diff --git a/Components/9918/Implementation/9918.cpp b/Components/9918/Implementation/9918.cpp
index ee91049a2..57c9c392a 100644
--- a/Components/9918/Implementation/9918.cpp
+++ b/Components/9918/Implementation/9918.cpp
@@ -885,9 +885,9 @@ void Base<personality>::commit_register(int reg, uint8_t value) {
 
 			case 45:
 				Storage<personality>::command_context_.arguments = value;
-				// b6: 0 = video RAM; 1 = expansion RAM.
-				// b5: MXD (???)
-				// b4: MXS
+				// b6: MXC, i.e. destination for INed/OUTed video data; 0 = video RAM; 1 = expansion RAM.
+				// b5: MXD, destination for command engine.
+				// b4: MXS, source for command engine.
 				// b3: DIY
 				// b2: DIX
 				// b1: EQ
diff --git a/Components/9918/Implementation/9918Base.hpp b/Components/9918/Implementation/9918Base.hpp
index 44dddc5e7..de92521ec 100644
--- a/Components/9918/Implementation/9918Base.hpp
+++ b/Components/9918/Implementation/9918Base.hpp
@@ -437,9 +437,14 @@ template <Personality personality> struct Base: public Storage<personality> {
 			return;
 		}
 
+		// Copy and mutate the RAM pointer.
 		AddressT address = ram_pointer_;
 		++ram_pointer_;
 
+		// Determine the relevant RAM and its mask.
+		uint8_t *ram = ram_.data();
+		AddressT mask = memory_mask(personality);
+
 		if constexpr (is_yamaha_vdp(personality)) {
 			// The Yamaha increments only 14 bits of the address in TMS-compatible modes.
 			if(this->underlying_mode_ < ScreenMode::YamahaText80) {
@@ -452,6 +457,12 @@ template <Personality personality> struct Base: public Storage<personality> {
 				// them as if linear.
 				address = (address >> 1) | (address << 16);
 			}
+
+			// Also check whether expansion RAM is the true target here.
+			if(Storage<personality>::command_context_.arguments & 0x40) {
+				ram = Storage<personality>::expansion_ram_.data();
+				mask = AddressT(Storage<personality>::expansion_ram_.size() - 1);
+			}
 		}
 
 		switch(queued_access_) {
@@ -490,10 +501,10 @@ template <Personality personality> struct Base: public Storage<personality> {
 						break;
 					}
 				}
-				ram_[address & memory_mask(personality)] = read_ahead_buffer_;
+				ram[address & mask] = read_ahead_buffer_;
 			break;
 			case MemoryAccess::Read:
-				read_ahead_buffer_ = ram_[address & memory_mask(personality)];
+				read_ahead_buffer_ = ram[address & mask];
 			break;
 		}
 		queued_access_ = MemoryAccess::None;
diff --git a/Components/9918/Implementation/Storage.hpp b/Components/9918/Implementation/Storage.hpp
index 1ffc2ce38..5ea6be717 100644
--- a/Components/9918/Implementation/Storage.hpp
+++ b/Components/9918/Implementation/Storage.hpp
@@ -32,6 +32,8 @@ template <> struct Storage<Personality::TMS9918A> {
 template <Personality personality> struct Storage<personality, std::enable_if_t<is_yamaha_vdp(personality)>> {
 	using AddressT = uint32_t;
 
+	std::array<uint8_t, 65536> expansion_ram_;
+
 	int selected_status_ = 0;
 
 	int indirect_register_ = 0;