From a9b4eac37fa8eacc8c48548ddb37a7f7a03d5c0d Mon Sep 17 00:00:00 2001
From: Richard Dymond <rjdymond@gmail.com>
Date: Wed, 14 Aug 2024 17:07:10 -0300
Subject: [PATCH] CSimulator: Fix calculation of IX/IY offset addresses

---
 c/csimulator.c              | 11 ++++++-----
 sphinx/source/changelog.rst |  2 ++
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/c/csimulator.c b/c/csimulator.c
index 4d43eb45..4498f69a 100644
--- a/c/csimulator.c
+++ b/c/csimulator.c
@@ -35,6 +35,7 @@
 #endif
 #define INC_PC(i) LD(PC, (REG(PC) + (i)) % 65536)
 #define OUT(p, v) if (mem == NULL && (p & 0x8002) == 0 && (self->out7ffd & 0x20) == 0) out7ffd(self, v)
+#define ADDR(a) (a) & 0xFFFF
 #define CHECK_SIGNALS if ((TIME & 0xFFFFFF) < 10) PyErr_CheckSignals()
 
 typedef unsigned char byte;
@@ -910,7 +911,7 @@ static void af_xy(CSimulatorObject* self, void* lookup, int args[]) {
 
     int xy = REG(xyl) + 256 * REG(xyh);
     int d = PEEK((REG(PC) + 2) % 65536);
-    int addr = (xy + (d < 128 ? d : d - 256)) % 65536;
+    int addr = ADDR(xy + (d < 128 ? d : d - 256));
 #ifdef CONTENTION
     CONTEND {
         unsigned pc = REG(PC);
@@ -1007,7 +1008,7 @@ static void afc_xy(CSimulatorObject* self, void* lookup, int args[]) {
 
     int xy = REG(xyl) + 256 * REG(xyh);
     int d = PEEK((REG(PC) + 2) % 65536);
-    int addr = (xy + (d < 128 ? d : d - 256)) % 65536;
+    int addr = ADDR(xy + (d < 128 ? d : d - 256));
 #ifdef CONTENTION
     CONTEND {
         unsigned pc = REG(PC);
@@ -1084,7 +1085,7 @@ static void f_xy(CSimulatorObject* self, void* lookup, int args[]) {
 
     int xy = REG(xyl) + 256 * REG(xyh);
     int d = PEEK((REG(PC) + 2) % 65536);
-    int addr = (xy + (d < 128 ? d : d - 256)) % 65536;
+    int addr = ADDR(xy + (d < 128 ? d : d - 256));
 #ifdef CONTENTION
     CONTEND {
         unsigned pc = REG(PC);
@@ -1177,7 +1178,7 @@ static void fc_xy(CSimulatorObject* self, void* lookup, int args[]) {
 
     int xy = REG(xyl) + 256 * REG(xyh);
     int d = PEEK((REG(PC) + 2) % 65536);
-    int addr = (xy + (d < 128 ? d : d - 256)) % 65536;
+    int addr = ADDR(xy + (d < 128 ? d : d - 256));
 #ifdef CONTENTION
     CONTEND {
         unsigned pc = REG(PC);
@@ -1338,7 +1339,7 @@ static void bit_xy(CSimulatorObject* self, void* lookup, int args[]) {
 
     int xy = REG(xyl) + 256 * REG(xyh);
     int d = PEEK((REG(PC) + 2) % 65536);
-    int addr = (xy + (d < 128 ? d : d - 256)) % 65536;
+    int addr = ADDR(xy + (d < 128 ? d : d - 256));
 #ifdef CONTENTION
     CONTEND {
         unsigned pc = REG(PC);
diff --git a/sphinx/source/changelog.rst b/sphinx/source/changelog.rst
index 94cd39c7..b264c793 100644
--- a/sphinx/source/changelog.rst
+++ b/sphinx/source/changelog.rst
@@ -7,6 +7,8 @@ Changelog
   affect the half-carry flag
 * Fixed how 'BIT n,(IX/Y+d)' affects bits 3 and 5 of the flags in the C version
   of the Z80 simulator
+* Fixed how IX/IY offset addresses are calculated in the C version of the Z80
+  simulator
 
 9.3 (2024-08-10)
 ----------------