diff --git a/src/sys/sop.s b/src/sys/sop.s
index d5c3dda..c941790 100644
--- a/src/sys/sop.s
+++ b/src/sys/sop.s
@@ -77,7 +77,7 @@ rrb = 0700112			" PTR: clear flag, or read buffer
 psf = 0700201			" PTP: skip if flag set
 pcf = 0700202			" PTP: clear flag
 psa = 0700204			" PTP: alphanumeric mode
-cdf = 0700501			" ???
+cdf = 0700501			" G-2: clear display flags
 lds = 0701052			" G-2: load display status
 lda = 0701012			" G-2: load display address
 wcga = 0704206			" G-2: ???