From 2cd99f0c705c9a40cbc4a11866c05c03b6704734 Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Wed, 19 Mar 2025 07:54:54 +0100
Subject: [PATCH] X86 segmentation: apparently, the CPU can execute a data
 segment in some cases, used by LINK and CodeView, fixes #5283.

---
 src/cpu/x86seg.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/src/cpu/x86seg.c b/src/cpu/x86seg.c
index 2a0601661..145752237 100644
--- a/src/cpu/x86seg.c
+++ b/src/cpu/x86seg.c
@@ -1334,6 +1334,12 @@ pmoderetf(int is32, uint16_t off)
     if (CPL == (seg & 0x0003)) {
         x86seg_log("RETF CPL = RPL  %04X\n", segdat[2]);
         switch (segdat[2] & 0x1f00) {
+            case 0x1000:
+            case 0x1100:
+            case 0x1200:
+            case 0x1300:
+                 /* Data segment, apparently valid when CPL is the same, used by MS LINK for DOS. */
+                 fallthrough;
             case 0x1800:
             case 0x1900:
             case 0x1a00:
@@ -1384,6 +1390,12 @@ pmoderetf(int is32, uint16_t off)
         cycles -= timing_retf_pm;
     } else {
         switch (segdat[2] & 0x1f00) {
+            case 0x1000:
+            case 0x1100:
+            case 0x1200:
+            case 0x1300:
+                 /* Data segment, apparently valid when CPL is the same, used by MS LINK for DOS. */
+                 fallthrough;
             case 0x1800:
             case 0x1900:
             case 0x1a00:
@@ -1605,6 +1617,12 @@ pmodeint(int num, int soft)
                 return;
             }
             switch (segdat2[2] & 0x1f00) {
+                case 0x1000:
+                case 0x1100:
+                case 0x1200:
+                case 0x1300:
+                     /* Data segment, apparently valid when CPL is the same, used by MS CodeView for DOS. */
+                     fallthrough;
                 case 0x1800:
                 case 0x1900:
                 case 0x1a00:
@@ -1983,6 +2001,12 @@ pmodeiret(int is32)
     }
 
     switch (segdat[2] & 0x1f00) {
+        case 0x1000:
+        case 0x1100:
+        case 0x1200:
+        case 0x1300:
+             /* Data segment, apparently valid when CPL is the same, used by MS CodeView for DOS. */
+             fallthrough;
         case 0x1800:
         case 0x1900:
         case 0x1a00: