SpritesMind.Net

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;drive command nibbles

CMD_NOP         EQU                $0                            ;nop command
CMD_STOP        EQU                $1                            ;stop motor
CMD_REPORTREQ   EQU                $2                            ;change report type
CMD_READ        EQU                $3                            ;read ROM data
CMD_SEEK        EQU                $4                            ;seek to a given location
CMD_PAUSE       EQU                $6                            ;pause the drive
CMD_PLAY        EQU                $7                            ;start playing from current location
CMD_FWD         EQU                $8                            ;forward skip and playback
CMD_RVS         EQU                $9                            ;reverse skip and playback
CMD_TRACKSKIP   EQU                $A                            ;start track skipping
CMD_TRACKCUE    EQU                $B                            ;track cueing
CMD_DOORCLOSE   EQU                $C                            ;close the door
CMD_DOOROPEN    EQU                $D                            ;open the door

;drive status nibbles

STATUS_STOP     EQU                $0                            ;disc is set, motor not in operation
STATUS_PLAY     EQU                $1                            ;data or audio playback in progress
STATUS_SEEK     EQU                $2                            ;seek operation by read or seek command
STATUS_SCAN     EQU                $3                            ;skipping to a specific track
STATUS_PAUSE    EQU                $4                            ;paused at designated track or frame
STATUS_DOOROPEN EQU                $5                            ;the door mechanism is open
STATUS_SUMERROR EQU                $6                            ;a checksum error occurred during communication
STATUS_COMMANDERROR    EQU         $7                            ;an invalid command has arrived
STATUS_FUNCTIONERROR    EQU        $8                            ;some sort of error occurred during command execution
STATUS_TOCREAD  EQU                $9                            ;TOC is being read at the moment
STATUS_TRACKING EQU                $A                            ;currently skipping tracks
STATUS_NODISC   EQU                $B                            ;door is closed, but cannot focus on disc
STATUS_DISCEND  EQU                $C                            ;paused in the leadout
STATUS_DISCIN   EQU                $D                            ;paused in the leadin
STATUS_TRAYMOVING   EQU            $E                            ;the disc door mechanism is in operation

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#2 - The attached document contains information regarding use of backup RAM on the Sega-CD
#3 - The attached document is a supplement to the sega-cd manual and contains notes nd updates on a variety of subjects
#4 - contains updates on basic sample playback frequency for the Sega -CD PCM sound chip
#5 - waiting during Vram reading and also ap roblem with the Super Target Development Systems
#6 - problems encountered while using Macintosh Sound Designer to produce Redbook audio
#7 - New standards for Sega-CD ID table
#16 - additional cd software development guidelins as well as updates to the CD-ROM Manual
#17 - contains info of sega approved write-once writers and discs
#18 - swapping PRG-RAM (program RAM) into the Genesis side by halding the SUB CPU the method is unsafe and unstable

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;drive command nibbles

CMD_NOP			EQU				$0				;nop command
CMD_STOP		EQU				$1				;stop motor
CMD_REPORTREQ	EQU				$2				;change report type
CMD_READ		EQU				$3				;read ROM data
CMD_SEEK		EQU				$4				;seek to a given location
CMD_PAUSE		EQU				$6				;pause the drive
CMD_PLAY		EQU				$7				;start playing from current location
CMD_FWD			EQU				$8				;forward skip and playback
CMD_RVS			EQU				$9				;reverse skip and playback
CMD_TRACKSKIP	EQU				$A				;start track skipping
CMD_TRACKCUE	EQU				$B				;track cueing
CMD_DOORCLOSE	EQU				$C				;close the door
CMD_DOOROPEN	EQU				$D				;open the door

