diff --git a/arch/arm64_ext.go b/arch/arm64_ext.go
index 89bef17..91d1b33 100644
--- a/arch/arm64_ext.go
+++ b/arch/arm64_ext.go
@@ -529,7 +529,10 @@ const (
 	// ExtFormWhile is the while-compare form, PWHILELT R2, R10, P10.H:
 	// P10 counts the lanes where the signed comparison holds.  Operands:
 	// Rm (R0-R30), Rn (R0-R30), Pd.T, the arrangement feeding the size
-	// field.
+	// field.  The first written register is the comparison bound and the
+	// second the start of the count, the order the toolchain's corpus
+	// spells and the JIT run under qemu-user confirmed: PWHILELT R1, R0,
+	// P0.S with R1=4 builds four active lanes.
 	ExtFormWhile
 	// ExtFormPredicateCounter is PTRUE's counter-register form, PPTRUE
 	// PN14.S: the destination is a predicate-as-counter register, spelled
@@ -655,7 +658,7 @@ func (f ExtForm) Arity() int {
 	case ExtFormImmediate, ExtFormSignedImmediate, ExtFormPredicateFFRRead,
 		ExtFormPredicateMove, ExtFormPredicateUnpack, ExtFormPredicateTest,
 		ExtFormZMoveBare, ExtFormZFromGeneral,
-		ExtFormZLastReg, ExtFormZMove, ExtFormZUnpack:
+		ExtFormZMove, ExtFormZUnpack:
 		return 2
 	case ExtFormAmdVec3, ExtFormAmdMask2, ExtFormAmdVecGprVec:
 		return 3
@@ -667,7 +670,7 @@ func (f ExtForm) Arity() int {
 		return 2
 	case ExtFormPredicateOne, ExtFormPredicateWrite, ExtFormPredicateCounter:
 		return 1
-	case ExtFormZReadBack, ExtFormCountP, ExtFormCountPnW, ExtFormReduce:
+	case ExtFormZReadBack, ExtFormCountP, ExtFormCountPnW, ExtFormReduce, ExtFormZLastReg:
 		return 3
 	case ExtFormZSameReg, ExtFormCountPn, ExtFormCTerm:
 		return 2
diff --git a/arch/arm64_ext_test.go b/arch/arm64_ext_test.go
index b86d9ed..17fb7bc 100644
--- a/arch/arm64_ext_test.go
+++ b/arch/arm64_ext_test.go
@@ -1060,6 +1060,12 @@ func TestArm64ExtZGolden(t *testing.T) {
 		{"ZSUBR $6, Z7.D, Z7.D", "ZSUBR", ExtFormZImmediateSource, 0, ExtArrNone,
 			[]ExtOperand{ExtImmediate(6), ExtVector(7, ExtArrD), ExtVector(7, ExtArrD)},
 			0x25e3c0c7},
+		{"ZLASTA Z10.D, P3, R15", "ZLASTA", ExtFormZLastReg, 0, ExtArrD,
+			[]ExtOperand{ExtVector(10, ExtArrD), ExtPredicateSized(3, ExtArrNone), ExtGeneral(15)},
+			0x05e0ad4f},
+		{"ZLASTB Z10.D, P3, R15", "ZLASTB", ExtFormZLastReg, 0, ExtArrD,
+			[]ExtOperand{ExtVector(10, ExtArrD), ExtPredicateSized(3, ExtArrNone), ExtGeneral(15)},
+			0x05e1ad4f},
 	} {
 		in := extInstructionMeta(t, tt.mnem, tt.form, tt.qual, tt.arr)
 		got, err := in.Encode(tt.ops)