=============================================================================

*** How to use ***

You need to take "src-68k/echo.68k" and include it in your program. Then you
need to take "built/prog-z80.bin" (or if you built Echo from source, the
generated binary). Finally, go to the echo.68k file, look for @Z80Program
(should be near the end of the file) and change the filename to point where
the prog-z80.bin file is.

Echo should now be inside your program. Now call Echo_Init (see below) to
initialize Echo and load the instrument list, and then you can proceed to use
Echo as needed (e.g. call Echo_PlayBGM to start playing music).

Unless stated otherwise, calling the API subroutines will *not* modify the
68000 registers.

=============================================================================

*** Initialization ***

Echo_Init
   in a0 = pointer to instrument list

   Initializes Echo. Loads the instrument list, loads the Z80 engine and gets
   it running. You need to call this before you can use Echo (usually when
   the program is just starting).
   
   The address of the instrument list is given in register a0. The instrument
   list can be built using the Echo_List* macros. An example of a short
   instrument list is as follows:
   
   Echo_ListEntry instrument1
   Echo_ListEntry instrument2
   Echo_ListEntry instrument3
   Echo_ListEnd
   
   Where the parameter for Echo_ListEntry is the address (e.g. a label) to
   the EIF/EEF/EWF data of the instrument.

=============================================================================

*** Background music ***

Echo_PlayBGM
   in a0 = pointer to ESF data to play

   Starts playback of the specified background music. The register a0 points
   to the ESF data for the background music.

Echo_StopBGM

   Stops playback of background music. Used both to stop and to pause music.

Echo_PauseBGM

   Pauses BGM playback (SFXs should be unaffected).

Echo_ResumeBGM

   Resumes BGM playback after it has been paused with Echo_PauseBGM.

=============================================================================

*** Sound effects ***

Echo_PlaySFX
   in a0 = pointer to ESF data to play

   Starts playback of the specified sound effect. The register a0 points to
   the ESF data for the sound effect.

Echo_StopSFX

   Stops playback of sound effects.

=============================================================================

*** Direct events ***

Echo_PlayDirect
   in a0 = pointer to ESF data to inject

   Injects events to be played as part of the BGM the next tick. The register
   a0 points to the ESF data to be injected.

   The injected events are a small stream on their own. The last event must
   be $FF (this will return back to the BGM). Do *not* issue $FC, $FD or $FE
   events, as you'll just break everything instead.

   The buffer is small, so don't go overboard. There's room for up to 128
   bytes (though again, each event is just 2-3 bytes). If there were direct
   events pending to play, the new events will be appended at the end, so
   take this into account when it comes to the buffer usage. You can check
   if there are pending events with Echo_GetStatus (see bit 14) if you're
   worried about running out of space.
   
   The buffer is only checked every tick.

=============================================================================

*** Control ***

Echo_GetStatus
   out d0 = status (see below)

   Gets the current status of Echo. The status is returned as a word in d0,
   with the following bits set as relevant:
   
   Bit 0 .... Sound effect is playing
   Bit 1 .... Background music is playing
   Bit 14 ... Echo isn't done parsing direct events
   Bit 15 ... Echo is busy (can't take commands)
   
   The API will automatically wait if you try to send a command while Echo is
   busy, so the only reason to check for that is if you don't want to halt
   the 68000 until Echo is ready to take more commands.

Echo_SetVolume
   in d0 = new volume ($00 = quietest, $FF = loudest)

   Sets the global volume. Register d0 is a byte value ranging from 0
   (quietest) to 255 (loudest), and every channel is affected immediately.
   The scale of the volume in this case is *linear*.
   
   Note that since PCM doesn't have volume, it gets toggled on/off depending
   on the volume value (the cut off point is at 25%).

Echo_SetVolumeEx
   in a0 = pointer to volume data (see below)

   Sets the global volume for each channel separately. Register a0 points to
   a list of 16 bytes (one for each Echo channel). Values for FM and PSG
   channels are given in the same way as in events, that is: logarithmic
   scale, 0..127 for FM, 0..15 for PSG, lower = louder.
   
   The last byte (the one belonging to the PCM channel) is used to toggle
   whether PCM plays, either 0 (disabled) or 1 (enabled).
   
   NOTE: the Echo 1.4 docs requested for 13 bytes instead of 16. This has
   been changed for the sake of expansion. Currently the extra bytes are
   ignored, but consider adapting your code (just set them to zero).

=============================================================================

*** Settings ***

Echo_SetPCMRate
   in d0 = new PCM rate (see below)

   Changes the sample rate of PCM. Note this is a global parameter as it
   affects both BGM and SFX. The value is what one would write in timer A of
   the YM2612 register. Here are the approximate frequencies for some values
   (default is $04):
   
              NTSC         PAL |           NTSC        PAL
   ----------------------------|--------------------------
   $01 ... 26632Hz ... 26389Hz | $07 ... 6658Hz ... 6597Hz
   $02 ... 17755Hz ... 17593Hz | $08 ... 5918Hz ... 5864Hz
   $03 ... 13316Hz ... 13194Hz | $09 ... 5326Hz ... 5278Hz
   $04 ... 10653Hz ... 10556Hz | $0A ... 4842Hz ... 4798Hz
   $05 .... 8877Hz .... 8796Hz | $0B ... 4439Hz ... 4398Hz
   $06 .... 7609Hz .... 7539Hz | $0C ... 4097Hz ... 4060Hz
   
   The higher the sample rate, the better quality, but also takes up more
   space and, more importantly, reduces CPU time available for other things
   (which can hamper Echo's ability to process complex streams). Be careful
   if you increase the sample rate.

=============================================================================

*** Raw access ***

Echo_SendCommand
   in d0 = command

   Sends an argument-less command to Echo. The command ID is given as a byte
   in register d0.

Echo_SendCommandAddr
   in d0 = command
   in a0 = address

   Sends a command to Echo that takes an address as its argument. The command
   ID is given as a byte in register d0, while the address argument is given
   in register a0.

Echo_SendCommandByte
   in d0 = command
   in d1 = argument

   Sends a command to Echo that takes a byte as its argument. The command ID
   is given as a byte in register d0, while the byte argument is given in
   register d1.

=============================================================================