From 1c5d432d3cf4332066ec48c0e572e11e95636ddb Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Sun, 16 Mar 2025 18:37:32 +0100
Subject: [PATCH] Disable special segment selector pushing behavior on Pentium
 onwards, fixes MSVC builds of ReactOS.

---
 src/cpu/cpu.c    | 79 +++++++++++++++++++++++++-----------------------
 src/cpu/cpu.h    |  1 +
 src/cpu/x86seg.c | 23 +++++++++-----
 3 files changed, 57 insertions(+), 46 deletions(-)

diff --git a/src/cpu/cpu.c b/src/cpu/cpu.c
index c4d084947..b821bc657 100644
--- a/src/cpu/cpu.c
+++ b/src/cpu/cpu.c
@@ -209,6 +209,7 @@ int is286;
 int is386;
 int is6117;
 int is486 = 1;
+int is586 = 0;
 int cpu_isintel;
 int cpu_iscyrix;
 int hascache;
@@ -552,6 +553,8 @@ cpu_set(void)
     cpu_16bitbus = (cpu_s->cpu_type == CPU_286) || (cpu_s->cpu_type == CPU_386SX) || (cpu_s->cpu_type == CPU_486SLC) || (cpu_s->cpu_type == CPU_IBM386SLC) || (cpu_s->cpu_type == CPU_IBM486SLC);
     cpu_64bitbus = (cpu_s->cpu_type >= CPU_WINCHIP);
 
+    is586    = cpu_64bitbus || (cpu_s->cpu_type == CPU_P24T);
+
     if (cpu_s->multi)
         cpu_busspeed = cpu_s->rspeed / cpu_s->multi;
     else
@@ -4303,47 +4306,47 @@ cpu_write(uint16_t addr, uint8_t val, UNUSED(void *priv))
 static uint8_t
 cpu_read(uint16_t addr, UNUSED(void *priv))
 {
+    uint8_t ret = 0xff;
+
     if (addr == 0xf007)
-        return 0x7f;
+        ret = 0x7f;
+    else if ((addr < 0xf0) && (addr & 1))  switch (cyrix_addr) {
+        case 0xc0:
+            ret = ccr0;
+            break;
+        case 0xc1:
+            ret = ccr1;
+            break;
+        case 0xc2:
+            ret = ccr2;
+            break;
+        case 0xc3:
+            ret = ccr3;
+            break;
+        case 0xe8:
+            ret = ((ccr3 & 0xf0) == 0x10) ? ccr4 : 0xff;
+            break;
+        case 0xe9:
+            ret = ((ccr3 & 0xf0) == 0x10) ? ccr5 : 0xff;
+            break;
+        case 0xea:
+            ret = ((ccr3 & 0xf0) == 0x10) ? ccr6 : 0xff;
+            break;
+        case 0xeb:
+            ret = ccr7;
+            break;
+        case 0xfe:
+            ret = cpu_s->cyrix_id & 0xff;
+            break;
+        case 0xff:
+            ret = cpu_s->cyrix_id >> 8;
+            break;
 
-    if (addr >= 0xf0)
-        return 0xff; /* FPU stuff */
-
-    if (addr & 1) {
-        switch (cyrix_addr) {
-            case 0xc0:
-                return ccr0;
-            case 0xc1:
-                return ccr1;
-            case 0xc2:
-                return ccr2;
-            case 0xc3:
-                return ccr3;
-            case 0xe8:
-                return ((ccr3 & 0xf0) == 0x10) ? ccr4 : 0xff;
-            case 0xe9:
-                return ((ccr3 & 0xf0) == 0x10) ? ccr5 : 0xff;
-            case 0xea:
-                return ((ccr3 & 0xf0) == 0x10) ? ccr6 : 0xff;
-            case 0xeb:
-                return ccr7;
-            case 0xfe:
-                return cpu_s->cyrix_id & 0xff;
-            case 0xff:
-                return cpu_s->cyrix_id >> 8;
-
-            default:
-                break;
-        }
-
-        if ((cyrix_addr & 0xf0) == 0xc0)
-            return 0xff;
-
-        if (cyrix_addr == 0x20 && (cpu_s->cpu_type == CPU_Cx5x86))
-            return 0xff;
+        default:
+            break;
     }
-
-    return 0xff;
+ 
+    return ret;
 }
 
 void
diff --git a/src/cpu/cpu.h b/src/cpu/cpu.h
index 8324c543e..d58da6998 100644
--- a/src/cpu/cpu.h
+++ b/src/cpu/cpu.h
@@ -513,6 +513,7 @@ extern int is286;
 extern int is386;
 extern int is6117;
 extern int is486;
+extern int is586;
 extern int is_am486;
 extern int is_am486dxl;
 extern int is_pentium;
diff --git a/src/cpu/x86seg.c b/src/cpu/x86seg.c
index 61c0edd9f..cd393812d 100644
--- a/src/cpu/x86seg.c
+++ b/src/cpu/x86seg.c
@@ -1113,7 +1113,7 @@ loadcscall(uint16_t seg)
 
                                 x86seg_log("Type %04X\n", type);
                                 if (type == 0x0c00) {
-                                    PUSHL_SEL(oldss);
+                                    is586 ? PUSHL(oldss) : PUSHL_SEL(oldss);
                                     PUSHL(oldsp2);
                                     if (cpu_state.abrt) {
                                         SS  = oldss;
@@ -1678,10 +1678,17 @@ pmodeint(int num, int soft)
                         cpl_override = 1;
                         if (type >= 0x0800) {
                             if (cpu_state.eflags & VM_FLAG) {
-                                PUSHL_SEL(GS);
-                                PUSHL_SEL(FS);
-                                PUSHL_SEL(DS);
-                                PUSHL_SEL(ES);
+                                if (is586) {
+                                    PUSHL(GS);
+                                    PUSHL(FS);
+                                    PUSHL(DS);
+                                    PUSHL(ES);
+                                } else {
+                                    PUSHL_SEL(GS);
+                                    PUSHL_SEL(FS);
+                                    PUSHL_SEL(DS);
+                                    PUSHL_SEL(ES);
+                                }
                                 if (cpu_state.abrt)
                                     return;
                                 op_loadseg(0, &cpu_state.seg_ds);
@@ -1689,10 +1696,10 @@ pmodeint(int num, int soft)
                                 op_loadseg(0, &cpu_state.seg_fs);
                                 op_loadseg(0, &cpu_state.seg_gs);
                             }
-                            PUSHL_SEL(oldss);
+                            is586 ? PUSHL(oldss) : PUSHL_SEL(oldss);
                             PUSHL(oldsp);
                             PUSHL(cpu_state.flags | (cpu_state.eflags << 16));
-                            PUSHL_SEL(CS);
+                            is586 ? PUSHL(CS) : PUSHL_SEL(CS);
                             PUSHL(cpu_state.pc);
                             if (cpu_state.abrt)
                                 return;
@@ -1728,7 +1735,7 @@ pmodeint(int num, int soft)
                     }
                     if (type > 0x0800) {
                         PUSHL(cpu_state.flags | (cpu_state.eflags << 16));
-                        PUSHL_SEL(CS);
+                        is586 ? PUSHL(CS) : PUSHL_SEL(CS);
                         PUSHL(cpu_state.pc);
                         if (cpu_state.abrt)
                             return;