/**
AstroLibre
An open source Bally Astrocade BIOS implementation in C
by Steven Hugg (@sehugg) for 8bitworkshop.com

To the extent possible under law, the author(s) have
dedicated all copyright and related and neighboring
rights to this software to the public domain worldwide.
This software is distributed without any warranty.

See: http://creativecommons.org/publicdomain/zero/1.0/
**/

//#resource "astrocade.inc"
//#link "biosasm.s"

// comment out to build BIOS without demo
//#link "test1.s"

#include <string.h>

typedef unsigned char byte;
typedef signed char sbyte;
typedef unsigned short word;
typedef signed short sword;

/// HARDWARE

__sfr __at(0x00) hw_col0r;	// palette 0
__sfr __at(0x01) hw_col1r;
__sfr __at(0x02) hw_col2r;
__sfr __at(0x03) hw_col3r;
__sfr __at(0x04) hw_col0l;
__sfr __at(0x05) hw_col1l;
__sfr __at(0x06) hw_col2l;
__sfr __at(0x07) hw_col3l;	// palette 7

__sfr __at(0x08) hw_intst;	// intercept test feedback
__sfr __at(0x09) hw_horcb;	// horiz color boundary
__sfr __at(0x0a) hw_verbl;	// vertical blanking line * 2
__sfr __at(0x0c) hw_magic;	// magic register
__sfr __at(0x0d) hw_infbk;	// interrupt feedback
__sfr __at(0x0e) hw_inmod;	// interrupt enable
__sfr __at(0x0f) hw_inlin;	// interrupt line
__sfr __at(0x19) hw_xpand;	// expander register

__sfr __at(0x10) hw_p1ctrl;	// player controls
__sfr __at(0x11) hw_p2ctrl;	// player controls
__sfr __at(0x12) hw_p3ctrl;	// player controls
__sfr __at(0x13) hw_p4ctrl;	// player controls

#define M_SHIFT0	0x00
#define M_SHIFT1	0x01
#define M_SHIFT2	0x02
#define M_SHIFT3	0x03
#define M_XPAND		0x08
#define M_MOVE		0x00
#define M_OR		0x10
#define M_XOR		0x20
#define M_FLOP		0x40
#define M_SHIFT(x)	((x)&3)
#define XPAND_COLORS(off,on) (((off)&3) | (((on)&3)<<2))

// font options
#define OPT_1x1		0x00
#define OPT_2x2		0x40
#define OPT_4x4		0x80
#define OPT_8x8		0xc0
#define OPT_XOR		0x20
#define OPT_OR		0x10
#define OPT_ON(n)	((n)<<2)
#define OPT_OFF(n)	((n))

// bcd options
#define DISBCD_SML	0x40
#define DISBCD_NOZERO	0x80

/// GRAPHICS FUNCTIONS

#define VHEIGHT 102	// number of scanlines
#define VBWIDTH 40	// number of bytes per scanline
#define PIXWIDTH 160	// 4 pixels per byte

//#define EXIT_CLIPDEST(addr) if ((((word)addr)&0xfff) >= 0xe10) return
#define EXIT_CLIPDEST(addr)

byte __at (0x0000) vmagic[VHEIGHT][VBWIDTH];
byte __at (0x4000) vidmem[VHEIGHT][VBWIDTH];

// start @ $4FCE
volatile word MUZPC;   // music PC
volatile word MUZSP;   // music SP
volatile byte PVOLAB;  // channels A and B volume
volatile byte PVOLMC;  // channel C volume and noise mask
volatile byte VOICES;  // voice smask

volatile byte CT[8];   // counter timers

volatile byte CNT;
volatile byte SEMI4S;
volatile byte OPOT[4];
volatile byte KEYSEX;
volatile byte OSW[4];
volatile word COLLST;

volatile byte DURAT;   // note duration
volatile byte TMR60;   // 1/60 sec timer
volatile byte TIMOUT;  // blackout timer
volatile byte GTSECS;  // seconds timer
volatile byte GTMINS;  // minutes timer

unsigned long RANSHT;	// RNG
byte NUMPLY;		// # players
byte ENDSCR[3];		// end score
byte MRLOCK;		// magic register lock out
byte GAMSTB;		// game status
byte PRIOR;		// music protect
byte SENFLG; 		// sentry control

byte* UMARGT;		// user mask table (-64 bytes)
word* USERTB;		// user routine table (-128 bytes)

typedef struct {
  byte base_ch;		// first char
  byte frame_x;		// frame width
  byte frame_y;		// frame height
  byte pattern_x;	// pattern width
  byte pattern_y;	// pattern height
  const byte* chartab;	// pointer to char data
} FontDescriptor;

