From 31edf022cc0685bb6f9003ffaecd52dadd1b02ac Mon Sep 17 00:00:00 2001
From: folkert van heusden <mail@vanheusden.com>
Date: Sat, 18 Jun 2022 18:05:31 +0200
Subject: [PATCH] RTI/RTT shall not change the PSW in user/supervisor mode

---
 cpu.cpp | 30 +++++++++++++++++++-----------
 cpu.h   |  2 +-
 2 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/cpu.cpp b/cpu.cpp
index 6d2c55b..e28aca5 100644
--- a/cpu.cpp
+++ b/cpu.cpp
@@ -246,17 +246,15 @@ int cpu::getPSW_spl() const
 	return (psw >> 5) & 7;
 }
 
-void cpu::setPSW(const uint16_t v, const bool limited)
+void cpu::setPSW(uint16_t v, const bool limited)
 {
 	if (limited) {
-		psw |= v & 0174000;  // current & previous mode can only be increased, 11 can only be set
+		v &= 0174037;
 
-		psw &= 0174000;  // retain upper 5 bit
-		psw |= v & ~0174000;
-	}
-	else {
-		psw = v;
+		v |= psw & 0174340;
 	}
+
+	psw = v;
 }
 
 bool cpu::check_queued_interrupts()
@@ -1548,10 +1546,15 @@ bool cpu::misc_operations(const uint16_t instr)
 		case 0b0000000000000001: // WAIT
 			return true;
 
-		case 0b0000000000000010: // RTI
+		case 0b0000000000000010: {  // RTI
 			setPC(popStack());
-			setPSW(popStack(), !!((getPSW() >> 12) & 3));
+
+			uint16_t replacement_psw = popStack();
+
+			setPSW(replacement_psw, true);
+
 			return true;
+		}
 
 		case 0b0000000000000011: // BPT
 			trap(014);
@@ -1561,10 +1564,15 @@ bool cpu::misc_operations(const uint16_t instr)
 			trap(020);
 			return true;
 
-		case 0b0000000000000110: // RTT
+		case 0b0000000000000110: {  // RTT
 			setPC(popStack());
-			setPSW(popStack(), !!((getPSW() >> 12) & 3));
+
+			uint16_t replacement_psw = popStack();
+
+			setPSW(replacement_psw, true);
+
 			return true;
+		}
 
 		case 0b0000000000000111: // MFPT
 			if (emulateMFPT)
diff --git a/cpu.h b/cpu.h
index ba8a79f..097c0c7 100644
--- a/cpu.h
+++ b/cpu.h
@@ -118,7 +118,7 @@ public:
 	void setBitPSW(const int bit, const bool v);
 
 	uint16_t getPSW() const { return psw; }
-	void setPSW(const uint16_t v, const bool limited);
+	void setPSW(uint16_t v, const bool limited);
 
 	uint16_t getStackLimitRegister() { return stackLimitRegister; }
 	void setStackLimitRegister(const uint16_t v) { stackLimitRegister = v; }