ISA 15MB memory hole option, especially for PC-98 games

CHANGELOG

@@ -1,5 +1,9 @@
 Next:
   - Change ISA memory hole 512kb option from boolean to true/false/auto.
+  - Add ISA memory hole 15mb option, make it true/false/auto. Auto
+    means off for IBM compatible mode and on for PC-9821 compatible mode.
+    This should allow some DOS games that depend on the linear framebuffer
+    to work properly even if memsize=16 or higher.
 
 2023.10.06
   - Add "VRD" debugger command to force redraw of the VGA screen.

src/dosbox.cpp

@@ -1804,6 +1804,12 @@ void DOSBOX_SetupConfigSections(void) {
     Pstring->Set_values(truefalseautoopt);
     Pstring->Set_help("If set, emulate an ISA memory hole at the 512KB to 640KB area (0x80000-0x9FFFF).");
 
+    Pstring = secprop->Add_string("isa memory hole at 15mb",Property::Changeable::WhenIdle,"auto");
+    Pstring->Set_values(truefalseautoopt);
+    Pstring->Set_help("If set, emulate an ISA memory hole at the 15MB to 16MB area (0xF00000-0xFFFFFF).\n"
+		    "If auto, hole is disabled by default for IBM compatible modes and enabled by default for NEC PC-98 compatible modes.\n"
+		    "The reason for this is that the hole is needed for the PC-9821 256-color mode linear framebuffer to work with some DOS games even when memsize >= 16.");
+
     Pint = secprop->Add_int("reboot delay", Property::Changeable::WhenIdle,-1);
     Pint->SetMinMax(-1,10000);
     Pint->Set_help(

src/hardware/memory.cpp

@@ -61,6 +61,8 @@ class MEM_callout_vector : public std::vector<MEM_CalloutObject> {
 
 static MEM_callout_vector MEM_callouts[MEM_callouts_max];
 
+extern bool isa_memory_hole_15mb;
+
 bool a20_guest_changeable = true;
 bool a20_fake_changeable = false;
 bool a20_fast_changeable = false;
@@ -279,8 +281,13 @@ static PageHandler *MEM_SlowPath(Bitu page) {
 
     /* TEMPORARY, REMOVE LATER. SHOULD NOT HAPPEN. */
     if (page < memory.reported_pages) {
-        LOG(LOG_MISC,LOG_WARN)("MEM_SlowPath called within system RAM at page %x",(unsigned int)page);
-        f = (PageHandler*)(&ram_page_handler);
+        if (page >= 0xf00 && page <= 0xfff && isa_memory_hole_15mb) { /* 0xF00000-0xFFFFFF (15MB-16MB) */
+            /* ignore, ISA memory hole */
+        }
+        else {
+            LOG(LOG_MISC,LOG_WARN)("MEM_SlowPath called within system RAM at page %x",(unsigned int)page);
+            f = (PageHandler*)(&ram_page_handler);
+        }
     }
 
     /* check motherboard devices (ROM BIOS, system RAM, etc.) */
@@ -1885,6 +1892,12 @@ void Init_RAM() {
     for (;i < memory.handler_pages;i++)
         memory.phandlers[i] = NULL;//&illegal_page_handler;
 
+    /* ISA 15MB memory hole? */
+    if (isa_memory_hole_15mb) {
+        for (i=0xf00;i <= 0xfff && i < memory.handler_pages;i++)
+            memory.phandlers[i] = NULL;//&illegal_page_handler;
+    }
+
     /* FIXME: VGA emulation will selectively respond to 0xA0000-0xBFFFF according to the video mode,
      *        what we want however is for the VGA emulation to assign illegal_page_handler for
      *        address ranges it is not responding to when mapping changes. */
@@ -2059,6 +2072,11 @@ void Init_MemHandles() {
 
     for (i = 0;i < memory.pages;i++)
         memory.mhandles[i] = 0;             //Set to 0 for memory allocation
+
+    // ISA memory hole awareness (15MB region). Block off 0xF00000-0xFFFFFF with a dummy handle.
+    if (isa_memory_hole_15mb) {
+        for (i=0xF00;i<=0xFFF && i < memory.pages;i++) memory.mhandles[i] = 0x7FFFFFFF;
+    }
 }
 
