furnace/papers/doc/7-systems/ay8910.md

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# General Instrument AY-3-8910
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this chip was used in many home computers (ZX Spectrum, MSX, Amstrad CPC, Atari ST, etc.), video game consoles (Intellivision and Vectrex), arcade boards and even slot machines!
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it is a 3-channel square/noise/envelope sound generator. the chip's powerful sound comes from the envelope...
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the AY-3-8914 variant was used in Intellivision, which is pretty much an AY with 4 level envelope volume per channel and different register format.
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# effects
- `20xx`: set channel mode. `xx` may be one of the following:
- `00`: square
- `01`: noise
- `02`: square and noise
- `03`: envelope
- `04`: envelope and square
- `05`: envelope and noise
- `06`: envelope and square and noise
- `07`: nothing
- `21xx`: set noise frequency. `xx` is a value between 00 and 1F.
- `22xy`: set envelope mode.
- `x` sets the envelope shape, which may be one of the following:
- `0: \___` decay
- `4: /___` attack once
- `8: \\\\` saw
- `9: \___` decay
- `A: \/\/` inverse obelisco
- `B: \¯¯¯` decay once
- `C: ////` inverse saw
- `D: /¯¯¯` attack
- `E: /\/\` obelisco
- `F: /___` attack once
- if `y` is 1 then the envelope will affect this channel.
- `23xx`: set envelope period low byte.
- `24xx`: set envelope period high byte.
- `25xx`: slide envelope period up.
- `26xx`: slide envelope period down.
- `29xy`: enable auto-envelope mode.
- in this mode the envelope period is set to the channel's notes, multiplied by a fraction.
- `x` is the numerator.
- `y` is the denominator.
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- if `x` or `y` are 0 this will disable auto-envelope mode.
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- `2Exx`: write to I/O port A.
- this changes the port's mode to "write". make sure you have connected something to it.
- `2Fxx`: write to I/O port B.
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- this changes the port's mode to "write". make sure you have connected something to it.