;drive status nibbles

STATUS_STOP		EQU				$0				;disc is set, motor not in operation
STATUS_PLAY		EQU				$1				;data or audio playback in progress
STATUS_SEEK		EQU				$2				;seek operation by read or seek command
STATUS_SCAN		EQU				$3				;skipping to a specific track
STATUS_PAUSE		EQU				$4				;paused at designated track or frame
STATUS_DOOROPEN	EQU				$5				;the door mechanism is open
STATUS_SUMERROR	EQU				$6				;a checksum error occurred during communication
STATUS_COMMANDERROR	EQU			$7				;an invalid command has arrived
STATUS_FUNCTIONERROR	EQU			$8				;some sort of error occurred during command execution
STATUS_TOCREAD	EQU				$9				;TOC is being read at the moment
STATUS_TRACKING	EQU				$A				;currently skipping tracks
STATUS_NODISC	EQU				$B				;door is closed, but cannot focus on disc
STATUS_DISCEND	EQU				$C				;paused in the leadout
STATUS_DISCIN		EQU				$D				;paused in the leadin
STATUS_TRAYMOVING	EQU			$E				;the disc door mechanism is in operation

;drive request format nibbles

RF_A			EQU				$0				;request absolute time
RF_R			EQU				$1				;request relative time
RF_T			EQU				$2				;request track info
RF_TOCO			EQU				$3				;request disc completion time
RF_TOCT			EQU				$4				;request start/end tracks on disc
RF_TOCN			EQU				$5				;request start time of track N
RF_E			EQU				$6				;Error information requested
RF_SUBCERROR		EQU				$E				;had subcode error
RF_NOTREADY		EQU				$F				;not ready to comply with command at the moment

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;;various timeout values

IRQTIMEOUT		EQU		2		;number of ms before HSCK required after IRQ
HSCKTIMEOUT		EQU		3		;number of ms to complete status/command sequence
H2ITIMEOUT		EQU		3		;number of ms to wait for handshake from the IBM
SCANPLAYTIME	EQU		10		;while scanning, play at least this many blocks before seeking again

;constants relating to emulation

TOCTVERSION		EQU		0	;version number to report to a TOCT request

ADJUSTBLOCKBACK	EQU		4	;number of blocks to subtract from seek request to guarantee that the requested block is seen

FWDOFFSETLOW	EQU		100		;low word of number of blocks to skip while in forward scan mode
FWDOFFSETHIGH	EQU		0	;high word of number of blocks to skip while in forward scan mode

RVSOFFSETLOW	EQU		140		;low word of number of blocks to skip while in reverse scan mode
RVSOFFSETHIGH	EQU		0		;high word of number of blocks to skip while in reverse scan mode

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        uint6 pixel = uint3(xpixel) + uint3(ypixel) * 8;
        xpixel = xpixel >> 3 & stampMask;
        ypixel = ypixel >> 3 & stampMask;
        uint4 cell = ypixel + xpixel * (1 + stampMask);
        output = read(index << 8 | cell << 6 | pixel);

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auto MCD::GPU::step(uint clocks) -> void {
  if(!active) return;

  counter += clocks;
  while(counter >= period) {
    counter -= period;
    render(image.address, image.hdots);
    image.address += 8;
    if(!--image.vdots) {
      active = 0;
      irq.raise();
    }
  }
}

auto MCD::GPU::read(uint19 address) -> uint4 {
  uint lo = 12 - ((address & 3) << 2), hi = lo + 3;
  return mcd.wram[address >> 2].bits(lo, hi);
}

auto MCD::GPU::write(uint19 address, uint4 data) -> void {
  uint lo = 12 - ((address & 3) << 2), hi = lo + 3;
  mcd.wram[address >> 2].bits(lo, hi) = data;
}

auto MCD::GPU::render(uint19 address, uint9 width) -> void {
   uint8 stampShift = 11 + 4 + stamp.tile.size;
   uint8 mapShift   = (4 << stamp.map.size) - stamp.tile.size;

   uint2 stampMask  = !stamp.tile.size ? 1 : 3;
  uint23 mapMask    = !stamp.map.size ? 0x07ffff : 0x7fffff;

  uint24 x = mcd.wram[vector.address++] << 8;  //13.3 -> 13.11
  uint24 y = mcd.wram[vector.address++] << 8;  //13.3 -> 13.11

   int16 xstep = mcd.wram[vector.address++];
   int16 ystep = mcd.wram[vector.address++];

