diff --git a/mach/i386/as/mach3.c b/mach/i386/as/mach3.c
index f2b1124c0..1a348b584 100644
--- a/mach/i386/as/mach3.c
+++ b/mach/i386/as/mach3.c
@@ -411,8 +411,8 @@
 /* Intel 486 instructions */
 0,	EXTOPBW,	0xC0,		"xaddb",
 0,	EXTOPBW,	0xC1,		"xadd",
-0,	EXTOPBW,	0xA6,		"cmpxchgb",
-0,	EXTOPBW,	0xA7,		"cmpxchg",
+0,	EXTOPBW,	0xB0,		"cmpxchgb",
+0,	EXTOPBW,	0xB1,		"cmpxchg",
 0,	BSWAP,		0xC8,		"bswap",
 0,	NOOP_2,		017+(010<<8),	"invd",
 0,	EXTOP1,		071,		"invlpg",
diff --git a/mach/i386/as/mach4.c b/mach/i386/as/mach4.c
index 711c3c71e..178300257 100644
--- a/mach/i386/as/mach4.c
+++ b/mach/i386/as/mach4.c
@@ -199,9 +199,9 @@ oper	:	NOOP_1
 	/* 486 instructions */
 	|	BSWAP R32
 			{	emit1(0xF); emit1($1|$2); }
-	|	EXTOPBW reg ',' ea_2
+	|	EXTOPBW ea_2 ',' reg
 			{	regsize($1);
-				emit1(0xF); emit1($1); ea_2($2<<3);
+				emit1(0xF); emit1($1); ea_2($4<<3);
 			}
 	;