 void Init_MemoryAccessArray() {

src/hardware/vga_memory.cpp

@@ -47,6 +47,7 @@ extern bool vga_ignore_extended_memory_bit;
 extern bool memio_complexity_optimization;
 extern bool enable_pc98_256color_planar;
 extern bool enable_pc98_256color;
+extern bool isa_memory_hole_15mb;
 
 extern unsigned int vbe_window_granularity;
 extern unsigned int vbe_window_size;
@@ -2725,10 +2726,8 @@ void VGA_SetupHandlers(void) {
 		else
 			MEM_ResetPageHandler_Unmapped(0xE0, 8);
 
-		// TODO: What about PC-9821 systems with more than 15MB of RAM? Do they maintain a "hole"
-		//       in memory for this linear framebuffer? Intel motherboard chipsets of that era do
-		//       support a 15MB memory hole.
-		if (MEM_TotalPages() <= 0xF00/*FIXME*/) {
+		/* If the system has 15MB or less RAM, OR the ISA memory hole at 15MB is enabled */
+		if (MEM_TotalPages() <= 0xF00 || isa_memory_hole_15mb) {
 			/* F00000-FF7FFFh linear framebuffer (256-packed)
 			 *  - Does not exist except in 256-color mode.
 			 *  - Switching from 256-color mode immediately unmaps this linear framebuffer.

src/ints/bios.cpp

@@ -136,6 +136,7 @@ bool APM_PowerButtonSendsSuspend = true;
 
 bool bochs_port_e9 = false;
 bool isa_memory_hole_512kb = false;
+bool isa_memory_hole_15mb = false;
 bool int15_wait_force_unmask_irq = false;
 
 int unhandled_irq_method = UNHANDLED_IRQ_SIMPLE;
@@ -7216,11 +7217,17 @@ void BIOS_ZeroExtendedSize(bool in) {
              * capacity does not include conventional memory below 1MB, nor any memory
              * above 16MB.
              *
-             * PC-98 systems may reserve the top 1MB, limiting the top to 15MB instead.
+             * PC-98 systems may reserve the top 1MB, limiting the top to 15MB instead,
+             * for the ISA memory hole needed for DOS games that use the 256-color linear framebuffer.
              *
              * 0x70 = 128KB * 0x70 = 14MB
              * 0x78 = 128KB * 0x70 = 15MB */
-            if (ext > 0x78) ext = 0x78;
+            if (isa_memory_hole_15mb) {
+                if (ext > 0x70) ext = 0x70;
+            }
+            else {
+                if (ext > 0x78) ext = 0x78;
+            }
 
             mem_writeb(0x401,ext);
         }
@@ -9929,6 +9936,20 @@ class BIOS:public Module_base{
                     isa_memory_hole_512kb = false;
             }
 
+            // TODO: motherboard init, especially when we get around to full Intel Triton/i440FX chipset emulation
+            {
+                std::string s = section->Get_string("isa memory hole at 15mb");
+
+                if (s == "true" || s == "1")
+                    isa_memory_hole_15mb = true;
+                else if (s == "false" || s == "0")
+                    isa_memory_hole_15mb = false;
+                else if (IS_PC98_ARCH)
+                    isa_memory_hole_15mb = true; // For the sake of some DOS games, enable by default
+                else
+                    isa_memory_hole_15mb = false;
+            }
+
             // FIXME: Erm, well this couldv'e been named better. It refers to the amount of conventional memory
             //        made available to the operating system below 1MB, which is usually DOS.
             dos_conventional_limit = (unsigned int)section->Get_int("dos mem limit");
@@ -10070,6 +10091,8 @@ class BIOS:public Module_base{
             if (start < end) MEM_ResetPageHandler_Unmapped(start,end-start);
         }
 
+        if (isa_memory_hole_15mb) MEM_ResetPageHandler_Unmapped(0xf00,0x100); /* 0xF00000-0xFFFFFF */
+
         if (machine == MCH_TANDY) {
             /* Take 16KB off the top for video RAM.
              * This value never changes after boot, even if you then use the 16-color modes which then moves