  while(width--) {
    if(stamp.repeat) {
      x &= mapMask;
      y &= mapMask;
    }

    uint4 output;
    if(bool outside = (x | y) & ~mapMask; !outside) {
      auto xstamp = x >> stampShift;
      auto ystamp = y >> stampShift << mapShift;

      auto data  = mcd.wram[stamp.map.address + xstamp + ystamp];
      auto index = uint10(data >>  0);
      auto lroll =  uint1(data >> 13);  //0 = 0 degrees; 1 =  90 degrees
      auto hroll =  uint1(data >> 14);  //0 = 0 degrees; 1 = 180 degrees
      auto hflip =  uint1(data >> 15);

      if(index) {  //stamp index 0 is not rendered
        auto xpixel = uint6(x >> 11);
        auto ypixel = uint6(y >> 11);

        if(hflip) { xpixel = ~xpixel; }
        if(hroll) { xpixel = ~xpixel; ypixel = ~ypixel; }
        if(lroll) { auto tpixel = xpixel; xpixel = ~ypixel; ypixel = tpixel; }

        uint6 pixel = uint3(xpixel) + uint3(ypixel) * 8;
        xpixel = xpixel >> 3 & stampMask;
        ypixel = ypixel >> 3 & stampMask;
        uint4 cell = ypixel + xpixel * (1 + stampMask);
        output = read(index << 8 | cell << 6 | pixel);
      }
    }

    uint4 input = read(address);
    switch(mcd.io.wramPriority) {
    case 0: output = output; break;
    case 1: output = input ? input : output; break;
    case 2: output = output ? output : input; break;
    case 3: output = input; break;
    }
    write(address, output);
    if(!(++address & 7)) address += (image.vcells + 1 << 6) - 8;

    x += xstep;
    y += ystep;
  }
}

auto MCD::GPU::start() -> void {
  if(mcd.io.wramMode) return;  //must be in 2mbit WRAM mode

  active = 1;
  period = 4 * 5 * image.hdots;
  counter = 0;

  image.address = (image.base << 1) + image.offset;
  vector.address = vector.base >> 1;
  stamp.map.address = stamp.map.base >> 1;
  if(stamp.map.size == 0 && stamp.tile.size == 0) stamp.map.address &= 0x1ff00;  // A9-A17
  if(stamp.map.size == 0 && stamp.tile.size == 1) stamp.map.address &= 0x1ffc0;  // A5-A17
  if(stamp.map.size == 1 && stamp.tile.size == 0) stamp.map.address &= 0x10000;  //    A17
  if(stamp.map.size == 1 && stamp.tile.size == 1) stamp.map.address &= 0x1c000;  //A15-A17
}

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auto MCD::external_read(uint1 upper, uint1 lower, uint22 address, uint16 data) -> uint16 {
  address.bits(18,20) = 0;  //mirrors
  if(address >= 0x200000 && address <= 0x23ffff) {
    if(io.wramMode == 0) {
    //if(io.wramSwitch == 1) return data;
      address = (uint18)address;
    } else {
      address = (uint17)address << 1 | io.wramSelect == 0;
    }
    //VDP DMA from Mega CD word RAM to VDP VRAM responds with a one-access delay
    //note: it is believed that the first transfer is the CPU prefetch, which isn't emulated here
    //games manually correct the first word transferred after VDP DMAs from word RAM
    data = io.wramLatch;
    io.wramLatch = wram[address >> 1];
    if(vdp.dma.active()) return data;
    return io.wramLatch;  //non-DMA accesses respond with the requested data correctly
  }
 ...
}

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auto VDP::DMA::load() -> void {
  cpu.wait |= Wait::VDP_DMA;

  io.active = 1;
  auto data = cpu.read(1, 1, io.mode.bit(0) << 23 | io.source << 1);
  vdp.writeDataPort(data);
  io.active = 0;

  io.source.bits(0,15)++;
  if(--io.length == 0) {
    vdp.io.command.bit(5) = 0;
    cpu.wait &=~ Wait::VDP_DMA;
  }
}