VGM Spec v1.71 beta
==============
VGM (Video Game Music) is a sample-accurate sound logging format for the Sega
Master System, the Sega Game Gear and possibly many other machines (e.g. Sega
Genesis).

The normal file extension is .vgm but files can also be GZip compressed into
.vgz files. However, a VGM player should attempt to support compressed and
uncompressed files with either extension. (ZLib's GZIO library makes this
trivial to implement.)

The format starts with a 256 byte header:

      00  01  02  03   04  05  06  07   08  09  0A  0B  0C  0D  0E  0F
0x00 ["Vgm " ident   ][EoF offset     ][Version        ][SN76489 clock  ]
0x10 [YM2413 clock   ][GD3 offset     ][Total # samples][Loop offset    ]
0x20 [Loop # samples ][Rate           ][SN FB ][SNW][SF][YM2612 clock   ]
0x30 [YM2151 clock   ][VGM data offset][Sega PCM clock ][SPCM Interface ]
0x40 [RF5C68 clock   ][YM2203 clock   ][YM2608 clock   ][YM2610/B clock ]
0x50 [YM3812 clock   ][YM3526 clock   ][Y8950 clock    ][YMF262 clock   ]
0x60 [YMF278B clock  ][YMF271 clock   ][YMZ280B clock  ][RF5C164 clock  ]
0x70 [PWM clock      ][AY8910 clock   ][AYT][AY Flags  ][VM] *** [LB][LM]
0x80 [GB DMG clock   ][NES APU clock  ][MultiPCM clock ][uPD7759 clock  ]
0x90 [OKIM6258 clock ][OF][KF][CF] *** [OKIM6295 clock ][K051649 clock  ]
0xA0 [K054539 clock  ][HuC6280 clock  ][C140 clock     ][K053260 clock  ]
0xB0 [Pokey clock    ][QSound clock   ][SCSP clock     ][Extra Hdr ofs  ]
0xC0 [WSwan clock    ][VSU clock      ][SAA1099 clock  ][ES5503 clock   ]
0xD0 [ES5506 clock   ][EC][EC][CD] *** [X1-010 clock   ][C352 clock     ]
0xE0 [GA20 clock     ] *** *** *** ***  *** *** *** ***  *** *** *** ***
0xF0  *** *** *** ***  *** *** *** ***  *** *** *** ***  *** *** *** ***

- Unused space (marked with *) is reserved for future expansion, and must be
  zero.
- All integer values are *unsigned* and written in "Intel" byte order (Little
  Endian), so for example 0x12345678 is written as 0x78 0x56 0x34 0x12.
- All pointer offsets are written as relative to the current position in the
  file, so for example the GD3 offset at 0x14 in the header is the file
  position of the GD3 tag minus 0x14.
- All header sizes are valid for all versions from 1.50 on, as long as header
  has at least 64 bytes. If the VGM data starts at an offset that is lower
  than 0x100, all overlapping header bytes have to be handled as they were
  zero.
- VGMs run with a rate of 44100 samples per second. All sample values use this
  unit.