#define LOCHAR 0x20
#define HICHAR 0x63

extern const char BIGFONT[HICHAR-LOCHAR+1][7];
extern const char SMLFONT[HICHAR-LOCHAR+1][5];

const FontDescriptor __at(0x206) FNTSYS; // = { 0x20, 8, 8, 1, 7, (byte*)BIGFONT };
const FontDescriptor __at(0x20d) FNTSML; // = { 0xa0, 4, 6, 1, 5, (byte*)SMLFONT };



// INTERRUPT HANDLERS
// must be in 0x200 page
void hw_interrupt() __interrupt {
  CT[0]++;
  CT[1]++;
  CT[2]++;
  CT[3]++;
  if (++TMR60 == 60) {
    TMR60 = 0;
    if (++GTSECS == 60) {
      GTMINS++;
    }
  }
}

// TODO
byte add_counters(byte mask, byte delta) {
  byte i = 0;
  byte any0 = 0;
  while (mask) {
    if (mask & 1) {
      if (CT[i]) {
        if (!(CT[i] += delta))
          any0 = 1;
      }
    }
    mask >>= 1;
    i++;
  }
  return any0;
}

void STIMER() {
}

void CTIMER() {
}

void TIMEZ() {
  // updates game timer, blackout timer, and music processor
}

void TIMEY() {
  CT[0]--;
  CT[1]--;
  CT[2]--;
  CT[3]--;
}

void TIMEX() {
}

///// INTERPRETER

typedef struct {
  union {
    struct {
      word iy,ix,de,bc,af,hl;
    } w;
    struct {
      byte iyl,iyh,ixl,ixh,e,d,c,b,f,a,l,h;
    } b;
  } regs;
  byte* params;
} ContextBlock;

#define REG_IY	0x1
#define REG_E	0x2
#define REG_D	0x4
#define REG_C	0x8
#define REG_B	0x10
#define REG_IX  0x20
#define REG_A	0x40
#define REG_HL	0x80
#define REG_DE	(REG_D|REG_E)
#define REG_BC	(REG_B|REG_C)

#define _IY	(ctx->regs.w.iy)
#define _IX	(ctx->regs.w.ix)
#define _DE	(ctx->regs.w.de)
#define _BC	(ctx->regs.w.bc)
#define _AF	(ctx->regs.w.af)
#define _HL	(ctx->regs.w.hl)
#define _IXL	(ctx->regs.b.ixl)
#define _IXH	(ctx->regs.b.ixh)
#define _IYL	(ctx->regs.b.iyl)
#define _IYH	(ctx->regs.b.iyh)
#define _E	(ctx->regs.b.e)
#define _D	(ctx->regs.b.d)
#define _C	(ctx->regs.b.c)
#define _B	(ctx->regs.b.b)
#define _A	(ctx->regs.b.a)
#define _L	(ctx->regs.b.l)
#define _H	(ctx->regs.b.h)

typedef void (Routine)();
typedef void (SysRoutine)(ContextBlock *ctx);

typedef struct {
  SysRoutine* routine;
  byte argmask;
} SysCallEntry;

void SYSCALL(ContextBlock *ctx);

void INTPC(ContextBlock *ctx) {
  while (ctx->params[0] != 2) { // 2 = exit
    SYSCALL(ctx);
  }
}

