop iy
  pop ix
  pop hl
  pop de
  pop bc
  pop af
  ex af,af'                  // exchange AF and AF'
  exx                        // exchange BE,DE,HL with BE',DE',HL'
  pop hl
  pop de
  pop bc
  pop af
end;

macro exitIRQ(); assembler;
asm
  ei
end;

procedure vbSyncIRQ; assembler; volatile;
asm
  :saveAllRegisters();

  // increment frame counter
  ld hl,(IRQ.framecounter)
  inc hl
  ld (IRQ.framecounter),hl

  // set vertical blank sync
  ld a,1
  ld (IRQ.vbSync),a

USER_IRQ:
  call $ffff

  :restoreAllRegisters();
  :exitIRQ();
end;

procedure waitFor_vblank(); assembler;
asm
  // wait for vbSync <> 0
!:
  ld a,(IRQ.vbSync)
  cp 0
  jp z,!-
  // reset vbSync to 0
  xor a
  ld (IRQ.vbSync),a
end;

procedure setupInterrupt(irqRoutineAddr : Word); assembler;
const
  IM2_TABLE  = $FE00; // 256 byte page (+ 1 byte) for IM2
  IM2_VECTOR = $FDFD; // 3 bytes for JP routine under IM2 table

asm
  // link user irq to our routine
  ld hl,(irqRoutineAddr)
  ld (vbSyncIRQ.USER_IRQ + 1),hl

  // zero counters
  ld hl,0
  ld (IRQ.framecounter),hl
  xor a
  ld (IRQ.vbSync),a

  // Setup Interrupt Mode 2
  di                         // disable interrupts
  ld de,IM2_TABLE            // de = beginning of 257 bytes to set to $FD
  ld hl,IM2_VECTOR           // hl = vector location ($FDFD)
  ld a,d                     //
  ld i,a                     // i = custom IM2 table page
  ld a,l                     // a = $FD
fill_loop:
  ld (de),a                  // set byte in IM2 table to $FD
  inc e                      // de = next byte (unless e = 0)
  jp nz,fill_loop            // loop until whole page is set to $FD (e = 0)
  inc d                      // de = 257th byte of table
  ld (de),a                  // set last byte to $FD
  ld (hl),$C3                // place unconditional JP opcode at vector
  inc l                      // hl = vector + 1
  ld (hl),<vbSyncIRQ        // low byte of custom handler address
  inc l                      // hl = vector + 2
  ld (hl),>vbSyncIRQ        // high byte of custom handler address
  im 2                       // set interrupt mode 2
  ei                         // re-enable interrupts
end;