0x00: "Vgm " (0x56 0x67 0x6d 0x20) file identification (32 bits)
0x04: Eof offset (32 bits)
        Relative offset to end of file (i.e. file length - 4).
        This is mainly used to find the next track when concatenating
        player stubs and multiple files.
0x08: Version number (32 bits)
        Version number in BCD-Code. e.g. Version 1.71 is stored as 0x00000171.
        This is used for backwards compatibility in players, and defines which
        header values are valid.
0x0C: SN76489 clock (32 bits)
        Input clock rate in Hz for the SN76489 PSG chip. A typical value is
        3579545. It should be 0 if there is no PSG chip used.
        Note: Bit 31 (0x80000000) is used on combination with the dual-chip-bit
        to indicate that this is a T6W28. (PSG variant used in NeoGeo Pocket)
0x10: YM2413 clock (32 bits)
        Input clock rate in Hz for the YM2413 chip. A typical value is 3579545.
        It should be 0 if there is no YM2413 chip used.
0x14: GD3 offset (32 bits)
        Relative offset to GD3 tag. 0 if no GD3 tag.
        GD3 tags are descriptive tags similar in use to ID3 tags in MP3 files.
        See the GD3 specification for more details. The GD3 tag is usually
        stored immediately after the VGM data.
0x18: Total # samples (32 bits)
        Total of all wait values in the file.
0x1C: Loop offset (32 bits)
        Relative offset to loop point, or 0 if no loop.
        For example, if the data for the one-off intro to a song was in bytes
        0x0040-0x3FFF of the file, but the main looping section started at
        0x4000, this would contain the value 0x4000-0x1C = 0x00003FE4.
0x20: Loop # samples (32 bits)
        Number of samples in one loop, or 0 if there is no loop.
        Total of all wait values between the loop point and the end of
        the file.
[VGM 1.01 additions:]
0x24: Rate (32 bits)
        "Rate" of recording in Hz, used for rate scaling on playback. It is
        typically 50 for PAL systems and 60 for NTSC systems. It should be set
        to zero if rate scaling is not appropriate - for example, if the game
        adjusts its music engine for the system's speed.
        VGM 1.00 files will have a value of 0.
[VGM 1.10 additions:]
0x28: SN76489 feedback (16 bits)
        The white noise feedback pattern for the SN76489 PSG. Known values are:
        0x0009  Sega Master System 2/Game Gear/Mega Drive
                (SN76489/SN76496 integrated into Sega VDP chip)
        0x0003  Sega Computer 3000H, BBC Micro
                (SN76489AN)
        0x0006  SN76494, SN76496
        For version 1.01 and earlier files, the feedback pattern should be
        assumed to be 0x0009. If the PSG is not used then this may be omitted
        (left at zero).
0x2A: SN76489 shift register width (8 bits)
        The noise feedback shift register width, in bits. Known values are:
        16  Sega Master System 2/Game Gear/Mega Drive
            (SN76489/SN76496 integrated into Sega VDP chip)
        15  Sega Computer 3000H, BBC Micro
            (SN76489AN)
        For version 1.01 and earlier files, the shift register width should be
        assumed to be 16. If the PSG is not used then this may be omitted (left
        at zero).
[VGM 1.51 additions:]
0x2B: SN76489 Flags (8 bits)
        Misc flags for the SN76489. Most of them don't make audible changes and
        can be ignored, if the SN76489 emulator lacks the features.
        bit 0   frequency 0 is 0x400
        bit 1   output negate flag
        bit 2   stereo on/off (on when bit clear)
        bit 3   /8 Clock Divider on/off (on when bit clear)
        bit 4-7 reserved (must be zero)
        For version 1.51 and earlier files, all the flags should not be set.
        If the PSG is not used then this may be omitted (left at zero).
[VGM 1.10 additions:]
0x2C: YM2612 clock (32 bits)
        Input clock rate in Hz for the YM2612 chip. A typical value is 7670454.
        It should be 0 if there us no YM2612 chip used.
        For version 1.01 and earlier files, the YM2413 clock rate should be
        used for the clock rate of the YM2612.
0x30: YM2151 clock (32 bits)
        Input clock rate in Hz for the YM2151 chip. A typical value is 3579545.
        It should be 0 if there us no YM2151 chip used.
        For version 1.01 and earlier files, the YM2413 clock rate should be
        used for the clock rate of the YM2151.
[VGM 1.50 additions:]
0x34: VGM data offset (32 bits)
        Relative offset to VGM data stream.
        If the VGM data starts at absolute offset 0x40, this will contain 
        value 0x0000000C. For versions prior to 1.50, it should be 0 and the
        VGM data must start at offset 0x40.
[VGM 1.51 additions:]
0x38: Sega PCM clock (32 bits)
        Input clock rate in Hz for the Sega PCM chip. A typical value is
        4000000. It should be 0 if there is no Sega PCM chip used.
0x3C: Sega PCM interface register (32 bits)
        The interface register for the Sega PCM chip. It should be 0 if there
        is no Sega PCM chip used.
0x40: RF5C68 clock (32 bits)
        Input clock rate in Hz for the RF5C68 PCM chip. A typical value is
        12500000. It should be 0 if there is no RF5C68 chip used.
0x44: YM2203 clock (32 bits)
        Input clock rate in Hz for the YM2203 chip. A typical value is 3000000.
        It should be 0 if there is no YM2203 chip used.
0x48: YM2608 clock (32 bits)
        Input clock rate in Hz for the YM2608 chip. A typical value is 8000000.