// never called, hopefully
void EXIT(ContextBlock *ctx) {
  ctx;
}

// jumps to HL
void RCALL(ContextBlock *ctx) {
  ((Routine*)_HL)();
}

// start interpreting at HL
void MCALL(ContextBlock *ctx) {
  ctx; // TODO
}

// exit MCALL loop
void MRET(ContextBlock *ctx) {
  ctx; // TODO
}

// jump within MCALL
void MJUMP(ContextBlock *ctx) {
  ctx->params = (byte*) _HL; // TODO?
}

void suckParams(ContextBlock *ctx, byte argmask) {
  byte* dest = (byte*) ctx;
  byte* src = ctx->params;
  if (argmask & REG_IX) {
    _IXL = *src++;
    _IXH = *src++;
  }
  if (argmask & REG_E)
    _E = *src++;
  if (argmask & REG_D)
    _D = *src++;
  if (argmask & REG_C)
    _C = *src++;
  if (argmask & REG_B)
    _B = *src++;
  if (argmask & REG_A)
    _A = *src++;
  if (argmask & REG_HL) {
    _L = *src++;
    _H = *src++;
  }
  ctx->params = src;
}

void SUCK(ContextBlock* ctx) {
  suckParams(ctx, _B);
}


const void* const actint_vec = &hw_interrupt;

void ACTINT(ContextBlock *ctx) {
  ctx;
__asm
  LD	BC,#(_actint_vec)  ; upper 8 bits of address
  LD	A,B
  LD	I,A
  IM	2	; mode 2
  EI		; enable interrupts
__endasm;
  hw_inlin = 200;
  hw_infbk = (byte) &actint_vec;
  hw_inmod = 0x8;
  TIMEZ();
  TIMEY();
}

// Outputs D to port 0A, B to port 09, A to port 0E.
void SETOUT(ContextBlock *ctx) {
  hw_verbl = _D;
  hw_horcb = _B;
  hw_inmod = _A;
}

// sets color palettes from (HL)
void COLSET(ContextBlock *ctx) {
  byte* palette = (byte*) _HL;
  hw_col3l = *palette++;
  hw_col2l = *palette++;
  hw_col1l = *palette++;
  hw_col0l = *palette++;
  hw_col3r = *palette++;
  hw_col2r = *palette++;
  hw_col1r = *palette++;
  hw_col0r = *palette++;
}

// Stores A in BC bytes starting at location DE.
void FILL(ContextBlock *ctx) {
  byte* dest = (byte*) _DE;
  word count = _BC;
  byte val = _A;
  memset(dest, val, count);
}

void hline(byte x1, byte x2, byte y, byte pattern) {
  byte xb1 = x1/4;
  byte xb2 = x2/4;
  byte* dest = &vmagic[y][xb1];
  signed char nbytes = xb2 - xb1;
  hw_magic = M_SHIFT(x1) | M_XOR;
  while (--nbytes > 0) {
    *dest++ = pattern;
  }
  if (x2&3) *dest = 0;
  // TODO
}

// Fill rect (E,D,C,B) color A
void RECTAN(const ContextBlock *ctx) {
  for (byte y=_D; y<_D+_B; y++) {
    hline(_E, _E+_C, y, _A);
  }
}

