Alternate A20 disable hack for EXEPACK error, as an alternative to LOADFIX

contrib/windows/installer/dosbox-x.reference.setup.conf

@@ -1707,6 +1707,7 @@ timeout     = 0
 #                                              lfn: Enable long filename support. If set to auto (default), it is enabled if the reported DOS version is at least 7.0.
 #                                                     If set to autostart, the builtin VER command won't activate/disactivate LFN support according to the reported DOS version.
 #                                                     Possible values: true, false, 1, 0, auto, autostart.
+#                                       autoa20fix: If set (default), DOSBox-X will automatically re-run the executable with the A20 gate disabled if it failed with the "Packed file is corrupt" error.
 #                                      autoloadfix: If set (default), DOSBox-X will automatically re-run the executable with LOADFIX if it failed with the "Packed file is corrupt" error.
 #                                        automount: Enable automatic drive mounting in Windows.
 #                                     automountall: Automatically mount all available Windows drives at start.
@@ -1801,6 +1802,7 @@ umb                                              = true
 quick reboot                                     = false
 ver                                              = 
 lfn                                              = auto
+autoa20fix                                       = true
 autoloadfix                                      = true
 automount                                        = true
 automountall                                     = false

dosbox-x.reference.conf

@@ -624,6 +624,7 @@ timeout     = 0
 #                             lfn: Enable long filename support. If set to auto (default), it is enabled if the reported DOS version is at least 7.0.
 #                                    If set to autostart, the builtin VER command won't activate/disactivate LFN support according to the reported DOS version.
 #                                    Possible values: true, false, 1, 0, auto, autostart.
+#                      autoa20fix: If set (default), DOSBox-X will automatically re-run the executable with the A20 gate disabled if it failed with the "Packed file is corrupt" error.
 #                     autoloadfix: If set (default), DOSBox-X will automatically re-run the executable with LOADFIX if it failed with the "Packed file is corrupt" error.
 #                       automount: Enable automatic drive mounting in Windows.
 #                    automountall: Automatically mount all available Windows drives at start.
@@ -651,6 +652,7 @@ umb                             = true
 quick reboot                    = false
 ver                             = 
 lfn                             = auto
+autoa20fix                      = true
 autoloadfix                     = true
 automount                       = true
 automountall                    = false

dosbox-x.reference.full.conf

@@ -1707,6 +1707,7 @@ timeout     = 0
 #                                              lfn: Enable long filename support. If set to auto (default), it is enabled if the reported DOS version is at least 7.0.
 #                                                     If set to autostart, the builtin VER command won't activate/disactivate LFN support according to the reported DOS version.
 #                                                     Possible values: true, false, 1, 0, auto, autostart.
+#                                       autoa20fix: If set (default), DOSBox-X will automatically re-run the executable with the A20 gate disabled if it failed with the "Packed file is corrupt" error.
 #                                      autoloadfix: If set (default), DOSBox-X will automatically re-run the executable with LOADFIX if it failed with the "Packed file is corrupt" error.
 #                                        automount: Enable automatic drive mounting in Windows.
 #                                     automountall: Automatically mount all available Windows drives at start.
@@ -1801,6 +1802,7 @@ private area in umb                              = true
 quick reboot                                     = false
 ver                                              = 
 lfn                                              = auto
+autoa20fix                                       = true
 autoloadfix                                      = true
 automount                                        = true
 automountall                                     = false

src/dos/dos.cpp

@@ -379,8 +379,10 @@ static inline void overhead() {
 #define BCD2BIN(x)	((((unsigned int)(x) >> 4u) * 10u) + ((x) & 0x0fu))
 #define BIN2BCD(x)	((((x) / 10u) << 4u) + (x) % 10u)
 extern bool date_host_forced;
+extern bool dos_a20_disable_on_exec;
 