        It should be 0 if there is no YM2608 chip used.
0x4C: YM2610/YM2610B clock (32 bits)
        Input clock rate in Hz for the YM2610/B chip. A typical value is
        8000000. It should be 0 if there is no YM2610/B chip used.
        Note: Bit 31 is used to set whether it is a YM2610 or a YM2610B chip.
        If bit 31 is set it is an YM2610B, if bit 31 is clear it is an YM2610.
0x50: YM3812 clock (32 bits)
        Input clock rate in Hz for the YM3812 chip. A typical value is 3579545.
        It should be 0 if there is no YM3812 chip used.
0x54: YM3526 clock (32 bits)
        Input clock rate in Hz for the YM3526 chip. A typical value is 3579545.
        It should be 0 if there is no YM3526 chip used.
0x58: Y8950 clock (32 bits)
        Input clock rate in Hz for the Y8950 chip. A typical value is 3579545.
        It should be 0 if there is no Y8950 chip used.
0x5C: YMF262 clock (32 bits)
        Input clock rate in Hz for the YMF262 chip. A typical value is
        14318180. It should be 0 if there is no YMF262 chip used.
0x60: YMF278B clock (32 bits)
        Input clock rate in Hz for the YMF278B chip. A typical value is
        33868800. It should be 0 if there is no YMF278B chip used.
0x64: YMF271 clock (32 bits)
        Input clock rate in Hz for the YMF271 chip. A typical value is
        16934400. It should be 0 if there is no YMF271 chip used.
0x68: YMZ280B clock (32 bits)
        Input clock rate in Hz for the YMZ280B chip. A typical value is
        16934400. It should be 0 if there is no YMZ280B chip used.
0x6C: RF5C164 clock (32 bits)
        Input clock rate in Hz for the RF5C164 PCM chip. A typical value is
        12500000. It should be 0 if there is no RF5C164 chip used.
0x70: PWM clock (32 bits)
        Input clock rate in Hz for the PWM chip. A typical value is
        23011361. It should be 0 if there is no PWM chip used.
0x74: AY8910 clock (32 bits)
        Input clock rate in Hz for the AY8910 chip. A typical value is 1789750.
        It should be 0 if there is no AY8910 chip used.
0x78: AY8910 Chip Type (8 bits)
        Defines the exact type of AY8910. The values are:
        0x00 - AY8910
        0x01 - AY8912
        0x02 - AY8913
        0x03 - AY8930
        0x10 - YM2149
        0x11 - YM3439
        0x12 - YMZ284
        0x13 - YMZ294
        If the AY8910 is not used then this may be omitted (left at zero).
0x79: AY8910 Flags (8 bits)
        Misc flags for the AY8910. Default is 0x01.
        For additional description see ay8910.h in MAME source code.
        bit 0   Legacy Output
        bit 1   Single Output
        bit 2   Discrete Output
        bit 3   RAW Output
        bit 4   YM2149 Pin 26 (additional /2 clock divider)
        bit 5-7 reserved (must be zero)
        If the AY8910 is not used then this may be omitted (left at zero).
0x7A: YM2203/AY8910 Flags (8 bits)
        Misc flags for the AY8910. This one is specific for the AY8910 that's
        connected with/part of the YM2203.
0x7B: YM2608/AY8910 Flags (8 bits)
        Misc flags for the AY8910. This one is specific for the AY8910 that's
        connected with/part of the YM2608.
[VGM 1.60 additions:]
0x7C: Volume Modifier (8 bits)
        Volume = 2 ^ (VolumeModifier / 0x20) where VolumeModifier is a number
        from -63 to 192 (-63 = 0xC1, 0 = 0x00, 192 = 0xC0). Also the value -63
        gets replaced with -64 in order to make factor of 0.25 possible.
        Therefore the volume can reach levels between 0.25 and 64.
        Default is 0, which is equal to a factor of 1 or 100%.
        Note: Players should support the Volume Modifier in v1.50 files and
        higher. This way MegaDrive VGMs can use the Volume Modifier without
        breaking compatibility with old players.
0x7D: reserved
        Reserved byte for future use. It must be 0.
0x7E: Loop Base (8 bits)
        Modifies the number of loops that are played before the playback ends.
        Set this value to eg. 1 to reduce the number of played loops by one.
        This is useful, if the song is looped twice in the vgm, because there
        are minor differences between the first and second loop and the song
        repeats just the second loop.
        The resulting number of loops that are played is calculated as
        following: NumLoops = NumLoopsModified - LoopBase
        Default is 0. Negative numbers are possible (80h...FFh = -128...-1)
[VGM 1.51 additions:]
0x7F: Loop Modifier (8 bits)
        Modifies the number of loops that are played before the playback ends.
        You may want to use this, e.g. if a tune has a very short, but non-
        repetive loop (then set it to 0x20 double the loop number).
        The resulting number of loops that are played is calculated as
        following: NumLoops = ProgramNumLoops * LoopModifier / 0x10
        Default is 0, which is equal to 0x10.
[VGM 1.61 additions:]
0x80: GameBoy DMG clock (32 bits)
        Input clock rate in Hz for the GameBoy DMG chip, LR35902. A typical
        value is 4194304. It should be 0 if there is no GB DMG chip used.
0x84: NES APU clock (32 bits)
        Input clock rate in Hz for the NES APU chip, N2A03. A typical value is
        1789772. It should be 0 if there is no NES APU chip used.
        Note: Bit 31 is used to enable the FDS sound addon. Set to enable,
        clear to disable.
0x88: MultiPCM clock (32 bits)
        Input clock rate in Hz for the MultiPCM chip. A typical value is
        8053975. It should be 0 if there is no MultiPCM chip used.
0x8C: uPD7759 clock (32 bits)
