diff --git a/doc/7-systems/ay8910.md b/doc/7-systems/ay8910.md
index 393b3f341..bc7f65b5e 100644
--- a/doc/7-systems/ay8910.md
+++ b/doc/7-systems/ay8910.md
@@ -42,6 +42,8 @@ Furnace is capable of doing software sample playback on AY-3-8910, where all 3 c
   - `x` is the numerator.
   - `y` is the denominator.
   - if `x` or `y` are 0 this will disable auto-envelope mode.
+- `2Cxx`: **set timer period offset.**
+  - bit 7 is the sign.
 - `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.**
diff --git a/doc/7-systems/ay8930.md b/doc/7-systems/ay8930.md
index a72071fd8..50ab6afc4 100644
--- a/doc/7-systems/ay8930.md
+++ b/doc/7-systems/ay8930.md
@@ -55,6 +55,17 @@ Furnace is able to do software PCM on AY8930, where all 3 channels can play 5-bi
   - `x` is the numerator.
   - `y` is the denominator.
   - if `x` or `y` are 0 this will disable auto-envelope mode.
+- `2Cxx`: **automatic noise frequency.**
+  - `x` sets the mode:
+    - `0`: disabled
+    - `1`: alter frequency
+    - `2`: alter frequency and OR mask
+  - `y` sets the offset.
+  - this can be used to make a pulse-width modulation (PWM) effect.
+- `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.**
+  - this changes the port's mode to "write". make sure you have connected something to it.
 
 ## info