From 98b757f83d9e4be02e9373f3279d432d3f7113f5 Mon Sep 17 00:00:00 2001
From: Hugo Villeneuve <hugo@hugovil.com>
Date: Sun, 17 Nov 2013 22:52:00 -0500
Subject: [PATCH] Add parity bit update each instruction cycle

---
 src/cpu8051.c |  7 +++++++
 src/psw.c     | 32 ++++++++++++++++++++++++++++++++
 src/psw.h     |  6 ++++++
 3 files changed, 45 insertions(+)

diff --git a/src/cpu8051.c b/src/cpu8051.c
index 58a0e79..55e761a 100644
--- a/src/cpu8051.c
+++ b/src/cpu8051.c
@@ -28,6 +28,7 @@
 #include "reg8051.h"
 #include "cpu8051.h"
 #include "memory.h"
+#include "psw.h"
 #include "disasm.h"
 #include "instructions_8051.h"
 
@@ -374,6 +375,12 @@ cpu8051_Exec(void)
 	cpu8051.pc++;
 	insttiming = (*opcode_table[opcode])(); /* Function callback. */
 
+	/*
+	 * Parity bit (p): is automatically set or cleared in each machine
+	 * cycle to establish even parity in the accumulator.
+	 */
+	psw_compute_parity_bit();
+
 	for (i = 0; i < insttiming; i++) {
 		cpu8051_CheckInterrupts();
 		cpu8051_DoTimers();
diff --git a/src/psw.c b/src/psw.c
index 4dd2a67..2d9c157 100644
--- a/src/psw.c
+++ b/src/psw.c
@@ -117,3 +117,35 @@ psw_read_ov(void)
 {
 	return memory_read8(INT_MEM_ID, _PSW_) >> PSW_BIT_OV;
 }
+
+void
+psw_write_p(int p)
+{
+	u_int8_t psw = memory_read8(INT_MEM_ID, _PSW_);
+
+	if (p)
+		psw |= PSW_FLAG_P;  /* Set */
+	else
+		psw &= ~PSW_FLAG_P; /* Clear */
+
+	memory_write8(INT_MEM_ID, _PSW_, psw); /* Save updated value */
+}
+
+/*
+ * Compute parity of bits in accumulator:
+ *   parity = 0: even number of ones in accumulator
+ *   parity = 1: odd  number of ones in accumulator
+ */
+void
+psw_compute_parity_bit(void)
+{
+	int parity = 0;
+	uint8_t acc = memory_read8(INT_MEM_ID, _ACC_);
+
+	while (acc) {
+		parity = !parity;
+		acc = acc & (acc - 1);
+	}
+
+	psw_write_p(parity);
+}
diff --git a/src/psw.h b/src/psw.h
index f4c1bb4..1c87524 100644
--- a/src/psw.h
+++ b/src/psw.h
@@ -59,4 +59,10 @@ psw_write_ov(int ov);
 int
 psw_read_ov(void);
 
+void
+psw_write_p(int p);
+
+void
+psw_compute_parity_bit(void);
+
 #endif /* PSW_H */
-- 
2.20.1