        Input clock rate in Hz for the uPD7759 chip. A typical value is 640000.
        It should be 0 if there is no uPD7759 chip used.
0x90: OKIM6258 clock (32 bits)
        Input clock rate in Hz for the OKIM6258 chip. A typical value is
        4000000. It should be 0 if there is no OKIM6258 chip used.
0x94: OKIM6258 Flags (8 bits)
        Misc flags for the OKIM6258. Default is 0x00.
        bit 0-1 Clock Divider (clock dividers are 1024, 768, 512, 512)
        bit 2   3/4-bit ADPCM select (default is 4-bit, doesn't work currently)
        bit 3   10/12-bit Output (default is 10-bit)
        bit 4-7 reserved (must be zero)
        If the OKIM6258 is not used then this may be omitted (left at zero).
0x95: K054539 Flags (8 bits)
        Misc flags for the K054539. Default is 0x01.
        See also k054539.h in MAME source code.
        bit 0   Reverse Stereo
        bit 1   Disable Reverb
        bit 2   Update at KeyOn
        bit 3-7 reserved (must be zero)
        If the K054539 is not used then this may be omitted (left at zero).
0x96: C140 Chip Type (8 bits)
        Defines the exact type of C140 and its banking method. The values are:
        0x00 - C140, Namco System 2
        0x01 - C140, Namco System 21
        0x02 - 219 ASIC, Namco NA-1/2
        If the C140 is not used then this may be omitted (left at zero).
0x97: reserved
        Reserved byte for future use. It must be 0.
0x98: OKIM6295 clock (32 bits)
        Input clock rate in Hz for the OKIM6295 chip. A typical value is
        8000000. It should be 0 if there is no OKIM6295 chip used.
0x9C: K051649 clock (32 bits)
        Input clock rate in Hz for the K051649 chip. A typical value is
        1500000. It should be 0 if there is no K051649 chip used.
0xA0: K054539 clock (32 bits)
        Input clock rate in Hz for the K054539 chip. A typical value is
        18432000. It should be 0 if there is no K054539 chip used.
0xA4: HuC6280 clock (32 bits)
        Input clock rate in Hz for the HuC6280 chip. A typical value is
        3579545. It should be 0 if there is no HuC6280 chip used.
0xA8: C140 clock (32 bits)
        Input clock rate in Hz for the C140 chip. A typical value is 8000000.
        It should be 0 if there is no C140 chip used.
0xAC: K053260 clock (32 bits)
        Input clock rate in Hz for the K053260 chip. A typical value is 
        3579545. It should be 0 if there is no K053260 chip used.
0xB0: Pokey clock (32 bits)
        Input clock rate in Hz for the Pokey chip. A typical value is 1789772.
        It should be 0 if there is no Pokey chip used.
0xB4: QSound clock (32 bits)
        Input clock rate in Hz for the QSound chip. A typical value is 4000000.
        It should be 0 if there is no QSound chip used.
[VGM 1.71 additions:]
0xB8: SCSP clock (32 bits)
        Input clock rate in Hz for the SCSP chip. A typical value is 22579200.
        It should be 0 if there is no SCSP chip used.
[VGM 1.70 additions:]
0xBC: Extra Header Offset (32 bits)
        Relative offset to the extra header or 0 if no extra header is present.
[VGM 1.71 additions:]
0xC0: WonderSwan clock (32 bits)
        Input clock rate in Hz for the WonderSwan chip. A typical value is
        3072000. It should be 0 if there is no WonderSwan chip used.
0xC4: Virtual Boy VSU clock (32 bits)
        Input clock rate in Hz for the VSU chip. A typical value is 5000000.
        It should be 0 if there is no VSU chip used.
0xC8: SAA1099 clock (32 bits)
        Input clock rate in Hz for the SAA1099 chip. A typical value is
        8000000. It should be 0 if there is no SAA1099 chip used.
0xCC: ES5503 clock (32 bits)
        Input clock rate in Hz for the ES5503 chip. A typical value is 7159090.
        It should be 0 if there is no ES5503 chip used.
0xD0: ES5505/ES5506 clock (32 bits)
        Input clock rate in Hz for the ES5506 chip. A typical value is
        16000000. It should be 0 if there is no ES5506 chip used.
        Note: Bit 31 is used to set whether it is an ES5505 or an ES5506 chip.
        If bit 31 is set it is an ES5506, if bit 31 is clear it is an ES5505.
0xD4: ES5503 Channels (8 bits)
        Defines the internal number of output channels for the ES5503.
        Possible values are 1 to 8. A typical value is 2.
        If the ES5503 is not used then this may be omitted (left at zero).
0xD5: ES5505/6 Channels (8 bits)
        Defines the internal number of output channels for the ES5506.
        Possible values are 1 to 4 for the ES5505 and 1 to 8 for the ES5506.
        A typical value is 1.
        If the ES5506 is not used then this may be omitted (left at zero).
0xD6: C352 Clock Divider (8 bits)
        Defines the clock divider for the C352 chip, divided by 4 in order
        to achieve a divider range of 0 to 1020. A typical value is 288.
        If the C352 is not used then this may be omitted (left at zero).
0xD7: reserved
        Reserved byte for future use. It must be 0.
0xD8: Seta X1-010 clock (32 bits)
        Input clock rate in Hz for the X1-010 chip. A typical value is
        16000000. It should be 0 if there is no X1-010 chip used.
0xDC: Namco C352 clock (32 bits)
        Input clock rate in Hz for the C352 chip. A typical value is 24192000.
        It should be 0 if there is no C352 chip used.
0xE0: Irem GA20 clock (32 bits)
        Input clock rate in Hz for the GA20 chip. A typical value is 3579545.
        It should be 0 if there is no GA20 chip used.
0xE4: reserved
        Reserved bytes for future use. They must be 0.


