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

# General Instrument AY-3-8910

this chip was used in several home computers (ZX Spectrum, MSX, Amstrad CPC, Atari ST, etc.), video game consoles (Intellivision and Vectrex), arcade boards and even slot machines!

the chip's powerful sound comes from the envelope...

# 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.
  - if `x` or `y` are 0 this will disable auto-envelope mode.