diff --git a/InstructionSets/x86/Implementation/PerformImplementation.hpp b/InstructionSets/x86/Implementation/PerformImplementation.hpp
index e1ff68393..1d1422fe5 100644
--- a/InstructionSets/x86/Implementation/PerformImplementation.hpp
+++ b/InstructionSets/x86/Implementation/PerformImplementation.hpp
@@ -317,8 +317,8 @@ template <
 				Primitive::setmo<IntT>(destination_w(), context);
 				break;
 			} else {
-				// TODO: perform ENTER as of the 80186.
 				static_assert(int(Operation::SETMO) == int(Operation::ENTER));
+				Primitive::enter<IntT>(instruction, context);
 			}
 		return;
 		case Operation::SETMOC:
diff --git a/InstructionSets/x86/Implementation/Stack.hpp b/InstructionSets/x86/Implementation/Stack.hpp
index 892b2f4e3..dfefb18d2 100644
--- a/InstructionSets/x86/Implementation/Stack.hpp
+++ b/InstructionSets/x86/Implementation/Stack.hpp
@@ -22,13 +22,13 @@ void push(
 	IntT &value,
 	ContextT &context
 ) {
-	context.registers.sp_ -= sizeof(IntT);
+	context.registers.sp() -= sizeof(IntT);
 	if constexpr (preauthorised) {
-		context.memory.template preauthorised_write<IntT>(Source::SS, context.registers.sp_, value);
+		context.memory.template preauthorised_write<IntT>(Source::SS, context.registers.sp(), value);
 	} else {
 		context.memory.template access<IntT, AccessType::Write>(
 			Source::SS,
-			context.registers.sp_) = value;
+			context.registers.sp()) = value;
 	}
 	context.memory.template write_back<IntT>();
 }
@@ -39,8 +39,8 @@ IntT pop(
 ) {
 	const auto value = context.memory.template access<IntT, preauthorised ? AccessType::PreauthorisedRead : AccessType::Read>(
 		Source::SS,
-		context.registers.sp_);
-	context.registers.sp_ += sizeof(IntT);
+		context.registers.sp());
+	context.registers.sp() += sizeof(IntT);
 	return value;
 }
 
@@ -144,12 +144,49 @@ void pusha(
 	}
 }
 
+template <typename IntT, typename InstructionT, typename ContextT>
+void enter(
+	const InstructionT &instruction,
+	ContextT &context
+) {
+	// TODO: all non-16bit address sizes.
+	const auto alloc_size = instruction.dynamic_storage_size();
+	const auto nesting_level = instruction.nesting_level() & 0x1f;
+
+	// Preauthorse contents that'll be fetched via BP.
+	const auto copied_pointers = nesting_level - 2;
+	if(copied_pointers > 0) {
+		context.memory.preauthorise_read(
+			Source::SS,
+			context.registers.bp() - copied_pointers * sizeof(uint16_t),
+			copied_pointers * sizeof(uint16_t)
+		);
+	}
+
+	// Preauthorse stack activity.
+	context.memory.preauthorise_stack_write((1 + copied_pointers) * sizeof(uint16_t));
+
+	// Push BP and grab the end of frame.
+	push<uint16_t, true>(context.registers.bp(), context);
+	const auto frame = context.registers.sp();
+
+	// Copy data as per the nesting level.
+	for(int c = 1; c < nesting_level; c++) {
+		context.registers.bp() -= 2;
+
+		const auto value = context.memory.template preauthorised_read<uint16_t>(Source::SS, context.registers.bp());
+		push<uint16_t, true>(value);
+	}
+
+	// Set final BP.
+	context.registers.bp() = frame;
+}
+
 template <typename IntT, typename ContextT>
 void leave(
 	ContextT &context
 ) {
 	// TODO: should use StackAddressSize to determine assignment of bp to sp.
-	// Probably make that a static constexpr on registers.
 	if constexpr (std::is_same_v<IntT, uint32_t>) {
 		context.registers.esp() = context.registers.ebp();
 		context.registers.ebp() = pop<uint32_t, false>(context);
diff --git a/InstructionSets/x86/Instruction.hpp b/InstructionSets/x86/Instruction.hpp
index da62be4d1..baaf3ff6f 100644
--- a/InstructionSets/x86/Instruction.hpp
+++ b/InstructionSets/x86/Instruction.hpp
@@ -245,8 +245,8 @@ enum class Operation: uint8_t {
 	/// Raises a bounds exception if not.
 	BOUND = SETMOC,
 
-	/// Create stack frame. See operand() for the nesting level and offset()
-	/// for the dynamic storage size.
+	/// Create stack frame. See the Instruction getters `nesting_level()`
+	/// and `dynamic_storage_size()`.
 	ENTER,
 	/// Procedure exit; copies BP to SP, then pops a new BP from the stack.
 	LEAVE,
@@ -815,6 +815,11 @@ template<bool is_32bit> class Instruction {
 			return ops[has_operand()];
 		}
 
+		/// @returns The nesting level argument supplied to an ENTER.
+		constexpr ImmediateT nesting_level() const	{
+			return operand();
+		}
+
 		/// @returns The immediate segment value provided with this instruction, if any. Relevant for far calls and jumps; e.g.  `JMP 1234h:5678h` will
 		/// have a segment value of `1234h`.
 		constexpr uint16_t segment() const		{
@@ -833,6 +838,11 @@ template<bool is_32bit> class Instruction {
 			return DisplacementT(offset());
 		}
 
+		/// @returns The dynamic storage size argument supplied to an ENTER.
+		constexpr ImmediateT dynamic_storage_size() const	{
+			return displacement();
+		}
+
 		// Standard comparison operator.
 		constexpr bool operator ==(const Instruction<is_32bit> &rhs) const {
 			if(	operation_ != rhs.operation_ ||