Starting at the location specified by the VGM data offset (or, offset 0x40 for
file versions below 1.50) is found a sequence of commands containing data
written to the chips or timing information. A command is one of:

  0x4F dd    : Game Gear PSG stereo, write dd to port 0x06
  0x50 dd    : PSG (SN76489/SN76496) write value dd
  0x51 aa dd : YM2413, write value dd to register aa
  0x52 aa dd : YM2612 port 0, write value dd to register aa
  0x53 aa dd : YM2612 port 1, write value dd to register aa
  0x54 aa dd : YM2151, write value dd to register aa
  0x55 aa dd : YM2203, write value dd to register aa
  0x56 aa dd : YM2608 port 0, write value dd to register aa
  0x57 aa dd : YM2608 port 1, write value dd to register aa
  0x58 aa dd : YM2610 port 0, write value dd to register aa
  0x59 aa dd : YM2610 port 1, write value dd to register aa
  0x5A aa dd : YM3812, write value dd to register aa
  0x5B aa dd : YM3526, write value dd to register aa
  0x5C aa dd : Y8950, write value dd to register aa
  0x5D aa dd : YMZ280B, write value dd to register aa
  0x5E aa dd : YMF262 port 0, write value dd to register aa
  0x5F aa dd : YMF262 port 1, write value dd to register aa
  0x61 nn nn : Wait n samples, n can range from 0 to 65535 (approx 1.49
               seconds). Longer pauses than this are represented by multiple
               wait commands.
  0x62       : wait 735 samples (60th of a second), a shortcut for
               0x61 0xdf 0x02
  0x63       : wait 882 samples (50th of a second), a shortcut for
               0x61 0x72 0x03
  0x64 cc nn nn : override length of 0x62/0x63
                  cc - command (0x62/0x63)
                  nn - delay in samples
                  [Note: Not yet implemented. Am I really sure about this?]
  0x66       : end of sound data
  0x67 ...   : data block: see below
  0x68 ...   : PCM RAM write: see below
  0x7n       : wait n+1 samples, n can range from 0 to 15.
  0x8n       : YM2612 port 0 address 2A write from the data bank, then wait 
               n samples; n can range from 0 to 15. Note that the wait is n,
               NOT n+1. (Note: Written to first chip instance only.)
  0x90-0x95  : DAC Stream Control Write: see below
  0xA0 aa dd : AY8910, write value dd to register aa
  0xB0 aa dd : RF5C68, write value dd to register aa
  0xB1 aa dd : RF5C164, write value dd to register aa
  0xB2 ad dd : PWM, write value ddd to register a (d is MSB, dd is LSB)
  0xB3 aa dd : GameBoy DMG, write value dd to register aa
               Note: Register 00 equals GameBoy address FF10.
  0xB4 aa dd : NES APU, write value dd to register aa
               Note: Registers 00-1F equal NES address 4000-401F,
                     registers 20-3E equal NES address 4080-409E,
                     register 3F equals NES address 4023,
                     registers 40-7F equal NES address 4040-407F.
  0xB5 aa dd : MultiPCM, write value dd to register aa
  0xB6 aa dd : uPD7759, write value dd to register aa
  0xB7 aa dd : OKIM6258, write value dd to register aa
  0xB8 aa dd : OKIM6295, write value dd to register aa
  0xB9 aa dd : HuC6280, write value dd to register aa
  0xBA aa dd : K053260, write value dd to register aa
  0xBB aa dd : Pokey, write value dd to register aa
  0xBC aa dd : WonderSwan, write value dd to register aa
  0xBD aa dd : SAA1099, write value dd to register aa
  0xBE aa dd : ES5506, write 8-bit value dd to register aa
  0xBF aa dd : GA20, write value dd to register aa
  0xC0 bbaa dd : Sega PCM, write value dd to memory offset aabb
  0xC1 bbaa dd : RF5C68, write value dd to memory offset aabb
  0xC2 bbaa dd : RF5C164, write value dd to memory offset aabb
  0xC3 cc bbaa : MultiPCM, write set bank offset aabb to channel cc
  0xC4 mmll rr : QSound, write value mmll to register rr
                 (mm - data MSB, ll - data LSB)
  0xC5 mmll dd : SCSP, write value dd to memory offset mmll
                 (mm - offset MSB, ll - offset LSB)
  0xC6 mmll dd : WonderSwan, write value dd to memory offset mmll
                 (mm - offset MSB, ll - offset LSB)
  0xC7 mmll dd : VSU, write value dd to register mmll
                 (mm - MSB, ll - LSB)
  0xC8 mmll dd : X1-010, write value dd to memory offset mmll
                 (mm - offset MSB, ll - offset LSB)
  0xD0 pp aa dd : YMF278B port pp, write value dd to register aa
  0xD1 pp aa dd : YMF271 port pp, write value dd to register aa
  0xD2 pp aa dd : SCC1 port pp, write value dd to register aa
  0xD3 pp aa dd : K054539 write value dd to register ppaa
  0xD4 pp aa dd : C140 write value dd to register ppaa
  0xD5 pp aa dd : ES5503 write value dd to register ppaa
  0xD6 aa ddee  : ES5506 write 16-bit value ddee to register aa
  0xE0 dddddddd : seek to offset dddddddd (Intel byte order) in PCM data bank
  0xE1 aabb ddee: C352 write 16-bit value ddee to register aabb

