diff --git a/src/asm/parser.y b/src/asm/parser.y
index 74a40568..39d14618 100644
--- a/src/asm/parser.y
+++ b/src/asm/parser.y
@@ -392,6 +392,7 @@ enum {
 	REG_BC = 0,
 	REG_DE = 1,
 	REG_HL = 2,
+	// LD/INC/ADD/DEC allow SP, PUSH/POP allow AF
 	REG_SP = 3,
 	REG_AF = 3
 };