diff --git a/bus.cpp b/bus.cpp
index 5a15bc3..599a2b6 100644
--- a/bus.cpp
+++ b/bus.cpp
@@ -51,7 +51,7 @@ void bus::init()
 	MMR3 = 0;
 }
 
-uint16_t bus::read(const uint16_t a, const bool word_mode, const bool use_prev, const bool peek_only)
+uint16_t bus::read(const uint16_t a, const bool word_mode, const bool use_prev, const bool peek_only, const d_i_space_t space)
 {
 	uint16_t temp = 0;
 
@@ -279,7 +279,7 @@ uint16_t bus::read(const uint16_t a, const bool word_mode, const bool use_prev,
 
 	int run_mode = (c->getPSW() >> (use_prev ? 12 : 14)) & 3;
 
-	uint32_t m_offset = calculate_physical_address(run_mode, a, !peek_only, false, peek_only, false);
+	uint32_t m_offset = calculate_physical_address(run_mode, a, !peek_only, false, peek_only, space == d_space);
 
 	if (word_mode)
 		temp = m -> readByte(m_offset);
@@ -393,7 +393,7 @@ void bus::addToMMR1(const int8_t delta, const uint8_t reg)
 	MMR1 |= reg;
 }
 
-uint16_t bus::write(const uint16_t a, const bool word_mode, uint16_t value, const bool use_prev)
+uint16_t bus::write(const uint16_t a, const bool word_mode, uint16_t value, const bool use_prev, const d_i_space_t space)
 {
 	if (a >= 0160000) {
 		if (word_mode) {
@@ -666,7 +666,7 @@ uint16_t bus::write(const uint16_t a, const bool word_mode, uint16_t value, cons
 
 	int run_mode = (c->getPSW() >> (use_prev ? 12 : 14)) & 3;
 
-	uint32_t m_offset = calculate_physical_address(run_mode, a, true, true, false, false);
+	uint32_t m_offset = calculate_physical_address(run_mode, a, true, true, false, space == d_space);
 
 	DOLOG(debug, true, "WRITE to %06o/%07o: %o", a, m_offset, value);
 
diff --git a/bus.h b/bus.h
index 8dee308..0321134 100644
--- a/bus.h
+++ b/bus.h
@@ -13,6 +13,8 @@ class cpu;
 class memory;
 class tty;
 
+typedef enum { d_space, i_space } d_i_space_t;
+
 typedef struct
 {
 	uint16_t par, pdr;
@@ -63,14 +65,14 @@ public:
 	void    set_lf_crs_b7();
 	uint8_t get_lf_crs();
 
-	uint16_t read(const uint16_t a, const bool word_mode, const bool use_prev, const bool peek_only=false);
+	uint16_t read(const uint16_t a, const bool word_mode, const bool use_prev, const bool peek_only=false, const d_i_space_t s = i_space);
 	uint16_t readByte(const uint16_t a) { return read(a, true, false); }
 	uint16_t readWord(const uint16_t a);
 	uint16_t peekWord(const uint16_t a);
 
 	uint16_t readUnibusByte(const uint16_t a);
 
-	uint16_t write(const uint16_t a, const bool word_mode, uint16_t value, const bool use_prev);
+	uint16_t write(const uint16_t a, const bool word_mode, uint16_t value, const bool use_prev, const d_i_space_t s = i_space);
 	uint8_t writeByte(const uint16_t a, const uint8_t value) { return write(a, true, value, false); }
 	uint16_t writeWord(const uint16_t a, const uint16_t value);