Some ranges are reserved for future use, with different numbers of operands:

  0x30..0x3F dd          : one operand, reserved for future use
                           Note: used for dual-chip support (see below)
  0x40..0x4E dd dd       : two operands, reserved for future use
                           Note: was one operand only til v1.60
  0xA1..0xAF dd dd       : two operands, reserved for future use
                           Note: used for dual-chip support (see below)
  0xC9..0xCF dd dd dd    : three operands, reserved for future use
  0xD7..0xDF dd dd dd    : three operands, reserved for future use
  0xE2..0xFF dd dd dd dd : four operands, reserved for future use

On encountering these, the correct number of bytes should be skipped.


Data blocks
-----------
VGM command 0x67 specifies a data block. These are used to store large amounts
of data, which can be used in parallel with the normal VGM data stream. The
data block format is:

  0x67 0x66 tt ss ss ss ss (data)

where:
  0x67 = VGM command
  0x66 = compatibility command to make older players stop parsing the stream
  tt   = data type
  ss ss ss ss (32 bits) = size of data, in bytes
  (data) = data, of size previously specified

Data blocks of recorded streams, if present, should be at the very start of the
VGM data. Multiple data blocks expand the data bank. (The start offset and
length of the block in the data bank should be saved for command 0x95.)
Because data blocks can happen anywhere in the stream, players must be able to
parse data blocks anywhere in the stream. 

The data block type specifies what type of data it contains. Currently defined
types are:

00..3F : data of recorded streams (uncompressed)
40..7E : data of recorded streams (compressed)
  data block format for compressed streams:
    tt (8 bits) = compression type
                    00 - n-Bit-Compression
                    01 - DPCM-Compression
    ss ss ss ss (32 bits) = size of uncompressed data (for memory allocation)
                            It is assumed that each decompressed value uses
                            ceil(bd/8) bytes.
    (attr) = attribute bytes used by the decompression-algorithm
    n-Bit-Compression:
        bd (8 bits) = Bits decompressed
        bc (8 bits) = Bits compressed
        st (8 bits) = compression sub-type
                        00 - copy (high bits aren't used)
                        01 - shift left (low bits aren't used)
                        02 - use table (data = index into decompression table,
                                        see data block 7F)
        aa aa (16 bits) = value that is added (ignored if table is used)
        The data block is treated as a bitstream with bc bits per value. The
        top bits in each byte are read first. The extracted bits of each value
        are transformed into a value with at least bd bits using method st.
        Finally, aaaa is added to get the resulting value.
    DPCM-Compression: (uses a decompression table)
        bd (8 bits) = Bits decompressed
        bc (8 bits) = Bits compressed
        st (8 bits) = [reserved for future use, must be 00]
        aa aa (16 bits) = start value
        The data is read as a bitstream (see n-Bit). The read value is used as
        index into a delta-table (defined by data block 7F). The delta value
        is added to the "state" value, which is also the result value.
        The "state" value is initialized with aaaa at the beginning.
    (data) = compressed data, of size (block size - 0x0A - attr size)
7F     : Decompression Table
    tt (8 bits) = compression type (see data block 40..7E)
    st (8 bits) = compression sub-type (see data block 40..7E)
    bd (8 bits) = Bits decompressed
    bc (8 bits) = Bits compressed (only used for verifying against
                  block 40..7E)
    cc cc (16 bits) = number of following values (with each of size
                      ceil(bd / 8))
    (data) = table data, cccc values with a total size of (block size - 0x06)
    Note: Multiple decompression tables are valid. The player should keep a
          list of one table per tt and st combination. If there are multiple
          tables of the same tt/st type, the new one overrides the old one and
          all following compressed data blocks will use the new table.
80..BF : ROM/RAM Image dumps (contain usually samples)
  data block format for ROM dumps:
    rr rr rr rr (32 bits) = size of the entire ROM
    ss ss ss ss (32 bits) = start address of data
    (data) = ROM data, of size (block size - 0x08)
  The size of the VGM can be decreased a lot by saving only the used parts
  of the ROM. This is done by saving multiple small ROM data blocks.
  The start address is the ROM offset where the data will be written, the
  ROM size is used to allocate space for the ROM (and some chips rely on it).
C0..DF : RAM writes (for RAM with up to 64 KB)
  data block format for direct RAM writes:
    ss ss (16 bits) = start address of data (affected by a chip's banking
                      registers)
    (data) = RAM data, of size (block size - 0x02)
E0..FF : RAM writes (for RAM with more than 64 KB)
  data block format for direct RAM writes:
    ss ss ss ss (32 bits) = start address of data (affected by a chip's banking
                            registers)
    (data) = RAM data, of size (block size - 0x04)

00 = YM2612 PCM data for use with associated commands
01 = RF5C68 PCM data for use with associated commands
02 = RF5C164 PCM data for use with associated commands
03 = PWM PCM data for use with associated commands
04 = OKIM6258 ADPCM data for use with associated commands
05 = HuC6280 PCM data for use with associated commands
06 = SCSP PCM data for use with associated commands
07 = NES APU DPCM data for use with associated commands
40..7E = same as 00..3E, but compressed
80 = Sega PCM ROM data
81 = YM2608 DELTA-T ROM data
82 = YM2610 ADPCM ROM data
83 = YM2610 DELTA-T ROM data
84 = YMF278B ROM data
85 = YMF271 ROM data
86 = YMZ280B ROM data
87 = YMF278B RAM data
88 = Y8950 DELTA-T ROM data
89 = MultiPCM ROM data
8A = uPD7759 ROM data
8B = OKIM6295 ROM data
8C = K054539 ROM data
8D = C140 ROM data
8E = K053260 ROM data
8F = Q-Sound ROM data
90 = ES5506 ROM data
91 = X1-010 ROM data
92 = C352 ROM data
93 = GA20 ROM data
C0 = RF5C68 RAM write
C1 = RF5C164 RAM write
C2 = NES APU RAM write
E0 = SCSP RAM write
E1 = ES5503 RAM write

All unknown types must be skipped by the player.


