diff --git a/src/apu/README.md b/src/apu/README.md
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+# APU - Audio Processing Unit
+
+
+
+
+Rust libs:
+- synthesis, has osc for common wavs.. will it work well to generate one for each register?
+https://docs.rs/twang/latest/twang/
+- SDL2 can play sounds
+  - ex of squarewave: https://github.com/Rust-SDL2/rust-sdl2/blob/master/examples/audio-queue-squarewave.rs
+  - can play multiple sounds with mixer:
+    - https://rust-sdl2.github.io/rust-sdl2/sdl2/mixer/index.html
+    - usage ex: https://github.com/Rust-SDL2/rust-sdl2/blob/master/examples/mixer-demo.rs
+
+Combing wave forms:
+https://0xc45.com/blog/digital-audio-synthesizer-in-rust/
+
+Other synthesizer examples:
+- https://www.reddit.com/r/rust/comments/mcbx48/fullyfeatured_fm_synthesizer_running_in_the/
+
+Very basic synthesis:
+- https://thewolfsound.com/sound-synthesis/wavetable-synth-in-rust/
diff --git a/src/apu/mod.rs b/src/apu/mod.rs
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+use crate::core::Mem;
+
+use bitflags::bitflags;
+
+/// Apu is the Audio Processing Unit
+pub struct Apu {
+    // pulse_register:
+    // Pulse ($4000–$4007)
+    pulse1: PulseRegister,
+    pulse2: PulseRegister,
+    // Triangle ($4008–$400B)
+    // 2.4	Noise ($400C–$400F)
+    // 2.5	DMC ($4010–$4013)
+    // 2.5.1	Other Uses
+    // 2.6	Status ($4015)
+    // 2.7	Frame Counter ($4017)
+    // 2.7.1	Length Counter
+    length_counter: usize,
+}
+
+bitflags! {
+    /// Pulse
+    /// https://www.nesdev.org/wiki/APU_Sweep
+    struct Pulse: u8 {
+        /// Duty Cycle
+        const D = 0b1100_0000;
+        /// Envelope loop / length counter halt
+        const L = 0b0010_0000;
+        /// Constant volume/envelope flag
+        const C = 0b0001_0000;
+        /// Volume/envelope divider period
+        const V = 0b0000_1111;
+    }
+}
+
+bitflags! {
+    /// APU Sweep
+    /// https://www.nesdev.org/wiki/APU_Sweep
+    struct Sweep: u8 {
+        /// Enabled flag
+        const E = 0b1000_0000;
+        /// Period
+        /// The divider's period is P + 1 half-frames
+        const P = 0b0111_0000;
+        /// Negate flag
+        /// 0: add to period, sweeping toward lower frequencies
+        /// 1: subtract from period, sweeping toward higher frequencies
+        const N = 0b0000_1000;
+        /// Shift count (number of bits).
+        /// If SSS is 0, then behaves like E=0.
+        const S = 0b0000_0111;
+    }
+}
+
+/// https://www.nesdev.org/wiki/APU_Pulse
+struct PulseRegister {
+    data: [u8; 4],
+}
+
+/// https://www.nesdev.org/wiki/APU_Length_Counter
+const LENGTH_TABLE: [u8; 32] = [
+    10, 254, 20, 2, 40, 4, 80, 6, 160, 8, 60, 10, 14, 12, 26, 14, 12, 16, 24, 18, 48, 20, 96, 22,
+    192, 24, 72, 26, 16, 28, 32, 30,
+];
+
+impl PulseRegister {
+    fn duty_cycle(&self) -> u8 {
+        let duty = (self.data[0] & Pulse::D.bits()) >> 6;
+        assert!(duty < 4);
+        duty
+    }
+
+    fn is_length_counter_halted(&self) -> bool {
+        Pulse::from_bits_truncate(self.data[0]).contains(Pulse::L)
+    }
+
+    fn is_constant_volume(&self) -> bool {
+        Pulse::from_bits_truncate(self.data[1]).contains(Pulse::C)
+    }
+
+    /// The sequencer is clocked by an 11-bit timer.
+    /// The timer value t = HHHLLLLLLLL is formed by timer high and timer low.
+    fn timer(&self) -> u16 {
+        let low8 = self.data[2] as u16;
+        let hi3 = ((self.data[3] & 0b0000_0111) as u16) << 8;
+        hi3 + low8
+    }
+
+    /// Gives an index into the LENGTH_TABLE
+    fn length_counter_load(&self) -> u8 {
+        (self.data[3] & 0b1111_1000) >> 3
+    }
+
+    fn is_sweep_enabled(&self) -> bool {
+        let ssc = self.sweep_shift_count();
+        let enabled = Sweep::from_bits_truncate(self.data[1]).contains(Sweep::E);
+        ssc > 0 && enabled
+    }
+
+    fn is_sweep_negated(&self) -> bool {
+        Sweep::from_bits_truncate(self.data[1]).contains(Sweep::N)
+    }
+
+    fn sweep_period(&self) -> u8 {
+        let s = Sweep::from_bits_truncate(self.data[1]);
+        Sweep::P.intersection(s).bits() >> 4
+    }
+
+    fn sweep_shift_count(&self) -> u8 {
+        let s = Sweep::from_bits_truncate(self.data[1]);
+        Sweep::S.intersection(s).bits()
+    }
+}
+
+// // https://www.nesdev.org/wiki/APU_registers
+
+impl Mem for Apu {
+    fn mem_read(&mut self, addr: u16) -> u8 {
+        match addr {
+            // They are write-only except $4015 which is read/write
+            0x4000..=0x4013 | 0x4017 => panic!(
+                "attempt to read from write only APU register: 0x{:04X}",
+                addr
+            ),
+            0x04015 => todo!("read APU status"),
+            _ => panic!("invalid lookup: 0x{:04X} is not in APU memory map", addr),
+        }
+    }
+
+    fn mem_write(&mut self, addr: u16, val: u8) {
+        match addr {
+            0x4000..=0x4003 => self.pulse1.data[(0x4000 - addr) as usize] = val,
+            0x4004..=0x4007 => self.pulse2.data[(0x4004 - addr) as usize] = val,
+            // 0x4008..=0x400B => panic!("APU write-only register: triangle register"),
+            // 0x400C..=0x400F => panic!("APU write-only register: noise register"),
+            // 0x4010..=0x4013 => todo!("dmc"),
+            // They are write-only except $4015 which is read/write
+            // 0x04015 => todo!("status"),
+            // 0x04017 => todo!("frame counter"),
+            _ => todo!(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::PulseRegister;
+
+    #[test]
+    fn test_pulse_register_getters() {
+        let mut p = PulseRegister { data: [0; 4] };
+        assert_eq!(p.timer(), 0);
+        assert_eq!(p.length_counter_load(), 0);
+        p.data[2] = 0xff;
+        p.data[3] = 0b0000_0111;
+        assert_eq!(p.length_counter_load(), 0);
+        assert_eq!(p.timer(), 2048 - 1);
+        p.data[3] = 0xff;
+        assert_eq!(p.length_counter_load(), 32 - 1);
+
+        assert_eq!(p.duty_cycle(), 0);
+        assert_eq!(p.is_constant_volume(), false);
+
+        assert_eq!(p.is_length_counter_halted(), true);
+    }
+}