diff --git a/bus.cpp b/bus.cpp
index e5f3bdf..680cc5e 100644
--- a/bus.cpp
+++ b/bus.cpp
@@ -416,7 +416,7 @@ uint16_t bus::write(const uint16_t a, const bool word_mode, uint16_t value, cons
 
 		if (a == 0172516) { // MMR3
 			D(fprintf(stderr, "write set MMR3: %o\n", value);)
-			MMR3 = value;
+			MMR3 = value & 067;
 			return MMR3;
 		}
 
@@ -571,9 +571,9 @@ uint16_t bus::write(const uint16_t a, const bool word_mode, uint16_t value, cons
 	m_offset += a & 8191;
 
 	if (word_mode)
-		m -> writeByte(m_offset, value);
+		m->writeByte(m_offset, value);
 	else
-		m -> writeWord(m_offset, value);
+		m->writeWord(m_offset, value);
 
 	return value;
 }
@@ -587,3 +587,13 @@ uint16_t bus::writeWord(const uint16_t a, const uint16_t value)
 {
 	return write(a, false, value);
 }
+
+uint16_t bus::readUnibusByte(const uint16_t a)
+{
+	return m->readByte(a);
+}
+
+void bus::writeUnibusByte(const uint16_t a, const uint8_t v)
+{
+	m->writeByte(a, v);
+}
diff --git a/bus.h b/bus.h
index 92441f3..ca8937a 100644
--- a/bus.h
+++ b/bus.h
@@ -55,10 +55,14 @@ public:
 	uint16_t readByte(const uint16_t a) { return read(a, true); }
 	uint16_t readWord(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=false);
 	uint8_t writeByte(const uint16_t a, const uint8_t value) { return write(a, true, value); }
 	uint16_t writeWord(const uint16_t a, const uint16_t value);
 
+	void writeUnibusByte(const uint16_t a, const uint8_t value);
+
 	void setMMR2(const uint16_t value) { MMR2 = value; }
 
 	uint16_t get_switch_register() const { return switch_register; }
diff --git a/rk05.cpp b/rk05.cpp
index 9edbae3..8d56828 100644
--- a/rk05.cpp
+++ b/rk05.cpp
@@ -178,7 +178,7 @@ void rk05::writeWord(const uint16_t addr, uint16_t v)
 
 				uint32_t p = reclen; // FIXME
 				for(size_t i=0; i<reclen; i++)
-					xfer_buffer[i] = b -> readByte(memoff + i);
+					xfer_buffer[i] = b->readUnibusByte(memoff + i);
 
 #if defined(ESP32)
 				if (!fh.seek(diskoffb))
@@ -242,7 +242,7 @@ void rk05::writeWord(const uint16_t addr, uint16_t v)
 
 					for(uint32_t i=0; i<cur; i++) {
 						if (p < 0160000)
-							b -> writeByte(p, xfer_buffer[i]);
+							b -> writeUnibusByte(p, xfer_buffer[i]);
 						p++;
 					}