PCM RAM writes
--------------
VGM command 0x68 specifies a PCM RAM write. These are used to write data from
data blocks to the RAM of a PCM chip. The data block format is:

  0x68 0x66 cc oo oo oo dd dd dd ss ss ss

where:
  0x68 = VGM command
  0x66 = compatibility command to make older players stop parsing the stream
  cc   = chip type (see data block types 00..3F)
  oo oo oo (24 bits) = read offset in data block
  dd dd dd (24 bits) = write offset in chip's ram (affected by chip's
                        registers)
  ss ss ss (24 bits) = size of data, in bytes
    Since size can't be zero, a size of 0 bytes means 0x0100 0000 bytes.

All unknown types must be skipped by the player.


DAC Stream Control Write
------------------------
VGM commands 0x90 to 0x95 specify writes to the DAC Stream Control Driver.
These are used to stream data from data blocks to the chips via chip writes.
To use it you must:
1. Setup the Stream (set chip type and command) - this activates the stream
2. Set the Stream Data Bank
3. Set the Stream Frequency
4. Now you can start the stream, change its frequency, start it again, stop it,
    etc ...

There are the following commands:

Note: Stream ID 0xFF is reserved and ignored unless noted otherwise.

Setup Stream Control:
  0x90 ss tt pp cc
      ss = Stream ID
      tt = Chip Type (see clock-order in header, e.g. YM2612 = 0x02)
            bit 7 is used to select the 2nd chip
      pp cc = write command/register cc at port pp
      Note: For chips that use Channel Select Registers (like the RF5C-family
            and the HuC6280), the format is pp cd where pp is the channel
            number, c is the channel register and d is the data register.
            If you set pp to FF, the channel select write is skipped.

Set Stream Data:
  0x91 ss dd ll bb
      ss = Stream ID
      dd = Data Bank ID (see data block types 0x00..0x3f)
      ll = Step Size (how many data is skipped after every write, usually 1)
            Set to 2, if you're using an interleaved stream (e.g. for
             left/right channel).
      bb = Step Base (data offset added to the Start Offset when starting
            stream playback, usually 0)
            If you're using an interleaved stream, set it to 0 in one stream
            and to 1 in the other one.
      Note: Step Size/Step Step are given in command-data-size
             (i.e. 1 for YM2612, 2 for PWM), not bytes

Set Stream Frequency:
  0x92 ss ff ff ff ff
      ss = Stream ID
      ff = Frequency (or Sample Rate, in Hz) at which the writes are done

Start Stream:
  0x93 ss aa aa aa aa mm ll ll ll ll
      ss = Stream ID
      aa = Data Start offset in data bank (byte offset in data bank)
            Note: if set to -1, the Data Start offset is ignored
      mm = Length Mode (how the Data Length is calculated)
            00 - ignore (just change current data position)
            01 - length = number of commands
            02 - length in msec
            03 - play until end of data
            1? - (bit 4) Reverse Mode
            8? - (bit 7) Loop (automatically restarts when finished)
      ll = Data Length

Stop Stream:
  0x94 ss
      ss = Stream ID
            Note: 0xFF stops all streams

Start Stream (fast call):
  0x95 ss bb bb ff
      ss = Stream ID
      bb = Block ID (number of the data block that is part of the data bank set
            with command 0x91)
      ff = Flags
            bit 0 - Loop (see command 0x93, mm bit 7)
            bit 4 - Reverse Mode (see command 0x93)

General Note to the DAC Stream Control:
Although it may be quite hard to press already streamed data into these
commands, it makes it very easy to write vgm-creation tools that need to stream
something. (like YM2612 DAC drums/voices/etc.)
The DAC Stream Control can use with almost all chips and is NOT limited to
chips such as YM2612 and PWM.


Dual Chip Support
-----------------
These chips support two instances of a chip in one vgm:
PSG, YM2413, YM2612, YM2151, YM2203, YM2608, YM2610, YM3812, YM3526, Y8950,
YMZ280B, YMF262, YMF278B, YMF271, AY8910, GameBoy DMG, NES APU, MultiPCM,
uPD7759, OKIM6258, OKIM6295, K051649, K054539, HuC6280, C140, K053260, Pokey,
SCSP

Dual chip support is activated by setting bit 30 (0x40000000) in the chip's
clock value. (Note: The PSG needs this bit set for T6W28 mode.)

Dual Chip Support #1:
The second chip instance is controlled via separate commands.

