From cb275dd6a3fa34f0026d796d9789503d0344e577 Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Mon, 29 Jul 2024 04:34:47 +0200
Subject: [PATCH] Fixes the behavior of F(N)INIT and F(N)SAVE on 8087, fixes
 the only MCPDIAG failure that was actually an 86Box bug, closes #4518.

---
 src/cpu/x87_ops_misc.h |  8 ++++++++
 src/cpu/x87_ops_sf.h   | 10 ++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/cpu/x87_ops_misc.h b/src/cpu/x87_ops_misc.h
index d40d70140..96750468a 100644
--- a/src/cpu/x87_ops_misc.h
+++ b/src/cpu/x87_ops_misc.h
@@ -78,7 +78,11 @@ opFINIT(uint32_t fetchdat)
     cpu_state.npxc                                = 0x37F;
 #endif
     codegen_set_rounding_mode(X87_ROUNDING_NEAREST);
+#ifdef FPU_8087
+    cpu_state.npxs &= 0x4700;
+#else
     cpu_state.npxs = 0;
+#endif
     p              = (uint64_t *) cpu_state.tag;
 #ifdef USE_NEW_DYNAREC
     *p = 0;
@@ -406,7 +410,11 @@ FSAVE(void)
 
     cpu_state.npxc = 0x37F;
     codegen_set_rounding_mode(X87_ROUNDING_NEAREST);
+#ifdef FPU_8087
+    cpu_state.npxs &= 0x4700;
+#else
     cpu_state.npxs = 0;
+#endif
     p              = (uint64_t *) cpu_state.tag;
 #ifdef USE_NEW_DYNAREC
     *p = 0;
diff --git a/src/cpu/x87_ops_sf.h b/src/cpu/x87_ops_sf.h
index af94897c9..31416447e 100644
--- a/src/cpu/x87_ops_sf.h
+++ b/src/cpu/x87_ops_sf.h
@@ -431,11 +431,12 @@ sf_FNSAVE_a16(uint32_t fetchdat)
     }
 
 #ifdef FPU_8087
-    fpu_state.swd = 0x3FF;
+    fpu_state.cwd = 0x3FF;
+    fpu_state.swd  &= 0x4700;
 #else
     fpu_state.cwd = 0x37F;
-#endif
     fpu_state.swd   = 0;
+#endif
     fpu_state.tos   = 0;
     fpu_state.tag   = 0xffff;
     cpu_state.ismmx = 0;
@@ -467,7 +468,7 @@ sf_FNSAVE_a32(uint32_t fetchdat)
     }
 
 #    ifdef FPU_8087
-    fpu_state.swd = 0x3FF;
+    fpu_state.cwd = 0x3FF;
 #    else
     fpu_state.cwd = 0x37F;
 #    endif
@@ -504,10 +505,11 @@ sf_FNINIT(uint32_t fetchdat)
     cpu_state.pc++;
 #ifdef FPU_8087
     fpu_state.cwd = 0x3FF;
+    fpu_state.swd  &= 0x4700;
 #else
     fpu_state.cwd = 0x37F;
-#endif
     fpu_state.swd   = 0;
+#endif
     fpu_state.tos   = 0;
     fpu_state.tag   = 0xffff;
     fpu_state.foo   = 0;