const char BIGFONT[HICHAR-LOCHAR+1][7] = {/*{count:68,w:8,h:7,brev:1}*/
{0x00,0x00,0x00,0x00,0x00,0x00,0x00},{0x20,0x20,0x20,0x20,0x00,0x00,0x20},{0x50,0x50,0x00,0x00,0x00,0x00,0x00},{0x48,0xFC,0x48,0x48,0x48,0xFC,0x48},{0x20,0x78,0x80,0x70,0x08,0xF0,0x20},{0x00,0x48,0x10,0x20,0x40,0x90,0x00},{0x60,0x90,0x60,0xA0,0xA8,0x90,0x68},{0x60,0x60,0x20,0x00,0x00,0x00,0x00},{0x20,0x40,0x40,0x40,0x40,0x40,0x20},{0x40,0x20,0x20,0x20,0x20,0x20,0x40},{0x00,0xA8,0x70,0xF8,0x70,0xA8,0x00},{0x00,0x20,0x20,0xF8,0x20,0x20,0x00},{0x00,0x00,0x00,0x00,0x60,0x20,0x40},{0x00,0x00,0x00,0xF0,0x00,0x00,0x00},{0x00,0x00,0x00,0x00,0x00,0x60,0x60},{0x00,0x08,0x10,0x20,0x40,0x80,0x00},{0x70,0x88,0x88,0xA8,0x88,0x88,0x70},{0x20,0x60,0x20,0x20,0x20,0x20,0x70},{0x70,0x88,0x08,0x30,0x40,0x80,0xF8},{0x70,0x88,0x08,0x70,0x08,0x88,0x70},{0x10,0x30,0x50,0x90,0xF8,0x10,0x10},{0xF8,0x80,0x80,0x70,0x08,0x88,0x70},{0x70,0x88,0x80,0xF0,0x88,0x88,0x70},{0xF8,0x88,0x10,0x20,0x20,0x20,0x20},{0x70,0x88,0x88,0x70,0x88,0x88,0x70},{0x70,0x88,0x88,0x78,0x08,0x88,0x70},{0x00,0x00,0x60,0x00,0x60,0x00,0x00},{0x00,0x00,0x60,0x00,0x60,0x20,0x40},{0x10,0x20,0x40,0x80,0x40,0x20,0x10},{0x00,0x00,0xF8,0x00,0xF8,0x00,0x00},{0x40,0x20,0x10,0x08,0x10,0x20,0x40},{0x70,0x08,0x08,0x30,0x20,0x00,0x20},{0x70,0x88,0xB8,0xA8,0x90,0x80,0x70},{0x70,0x88,0x88,0xF8,0x88,0x88,0x88},{0xF0,0x88,0x88,0xF0,0x88,0x88,0xF0},{0x70,0x88,0x80,0x80,0x80,0x88,0x70},{0xF0,0x88,0x88,0x88,0x88,0x88,0xF0},{0xF8,0x80,0x80,0xF0,0x80,0x80,0xF8},{0xF8,0x80,0x80,0xF0,0x80,0x80,0x80},{0x70,0x88,0x80,0xB0,0x88,0x88,0x70},{0x88,0x88,0x88,0xF8,0x88,0x88,0x88},{0x70,0x20,0x20,0x20,0x20,0x20,0x70},{0x08,0x08,0x08,0x08,0x88,0x88,0x70},{0x88,0x90,0xA0,0xC0,0xA0,0x90,0x88},{0x80,0x80,0x80,0x80,0x80,0x80,0xF8},{0x88,0xD8,0xA8,0x88,0x88,0x88,0x88},{0x88,0xC8,0xA8,0xA8,0x98,0x88,0x88},{0xF8,0x88,0x88,0x88,0x88,0x88,0xF8},{0xF0,0x88,0x88,0xF0,0x80,0x80,0x80},{0x70,0x88,0x88,0xA8,0xA8,0x90,0x68},{0xF0,0x88,0x88,0xF0,0x90,0x90,0x88},{0x70,0x88,0x80,0x70,0x08,0x88,0x70},{0xF8,0x20,0x20,0x20,0x20,0x20,0x20},{0x88,0x88,0x88,0x88,0x88,0x88,0x70},{0x88,0x88,0x88,0x88,0x88,0x50,0x20},{0x88,0x88,0x88,0x88,0xA8,0xD8,0x88},{0x88,0x88,0x50,0x20,0x50,0x88,0x88},{0x88,0x88,0x50,0x20,0x20,0x20,0x20},{0xF8,0x08,0x10,0x20,0x40,0x80,0xF8},{0x70,0x40,0x40,0x40,0x40,0x40,0x70},{0x00,0x80,0x40,0x20,0x10,0x08,0x00},{0x70,0x10,0x10,0x10,0x10,0x10,0x70},{0x20,0x70,0xA8,0x20,0x20,0x20,0x00},{0x00,0x20,0x40,0xF8,0x40,0x20,0x00},{0x00,0x20,0x20,0x20,0xA8,0x70,0x20},{0x00,0x20,0x10,0xF8,0x10,0x20,0x00},{0x00,0x88,0x50,0x20,0x50,0x88,0x00},{0x00,0x20,0x00,0xF8,0x00,0x20,0x00}};