The second SN76489 PSG uses 0x30 (0x3F for GG Stereo).
All chips of the YM-family that use command 0x5n use 0xAn for the second chip.
n is the last digit of the main command.
e.g. 0x52 (1st chip) -> 0xA2 (2nd chip)

Dual Chip Support #2:
All other chips use bit 7 (0x80) of the first parameter byte to distinguish
between the 1st and 2nd chip. (0x00-7F = Chip 1, 0x80-0xFF = chip 2)

Note: The SegaPCM chip has the 2nd-chip-bit in the high byte of the address
parameter. This is the second parameter byte.


Extra Header
------------
With VGM v1.70, there was an extra header added. This one has to be placed
between the usual header and the actual VGM data.

This is the format of the extra header:

      00  01  02  03   04  05  06  07   08  09  0A  0B  0C  0D  0E  0F
0x00 [Header Size    ][ChpClock Offset][ChpVol Offset  ]

Header Size is the size of the extra header. It has to be 4 or larger,
depending in the needed offsets.

Then there are two offsets that point to extra header data for:
- additional Chip Clocks for second chips
- user-defined chip volumes


Chip Clock Header
-----------------
1 byte   - Entry Count (chips with extra clocks)
[5 bytes - List Entry 1]
[5 bytes - List Entry 2]
...

Each list entry has the format:
1 byte  - Chip ID (chip order follows the header)
4 bytes - clock for second chip of the type above


Chip Volume Header
------------------
1 byte   - Entry Count (chips with user-defined volumes)
[4 bytes - List Entry 1]
[4 bytes - List Entry 2]
...

Each list entry has the format:
1 byte  - Chip ID (chip order follows the header)
          Note: If bit 7 is set, it's the volume for a paired chip.
                (e.g. the AY-part of the YM2203)
1 byte  - Flags
          Note: If bit 0 is set, it's the volume for the second chip.
2 bytes - volume for the chip
          Note: If Bit 15 is 0, this is an absolute volume setting.
                If Bit 15 is 1, it's relative and the chip volume gets
                multiplied by ((Value & 0x7FFF) / 0x0100).


History
-------
[1.00]
Initial public release by Dave

[1.01]
Rate value added by Maxim; 1.00 files are fully compatible

[1.10]
PSG white noise feedback and shift register width parameters added by Maxim,
with note on how to handle earlier version files.
Additional wait command added by Maxim with thanks to Steve Snake for the 
suggestion.
1.01 files are fully compatible but 1.01 players might have problems
with 1.10 files, hence the 0.1 version change.

[1.50]
VGM data offset added to header by Maxim.
Data block support added by blargg, to allow for better handling of YM2612 PCM
data.
Both of these changes have the potential to cause problems, but are really good
changes, so the version number has been increased all the way to 1.50.

[1.51]
Sega PCM, RF5C68, YM2203, YM2608, YM2610/B, YM3812, YM3526, Y8950, YMF262,
YMF278B, YMF271, YMZ280B, RF5C164, PWM and AY8910 chips and commands added.

Additional data block types RF5C68 RAM write, RF5C164 RAM write, Sega PCM ROM,
YM2608 DELTA-T ROM, YM2610 ADPCM ROM, YM2610 DELTA-T ROM, YMF278B ROM,
YMF271 ROM, YMF271 RAM, YMZ280B ROM and Y8950 DELTA-T ROM Data added.

Data Block Types splitted into 4 categories. (PCM Stream, compressed PCM
Stream, ROM/RAM Dump, RAM write)

SN76489 Flags and Loop Modifier added.

It is the first time the header size exceeds 0x40 bytes.
1.51 files are fully compatible to 1.50 players, but there may be problems with
the new commands.

Note: Dual chip support was added too, but as a "cheat"-feature. The dual-chip
-bits in the clock values are not compatible to 1.50, but the rest is.

All changes done by Valley Bell.

[1.60]
RF5C68, RF5C164 and PWM PCM blocks and compressed data blocks added.

A whole bunch of new commands (PCM RAM write and DAC Stream Control) added.

Volume Modifier and Loop Base added.

The new commands (especially 0x9?) may cause problems with older players.

All changes done by Valley Bell.

[1.61]
GameBoy DMG, NES APU, MultiPCM, uPD7759, OKIM6258, OKIM6295, K051649, K051649,
HuC6280, C140, K053260, Pokey and Q-Sound chips added.
(including necessary data blocks)

Changed number of operands from 1 to 2 for reserved commands 0x40-0x4E.
Although they're still unused, old players might handle future vgm versions
wrongly.

All changes done by Valley Bell.

[1.70]
Added extra header with seperate chip clocks for the second one of dual chips
and chip volume adjustments.

All changes done by Valley Bell.

[1.71]
SCSP, WonderSwan, Virtual Boy VSU, SAA1099, ES5503, ES5506, Seta X1-010,
Namco C352, Irem GA20 added.
(including necessary ROM data blocks)

Data blocks (type 0x) for OKIM6258, HuC6280, SCSP and NES added.
VGM v1.61 players should support the data block of their respective chips
despite their late addition.

All changes done by Valley Bell.
