diff --git a/cpu.cpp b/cpu.cpp
index 7ccc82a..ef72593 100644
--- a/cpu.cpp
+++ b/cpu.cpp
@@ -247,15 +247,17 @@ int cpu::getPSW_spl() const
 	return (psw >> 5) & 7;
 }
 
-void cpu::setPSW(uint16_t v, const bool limited)
+void cpu::setPSW(const uint16_t v, const bool limited)
 {
 	if (limited) {
-		v &= 0174037;
+		psw |= v & 0174000;  // current & previous mode can only be increased, 11 can only be set
 
-		v |= psw & 0174340;
+		psw &= 0174000;  // retain upper 5 bit
+		psw |= v & ~0174000;
+	}
+	else {
+		psw = v;
 	}
-
-	psw = v;
 }
 
 bool cpu::check_queued_interrupts()
@@ -1628,15 +1630,10 @@ bool cpu::misc_operations(const uint16_t instr)
 		case 0b0000000000000001: // WAIT
 			return true;
 
-		case 0b0000000000000010: {  // RTI
+		case 0b0000000000000010: // RTI
 			setPC(popStack());
-
-			uint16_t replacement_psw = popStack();
-
-			setPSW(replacement_psw, true);
-
+			setPSW(popStack(), !!((getPSW() >> 12) & 3));
 			return true;
-		}
 
 		case 0b0000000000000011: // BPT
 			trap(014);
@@ -1646,15 +1643,10 @@ bool cpu::misc_operations(const uint16_t instr)
 			trap(020);
 			return true;
 
-		case 0b0000000000000110: {  // RTT
+		case 0b0000000000000110: // RTT
 			setPC(popStack());
-
-			uint16_t replacement_psw = popStack();
-
-			setPSW(replacement_psw, true);
-
+			setPSW(popStack(), !!((getPSW() >> 12) & 3));
 			return true;
-		}
 
 		case 0b0000000000000111: // MFPT
 			if (emulateMFPT)
diff --git a/cpu.h b/cpu.h
index ec10c89..01508df 100644
--- a/cpu.h
+++ b/cpu.h
@@ -117,7 +117,7 @@ public:
 	void setBitPSW(const int bit, const bool v);
 
 	uint16_t getPSW() const { return psw; }
-	void setPSW(uint16_t v, const bool limited);
+	void setPSW(const uint16_t v, const bool limited);
 
 	uint16_t getStackLimitRegister() { return stackLimitRegister; }
 	void setStackLimitRegister(const uint16_t v) { stackLimitRegister = v; }