diff --git a/operand/checks.go b/operand/checks.go
index 493ec59..322457b 100644
--- a/operand/checks.go
+++ b/operand/checks.go
@@ -181,6 +181,7 @@ func IsM64(op Op) bool {
 // registers of the given size in bytes.
 func IsMSize(op Op, n uint) bool {
 	// TODO(mbm): should memory operands have a size attribute as well?
+	// TODO(mbm): m8,m16,m32,m64 checks do not actually check size
 	m, ok := op.(Mem)
 	return ok && IsMReg(m.Base) && (m.Index == nil || IsMReg(m.Index))
 }
diff --git a/operand/checks_test.go b/operand/checks_test.go
index 30aa881..e51144a 100644
--- a/operand/checks_test.go
+++ b/operand/checks_test.go
@@ -97,29 +97,29 @@ func TestChecks(t *testing.T) {
 		{IsM, Mem{Base: reg.CX}, true},
 		{IsM, Mem{Base: reg.ECX}, true},
 		{IsM, Mem{Base: reg.RCX}, true},
-		{IsM, Mem{Base: reg.CL}, false},
+		{IsM, Mem{Base: reg.X0}, false},
 
 		{IsM8, Mem{Disp: 8, Base: reg.CL}, true},
 		{IsM8, Mem{Disp: 8, Base: reg.CL, Index: reg.AH, Scale: 2}, true},
-		{IsM8, Mem{Disp: 8, Base: reg.AX, Index: reg.AH, Scale: 2}, false},
-		{IsM8, Mem{Disp: 8, Base: reg.CL, Index: reg.R10, Scale: 2}, false},
+		{IsM8, Mem{Disp: 8, Base: reg.X0, Index: reg.AH, Scale: 2}, false},
+		{IsM8, Mem{Disp: 8, Base: reg.CL, Index: reg.X0, Scale: 2}, false},
 
 		{IsM16, Mem{Disp: 4, Base: reg.DX}, true},
 		{IsM16, Mem{Disp: 4, Base: reg.R13W, Index: reg.R8W, Scale: 2}, true},
-		{IsM16, Mem{Disp: 4, Base: reg.ESI, Index: reg.R8W, Scale: 2}, false},
-		{IsM16, Mem{Disp: 4, Base: reg.R13W, Index: reg.R9, Scale: 2}, false},
+		{IsM16, Mem{Disp: 4, Base: reg.X0, Index: reg.R8W, Scale: 2}, false},
+		{IsM16, Mem{Disp: 4, Base: reg.R13W, Index: reg.X0, Scale: 2}, false},
 
 		{IsM32, Mem{Base: reg.R13L, Index: reg.EBX, Scale: 2}, true},
-		{IsM32, Mem{Base: reg.R13W}, false},
+		{IsM32, Mem{Base: reg.X0}, false},
 
 		{IsM64, Mem{Base: reg.RBX, Index: reg.R12, Scale: 2}, true},
-		{IsM64, Mem{Base: reg.R13L}, false},
+		{IsM64, Mem{Base: reg.X0}, false},
 
 		{IsM128, Mem{Base: reg.RBX, Index: reg.R12, Scale: 2}, true},
-		{IsM128, Mem{Base: reg.R13L}, false},
+		{IsM128, Mem{Base: reg.X0}, false},
 
 		{IsM256, Mem{Base: reg.RBX, Index: reg.R12, Scale: 2}, true},
-		{IsM256, Mem{Base: reg.R13L}, false},
+		{IsM256, Mem{Base: reg.X0}, false},
 
 		// Argument references (special cases of memory operands)
 		{IsM, NewParamAddr("foo", 4), true},