 static Bitu DOS_21Handler(void);
+void XMS_DOS_LocalA20DisableIfNotEnabled(void);
 void DOS_Int21_7139(char *name1, const char *name2);
 void DOS_Int21_713a(char *name1, const char *name2);
 void DOS_Int21_713b(char *name1, const char *name2);
@@ -1610,6 +1612,13 @@ static Bitu DOS_21Handler(void) {
         case 0x4b:                  /* EXEC Load and/or execute program */
             {
                 MEM_StrCopy(SegPhys(ds)+reg_dx,name1,DOSNAMEBUF);
+
+                /* A20 hack for EXEPACK'd executables */
+                if (dos_a20_disable_on_exec) {
+                    XMS_DOS_LocalA20DisableIfNotEnabled();
+                    dos_a20_disable_on_exec=false;
+                }
+
                 LOG(LOG_EXEC,LOG_NORMAL)("Execute %s %d",name1,reg_al);
                 if (!DOS_Execute(name1,SegPhys(es)+reg_bx,reg_al)) {
                     reg_ax=dos.errorcode;

src/dosbox.cpp

@@ -3660,6 +3660,10 @@ void DOSBOX_SetupConfigSections(void) {
                       "If set to autostart, the builtin VER command won't activate/disactivate LFN support according to the reported DOS version.");
     Pstring->SetBasic(true);
 
+    Pbool = secprop->Add_bool("autoa20fix",Property::Changeable::WhenIdle,true);
+    Pbool->Set_help("If set (default), DOSBox-X will automatically re-run the executable with the A20 gate disabled if it failed with the \"Packed file is corrupt\" error.");
+    Pbool->SetBasic(true);
+
     Pbool = secprop->Add_bool("autoloadfix",Property::Changeable::WhenIdle,true);
     Pbool->Set_help("If set (default), DOSBox-X will automatically re-run the executable with LOADFIX if it failed with the \"Packed file is corrupt\" error.");
     Pbool->SetBasic(true);

src/ints/xms.cpp

@@ -398,6 +398,15 @@ Bitu XMS_LocalDisableA20(void) {
     return 0;
 }
 
+void XMS_DOS_LocalA20DisableIfNotEnabled(void) {
+    /* This is one of two hacks to deal with EXEPACK'd executables loaded too low */
+    if (XMS_GetEnabledA20()) {
+        LOG(LOG_DOSMISC,LOG_DEBUG)("Temporarily disabling A20 gate. As a hack this will FORCE local A20 enable to zero (from count=%d)",xms_local_enable_count);
+        xms_local_enable_count = 1;
+        XMS_LocalDisableA20();
+    }
+}
+
 void XMS_DOS_LocalA20EnableIfNotEnabled(void) {
     /* Confirmed MS-DOS behavior if DOS=HIGH */
     if (!XMS_GetEnabledA20()) {

src/shell/shell_misc.cpp

@@ -747,6 +747,7 @@ void DOS_Shell::InputCommand(char * line) {
 	ProcessCmdLineEnvVarStitution(line);
 }
 
+void XMS_DOS_LocalA20DisableIfNotEnabled(void);
 
 /* WARNING: Substitution is carried out in-place!
  * Buffer pointed to by "line" must be at least CMD_MAXLINE+1 bytes long! */
@@ -885,6 +886,7 @@ overflow:
 
 std::string full_arguments = "";
 intptr_t hret=0;
+bool dos_a20_disable_on_exec=false;
 bool infix=false, winautorun=false;
 extern bool packerr, reqwin, startwait, startquiet, ctrlbrk, mountwarning;
 #if defined (WIN32) && !defined(HX_DOS)
@@ -1145,7 +1147,15 @@ continue_1:
 		reg_eip=oldeip;
 		SegSet16(cs,oldcs);
 #endif
-		if (packerr&&!infix&&sec->Get_bool("autoloadfix")) {
+		if (packerr&&!infix&&sec->Get_bool("autoa20fix")) {
+			WriteOut("\r\n\033[41;1m\033[1;37;1mDOSBox-X\033[0m Failed to load the executable\r\n\033[41;1m\033[37;1mDOSBox-X\033[0m Now try again with A20 disable...\r\n");
+			infix=true;
+			dos_a20_disable_on_exec=true;
+			Execute(name, args);
+			dos_a20_disable_on_exec=false;
+			infix=false;
+		}
+		else if (packerr&&!infix&&sec->Get_bool("autoloadfix")) {
 			uint16_t segment;
 			uint16_t blocks = (uint16_t)(64*1024/16);
 			if (DOS_AllocateMemory(&segment,&blocks)) {