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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
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- `20xx` : **set channel mode.**
- `0` : square
- `1` : noise
- `2` : square and noise
- `3` : envelope
- `4` : envelope and square
- `5` : envelope and noise
- `6` : envelope and square and noise
- `7` : nothing
- `21xx` : **set noise frequency.** range is `0` to `1F` .
- `22xy` : **set envelope mode.**
- `x` sets the envelope shape:
- `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
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- if `y` is 1 then the envelope will affect this channel.
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- `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.**
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- 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.**
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- this changes the port's mode to "write". make sure you have connected something to it.
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- `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.