const char SMLFONT[HICHAR-LOCHAR+1][5] = {/*{count:68,w:5,h:5,brev:1}*/
{ 0x00,0x00,0x00,0x00,0x00 },{ 0x40,0x40,0x00,0x40,0x00 },{ 0xA0,0xA0,0x00,0x00,0x00 },{ 0x60,0xF0,0xF0,0x60,0x00 },{ 0x40,0xE0,0xE0,0x40,0x00 },{ 0x90,0x20,0x40,0x90,0x00 },{ 0xC0,0xB0,0xE0,0xD0,0x00 },{ 0x20,0x40,0x00,0x00,0x00 },{ 0x20,0x40,0x40,0x20,0x00 },{ 0x40,0x20,0x20,0x40,0x00 },{ 0x40,0xE0,0x40,0xA0,0x00 },{ 0x00,0x40,0xE0,0x40,0x00 },{ 0x00,0x00,0x00,0x60,0x20 },{ 0x00,0x00,0xE0,0x00,0x00 },{ 0x00,0x00,0x00,0x40,0x00 },{ 0x20,0x20,0x40,0x40,0x00 },{ 0xE0,0xA0,0xA0,0xA0,0xE0 },{ 0xC0,0x40,0x40,0x40,0xE0 },{ 0xE0,0x20,0xE0,0x80,0xE0 },{ 0xE0,0x20,0x60,0x20,0xE0 },{ 0xA0,0xA0,0xE0,0x20,0x20 },{ 0xE0,0x80,0xE0,0x20,0xE0 },{ 0xE0,0x80,0xE0,0xA0,0xE0 },{ 0xE0,0x20,0x40,0x40,0x40 },{ 0xE0,0xA0,0xE0,0xA0,0xE0 },{ 0xE0,0xA0,0xE0,0x20,0xE0 },{ 0x00,0x40,0x00,0x40,0x00 },{ 0x00,0x40,0x00,0x60,0x20 },{ 0x00,0x20,0x40,0x20,0x00 },{ 0x00,0xE0,0x00,0xE0,0x00 },{ 0x00,0x40,0x20,0x40,0x00 },{ 0xE0,0x20,0x60,0x00,0x40 },{ 0xF0,0x90,0x10,0xD0,0xF0 },{ 0x60,0xA0,0xE0,0xA0,0x00 },{ 0xC0,0xE0,0xA0,0xE0,0x00 },{ 0x60,0x80,0x80,0xE0,0x00 },{ 0xC0,0xA0,0xA0,0xC0,0x00 },{ 0xE0,0xC0,0x80,0xE0,0x00 },{ 0xE0,0xC0,0x80,0x80,0x00 },{ 0x60,0x80,0xA0,0xE0,0x00 },{ 0xA0,0xA0,0xE0,0xA0,0x00 },{ 0xE0,0x40,0x40,0xE0,0x00 },{ 0x60,0x20,0xA0,0xE0,0x00 },{ 0xA0,0xC0,0xC0,0xA0,0x00 },{ 0x80,0x80,0x80,0xE0,0x00 },{ 0xE0,0xE0,0xE0,0xA0,0x00 },{ 0xE0,0xA0,0xA0,0xA0,0x00 },{ 0xE0,0xA0,0xA0,0xE0,0x00 },{ 0xE0,0xA0,0xE0,0x80,0x00 },{ 0xE0,0xA0,0xE0,0xF0,0x00 },{ 0xE0,0xA0,0xC0,0xA0,0x00 },{ 0xE0,0x80,0x60,0xE0,0x00 },{ 0xE0,0x40,0x40,0x40,0x00 },{ 0xA0,0xA0,0xA0,0xE0,0x00 },{ 0xA0,0xA0,0xC0,0x80,0x00 },{ 0xA0,0xE0,0xE0,0xE0,0x00 },{ 0xA0,0x40,0xA0,0xA0,0x00 },{ 0xA0,0xE0,0x40,0x40,0x00 },{ 0xE0,0x20,0x40,0xE0,0x00 },{ 0x60,0x40,0x40,0x60,0x00 },{ 0x40,0x40,0x20,0x20,0x00 },{ 0x60,0x20,0x20,0x60,0x00 },{ 0x40,0xA0,0x00,0x00,0x00 },{ 0x00,0x00,0x00,0x00,0xF0 },{ 0x80,0x40,0x00,0x00,0x00 },{ 0x00,0x60,0xA0,0xE0,0x00 },{ 0x80,0xE0,0xA0,0xE0,0x00 },{ 0x00,0x60,0x80,0xE0,0x00 }};

