diff --git a/doc/7-systems/opl.md b/doc/7-systems/opl.md
index b4a86e6d..80c00299 100644
--- a/doc/7-systems/opl.md
+++ b/doc/7-systems/opl.md
@@ -3,7 +3,7 @@
 a series of FM sound chips which were very popular in DOS land. it was so popular that even Yamaha made a logo for it!
 
 essentially a downgraded version of Yamaha's other FM chips, with only 2 operators per channel.
-however, it also had a drums mode, and later chips in the series added more waveforms (than just the typical sine) and even a 4-operator mode.
+however, it also had a [drums mode](opll.md), and later chips in the series added more waveforms (than just the typical sine) and even a 4-operator mode.
 
 the original OPL (Yamaha YM3526) was present as an expansion for the Commodore 64 and MSX computers (erm, a variant of it). it only had 9 two-operator channels and drums mode.
 
@@ -77,4 +77,4 @@ afterwards everyone moved to Windows and software mixed PCM streaming...
   - only in 4-op mode (OPL3).
 - `5Bxy`: set KSR of operator.
   - `x` is the operator (1-4; last 2 operators only in 4-op mode). a value of 0 means "all operators".
-  - `y` determines whether KSR is on.
\ No newline at end of file
+  - `y` determines whether KSR is on.