wdc65816: emulate (direct,X) wraparound bug in emulation mode

Manually cherry-picked ares commit ares-emulator/ares@be8fa76 Co-Authored-By: Adrian Siekierka <[email protected]>
bsnes-emu · Mar 1, 2024 · ccbe394 · ccbe394
1 parent 4faca65
commit ccbe394
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Showing 4 changed files with 16 additions and 8 deletions.
diff --git a/bsnes/processor/wdc65816/instructions-read.cpp b/bsnes/processor/wdc65816/instructions-read.cpp
@@ -113,8 +113,8 @@ auto WDC65816::instructionIndexedIndirectRead8(alu8 op) -> void {
   U.l = fetch();
   idle2();
   idle();
-  V.l = readDirect(U.l + X.w + 0);
-  V.h = readDirect(U.l + X.w + 1);
+  V.l = readDirectX(U.l + X.w, 0);
+  V.h = readDirectX(U.l + X.w, 1);
 L W.l = readBank(V.w + 0);
   alu(W.l);
 }
@@ -123,8 +123,8 @@ auto WDC65816::instructionIndexedIndirectRead16(alu16 op) -> void {
   U.l = fetch();
   idle2();
   idle();
-  V.l = readDirect(U.l + X.w + 0);
-  V.h = readDirect(U.l + X.w + 1);
+  V.l = readDirectX(U.l + X.w, 0);
+  V.h = readDirectX(U.l + X.w, 1);
   W.l = readBank(V.w + 0);
 L W.h = readBank(V.w + 1);
   alu(W.w);

diff --git a/bsnes/processor/wdc65816/instructions-write.cpp b/bsnes/processor/wdc65816/instructions-write.cpp
@@ -90,17 +90,17 @@ auto WDC65816::instructionIndexedIndirectWrite8() -> void {
   U.l = fetch();
   idle2();
   idle();
-  V.l = readDirect(U.l + X.w + 0);
-  V.h = readDirect(U.l + X.w + 1);
+  V.l = readDirectX(U.l + X.w, 0);
+  V.h = readDirectX(U.l + X.w, 1);
 L writeBank(V.w + 0, A.l);
 }
 
 auto WDC65816::instructionIndexedIndirectWrite16() -> void {
   U.l = fetch();
   idle2();
   idle();
-  V.l = readDirect(U.l + X.w + 0);
-  V.h = readDirect(U.l + X.w + 1);
+  V.l = readDirectX(U.l + X.w, 0);
+  V.h = readDirectX(U.l + X.w, 1);
   writeBank(V.w + 0, A.l);
 L writeBank(V.w + 1, A.h);
 }

diff --git a/bsnes/processor/wdc65816/memory.cpp b/bsnes/processor/wdc65816/memory.cpp
@@ -59,6 +59,13 @@ auto WDC65816::writeDirect(uint address, uint8 data) -> void {
   write(D.w + address & 0xffff, data);
 }
 
+auto WDC65816::readDirectX(uint address, uint offset) -> uint8 {
+  // The (direct,X) addressing mode has a bug in which the high byte is
+  // wrapped within the page if E = 1 and D&0xFF != 0.
+  if(EF && D.l) return read(((D.w + address) & 0xffff00) | ((D.w + address + offset) & 0xff));
+  else return readDirect(address + offset);
+}
+
 auto WDC65816::readDirectN(uint address) -> uint8 {
   return read(D.w + address & 0xffff);
 }

diff --git a/bsnes/processor/wdc65816/wdc65816.hpp b/bsnes/processor/wdc65816/wdc65816.hpp
@@ -58,6 +58,7 @@ struct WDC65816 {
   alwaysinline auto pushN(uint8 data) -> void;
   alwaysinline auto readDirect(uint address) -> uint8;
   alwaysinline auto writeDirect(uint address, uint8 data) -> void;
+  alwaysinline auto readDirectX(uint address, uint offset) -> uint8;
   alwaysinline auto readDirectN(uint address) -> uint8;
   alwaysinline auto readBank(uint address) -> uint8;
   alwaysinline auto writeBank(uint address, uint8 data) -> void;