// draw a letter
byte draw_char(const FontDescriptor* font, byte ch, byte x, byte y, byte op) {
  const byte* src = font->chartab + (ch-font->base_ch)*font->pattern_y;
  byte* dest = &vmagic[y][x>>2];// destination address
  byte magic = M_SHIFT(x) | M_XPAND | (op & 0x30);
  // big sizes?
  if (op & 0xc0) {
    char buf[8];		// expansion buffer
    char* mbuf = (buf - 0x4000);// make it magic
    byte sc = 1 << (op >> 6); // 2x2 = 2, 4x4 = 4, 8x8 = 8
    for (byte i=0; i<font->pattern_y; i++) {
      // expand into magic buffer onto stack
      hw_magic = M_XPAND;
      hw_xpand = 0b1100;	// on = 11, off = 00
      // 2x2 size
      mbuf[1] = mbuf[0] = *src++;
      // 4x4 size
      if (op & 0x80) {
        byte b = buf[0];
        mbuf[3] = mbuf[2] = buf[1];
        mbuf[1] = mbuf[0] = b;
      }
      // 8x8 size
      if ((op & 0xc0) == 0xc0) {
        byte b = buf[0];
        mbuf[7] = mbuf[6] = buf[3];
        mbuf[5] = mbuf[4] = buf[2];
        mbuf[3] = mbuf[2] = buf[1];
        mbuf[1] = mbuf[0] = b;
      }
      // draw to screen (magic, again)
      hw_xpand = op & 0xf;
      for (byte j=0; j<sc; j++) {
        hw_magic = magic; // reset flip flop
        EXIT_CLIPDEST(dest);
        for (byte k=0; k<sc; k++) {
          byte b = buf[k];
          *dest++ = b;
          *dest++ = b;
        }
        dest += VBWIDTH-sc*2;
      }
    }
  } else {
    hw_xpand = op & 0xf;
    for (byte i=0; i<font->pattern_y; i++) {
      char b = *src++;
      EXIT_CLIPDEST(dest);
      hw_magic = magic; // reset flip flop
      *dest++ = b;	// expand lower nibble -> 1st byte
      *dest++ = b;	// expand upper nibble -> 2nd byte
      *dest++ = 0;	// leftover -> 3rd byte
      dest += VBWIDTH-3;	// we incremented 3 bytes for this line
    }
  }
  return font->frame_x << (op >> 6);
}

#define FONT_IX ((const FontDescriptor*)ctx->regs.w.ix)

void draw_string(ContextBlock *ctx, const char* str, byte x, byte y, byte op) {
  do {
    byte ch = *str++;
    if (!ch) {
      _E = x;
      break;
    }
    if (ch < 0x20) {
      x += draw_char(&FNTSYS, ' ', x, y, op); // TODO
    } else if (ch < 0x64) {
      x += draw_char(&FNTSYS, ch, x, y, op);
    } else if (ch >= 0x80) {
      x += draw_char(FONT_IX, ch, x, y, op);
    } else {
      /*
      if (ch & 0x10) {
        ctx->regs.b.ixl = *str++;
        ctx->regs.b.ixh = *str++;
      }
      if (ch & 0x1)
        _E = *str++;
      if (ch & 0x2)
        _D = *str++;
      if (ch & 0x4)
        _C = *str++;
      */
      // TODO: only can change font
    }
  } while (1);
}

