update documentation

- 5E01
- mention software backend

doc/2-interface/settings.md

@@ -15,6 +15,7 @@ settings are saved when clicking the **OK** or **Apply** buttons at the bottom o
   - DirectX 11: works with the majority of graphics chips/cards and is optimized specifically for Windows.
   - SDL Renderer: this was the only available render backend prior to the addition of dedicated OpenGL/DirectX backends in 0.6. default on macOS.
     - it is slower than the other backends.
+  - Software: this is a last resort backend which renders the interface in software. very slow!
 - **Render driver**: this setting appears when using the SDL Renderer backend. it allows you to select an SDL render driver.
 - **VSync**: synchronizes rendering to VBlank and eliminates tearing.
 - **Frame rate limit**: allows you to set a frame rate limit (in frames per second).

doc/7-systems/5e01.md

@@ -0,0 +1,62 @@
+# 5E01
+
+a fantasy sound chip created by Euly, based on the Ricoh 2A03, with some improvements:
+
+- a 37.5% duty cycle,
+- 32 noise pitches instead of 16, and
+- triangle channel becomes a wave channel, with four available waveforms: triangle, saw, sine and square.
+
+## effects
+
+- `11xx`: **write to delta modulation counter.** range is `00` to `7F`.
+  - this may be used to attenuate the triangle and noise channels; at `7F`, they will be at about 57% volume.
+  - will not work if a sample is playing.
+- `12xx`: **set duty cycle/noise mode/waveform of channel.**
+  - may be `0` to `3` for the pulse channels:
+    - `0`: 12.5%
+    - `1`: 25%
+    - `2`: 37.5%
+    - `3`: 50%
+  - may be `0` or `1` for the noise channel:
+    - `0`: long (15-bit LFSR, 32767-step)
+    - `1`: short (9-bit LFSR, 93-step)
+  - may be `0` to `3` for the wave channel:
+    - `0`: triangle
+    - `1`: saw
+    - `2`: square
+    - `3`: sine
+- `13xy`: **setup sweep up.**
+  - `x` is the time.
+  - `y` is the shift.
+  - set to `0` to disable it.
+- `14xy`: **setup sweep down.**
+  - `x` is the time.
+  - `y` is the shift.
+  - set to `0` to disable it.
+- `15xx`: **set envelope mode.**
+  - `0`: envelope + length counter. volume represents envelope duration.
+  - `1`: length counter. volume represents output volume.
+  - `2`: looping envelope. volume represents envelope duration.
+  - `3`: constant volume. default value. volume represents output volume.
+  - pulse and noise channels only.
+  - you may need to apply a phase reset (using the macro) to make the envelope effective.
+- `16xx`: **set length counter.**
+  - see [NES](nes.md) for more information.
+  - this will trigger phase reset.
+- `17xx`: **set frame counter mode.**
+  - `0`: 4-step.
+  - `1`: 5-step.
+- `18xx`: **set PCM channel mode.**
+  - `00`: PCM (software).
+  - `01`: DPCM (hardware).
+  - when in DPCM mode, samples will sound muffled (due to its nature), availables pitches are limited, and loop point is ignored.
+- `19xx`: **set triangle linear counter.**
+  - `00` to `7F` set the counter.
+  - `80` and higher halt it.
+- `20xx`: **set DPCM frequency.**
+  - only works in DPCM mode.
+
+
+## info
+
+this chip uses the [NES](../4-instrument/nes.md) instrument editor.

doc/7-systems/nes.md

@@ -2,7 +2,9 @@
 
 the console from Nintendo that plays Super Mario Bros. and helped revive the agonizing video game market in the US during mid-80s.
 
-also known as Famicom. it is a five-channel sound generator: first two channels play pulse wave with three different duty cycles, third is a fixed-volume triangle channel, fourth is a noise channel (can work in both pseudo-random and periodic modes) and fifth is a (D)PCM sample channel.
+also known as Family Computer (Famicom), especially in Japan.
+
+the console is powered by the Ricoh 2A03, a CPU with sound generator built-in. it has five channels: first two channels play pulse wave with three different duty cycles, third is a fixed-volume triangle channel, fourth is a noise channel (can work in both pseudo-random and periodic modes) and fifth is a (D)PCM sample channel.
 
 ## effects