// String display routine (pass pointer to string)
void STRDIS2(ContextBlock *ctx, char *str) {
  byte opts = _C;
  byte x = _E;
  byte y = _D;
  void* fontdesc = (void*) ctx->regs.w.ix;
  draw_string(ctx, str, x, y, opts); // TODO: opts
}

// String display routine
void STRDIS(ContextBlock *ctx) {
  char* str = (char*) _HL;
  STRDIS2(ctx, str);
}

// Character display routine
void CHRDIS(ContextBlock *ctx) {
  char chstr[2];
  chstr[0] = _A;
  chstr[1] = 0;
  STRDIS2(ctx, chstr);
}

// BCD routine
const char BCDTAB[17] = "0123456789*+,-./";

// DISNUM - (E.D) x/y (C) options (B) ext (HL) BCD-addr
void DISNUM(ContextBlock *ctx) {
  // TODO: options, B
  byte x = _E;
  byte y = _D;
  byte opt = _C;
  byte ext = _B;
  byte ndigits = ext & 63;
  const FontDescriptor* font = (ext&64) ? &FNTSML : &FNTSYS;
  byte add = (ext&64) ? 128 : 0;
  byte noleadingzero = ext & 128;
  byte* pbcd = (byte*) _HL;
  pbcd += (ndigits-1)/2;
  while (ndigits--) {
    byte val = *pbcd;
    if (ndigits & 1) {
      val >>= 4;
    } else {
      val &= 15;
      pbcd--;
    }
    x += draw_char(font, BCDTAB[val]+add, x, y, opt);
  }
}

typedef struct {
  sbyte xofs, yofs;
  byte xsize, ysize;
  byte pattern[0];
} PatternBlock;

// write pattern (E,D,C,B) magic A @ HL
void WRIT(ContextBlock *ctx) {
  byte magic = _A;
  byte w = _C;
  byte h = _B;
  byte x = _E;
  byte y = _D;
  byte* src = (byte*) _HL;
  byte* dest = &vmagic[y][0]; // destination address
  byte xb = (magic & M_FLOP) ? (39-(x>>2)) : (x>>2);
  byte i,j,b;
  // iterate through all lines
  for (j=0; j<h; j++) {
    EXIT_CLIPDEST(dest);
    hw_magic = magic;
    if (magic & M_XPAND) {
      // when XPAND set, write twice as many bytes
      for (i=0; i<w*2; i+=2) {
        b = *src++;
        // when FLOP set, sprite position is also mirrored
        if (magic & M_FLOP) {
          dest[xb-i] = b;
          dest[xb-i-1] = b;
        } else {
          dest[xb+i] = b;
          dest[xb+i+1] = b;
        }
      }
    } else {
      for (i=0; i<w; i++) {
        // when FLOP set, sprite position is also mirrored
        if (magic & M_FLOP) {
          dest[xb-i] = *src++;
        } else {
          dest[xb+i] = *src++;
        }
      }
    }
    if (magic & M_FLOP)
      dest[xb-i] = 0;
    else
      dest[xb+i] = 0;
    dest += VBWIDTH;
  }
}

// write sized pattern (E,D) magic A @ HL
void WRITP(ContextBlock *ctx) {
  byte* src = (byte*) _HL;
  // get size
  _C = *src++;
  _B = *src++;
  // update src
  _HL = (word) src;
  WRIT(ctx);
}

// write relative pattern (E,D) magic A @ HL
void WRITR(ContextBlock *ctx) {
  byte* src = (byte*) _HL;
  // sub offset
  _E -= *src++;
  _D -= *src++;
  // update src
  _HL = (word) src;
  WRITP(ctx);
}

void wait_vsync() {
  byte x = TMR60;
  while (x == TMR60) ; // wait until timer/60 changes
}

void PAWS(ContextBlock *ctx) {
  while (_B--) {
    wait_vsync();
  }
}

// MATH

void MOVE(ContextBlock *ctx) {
  byte* dest = (byte*) _DE;
  const byte* src = (const byte*) _HL;
  word nb = _BC;
  memcpy(dest, src, nb);
}

word bcdadd8(byte a, byte b, byte c) {
  a;b;c;
__asm
  ld  a,6(ix)	; carry
  rrc a		; set carry bit
  ld  h,#0	; carry goes here
  ld  a,4(ix)	; a -> A
  adc a,5(ix)	; a + b -> A
  daa		; BCD conversion
  ld  l,a	; result -> L
  rl  h		; carry -> H
__endasm;
}

void BCDADD(ContextBlock *ctx) {
  byte* dest = (byte*) _DE;
  const byte* src = (const byte*) _HL;
  byte n = _B;
  byte carry = 0;
  while (n--) {
    byte a = *src++;
    byte b = *dest;
    word r = bcdadd8(a,b,carry);
    carry = (r>>8);
    *dest++ = r;
  }
}

void RANGED(ContextBlock *ctx) {
  /* Algorithm "xor" from p. 4 of Marsaglia, "Xorshift RNGs" */
  RANSHT ^= RANSHT << 13;
  RANSHT ^= RANSHT >> 17;
  RANSHT ^= RANSHT << 5;
  if (_A == 0) {
    _A = (byte)RANSHT;
  } else {
    _A = (byte)(RANSHT % _A);
  }
}

const SysCallEntry SYSCALL_TABLE[64] = {
  /* 0 */
  { &INTPC,	0 },
  { &EXIT,	0 },
  { &RCALL,	REG_HL },
  { &MCALL,	REG_HL },
  { &MRET, 	0 },
  /* 10 */
  { &MJUMP,	REG_HL },
  { &SUCK,	REG_B },
  { &ACTINT,	0 },
  { 0, 0 },
  { 0, 0 },
  /* 20 */
  { 0, 0 },
  { &SETOUT,	REG_D|REG_B|REG_A },
  { &COLSET,	REG_HL },
  { &FILL,	REG_A|REG_BC|REG_DE },
  { &RECTAN,	REG_A|REG_B|REG_C|REG_D|REG_E },
  /* 30 */
  { /*&VWRITR*/0, 0 },
  { &WRITR,  REG_E|REG_D|REG_A|REG_HL },
  { &WRITP,  REG_E|REG_D|REG_A|REG_HL },
  { &WRIT,   REG_E|REG_D|REG_C|REG_B|REG_A|REG_HL },
  { /*&WRITA*/0,  0 },
  /* 40 */
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  /* 50 */
  { &CHRDIS,	REG_E|REG_D|REG_C|REG_A },
  { &STRDIS,	REG_E|REG_D|REG_C|REG_HL },
  { &DISNUM,	REG_E|REG_D|REG_C|REG_B|REG_HL },
  { 0, 0 },
  { 0, 0 },
  /* 60 */
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  /* 70 */
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  /* 80 */
  { &PAWS,	REG_B },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  /* 90 */
  { 0, 0 },
  { 0, 0 },
  { &MOVE,	REG_DE|REG_BC|REG_HL },
  { 0, 0 },
  { &BCDADD,	REG_DE|REG_B|REG_HL },
  /* 100 */
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  /* 110 */
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { &RANGED,	REG_A },
  /* 120 */
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
};

void SYSCALL(ContextBlock *ctx) {
  byte op = *ctx->params++;
  byte argmask;
  SysRoutine* routine;
  // user-defined?
  if (op & 0x80) {
    argmask = UMARGT[op>>1];
    routine = (SysRoutine*) USERTB[op>>1];
  } else {
    const SysCallEntry* entry = &SYSCALL_TABLE[op>>1];
    argmask = entry->argmask;
    routine = entry->routine;
  }
  // suck params into context block?
  if (op & 1) {
    suckParams(ctx, argmask);
  }
  // call the routine
  if (routine) {
    routine(ctx);
  }
}

void bios_init() {
  memset((void*)0x4fce, 0, 0x5000-0x4fce);
  ACTINT(0);
  hw_verbl = 96*2;
  hw_horcb = 41;
  hw_inmod = 0x8;
  // clear shifter
  hw_magic = 0;
  *((byte*)0x4fce) = 0;
  hw_magic = 0;
}