phys_read/write functions are not used often enough to worry too much about performance, so to avoid emulator segfaults, put the system memory range check in the phys_read/write functions themselves. Add comments about the meaning of MemBase and MemSize and how they will be used when DOSBox-X eventually supports memory sizes larger than 4GB

include/mem.h

@@ -39,6 +39,7 @@ typedef uint64_t              PhysPt64;    /* guest physical memory pointer */
 typedef int32_t              MemHandle;
 
 extern HostPt               MemBase;
+extern size_t               MemSize;
 
 HostPt                      GetMemBase(void);
 bool                        MEM_A20_Enabled(void);
@@ -184,25 +185,59 @@ void phys_writes(PhysPt addr, const char* string, Bitu length);
 
 /* WARNING: These will cause a segfault or out of bounds access IF
  *          addr is beyond the end of memory */
+/* 2024/12/22: Looking across the DOSBox-X codebase, these functions
+ *             aren't used TOO often, and where they are used, some
+ *             code has memory range checks anyway. So it doesn't hurt
+ *             performance very much if at all to just put the range
+ *             check here instead, in order not to segfault if somehow
+ *             the address given is beyond end of system memory. --J.C.
+ *
+ *             There is one more important detail here. These functions
+ *             take only a 32-bit physical address. Which means if more
+ *             than 32 address bits are enabled on the CPU and the OS
+ *             has PSE/PAE page tables enabled, these functions will not
+ *             be able to reach above 4GB. Given how memory will be
+ *             segmented between the 'below 4GB' and 'above 4GB' regions,
+ *             if emulating 4GB or more, that is perfectly fine. S3 XGA
+ *             and ISA DMA emulation will never reach above 4GB anyway.
+ *
+ *             The way the range check is done is ideal for performance,
+ *             yet may fail to work correctly if MemSize is very close
+ *             to zero, low enough that subtraction would cause it to
+ *             wrap back around to the largest possible value. The code,
+ *             when MemBase is a valid pointer, will never set MemSize
+ *             that small. */
 
 static INLINE void phys_writeb(const PhysPt addr,const uint8_t val) {
-    host_writeb(MemBase+addr,val);
+    if (addr < MemSize)
+        host_writeb(MemBase+addr,val);
 }
 static INLINE void phys_writew(const PhysPt addr,const uint16_t val) {
-    host_writew(MemBase+addr,val);
+    if (addr < (MemSize-1u))
+        host_writew(MemBase+addr,val);
 }
 static INLINE void phys_writed(const PhysPt addr,const uint32_t val) {
-    host_writed(MemBase+addr,val);
+    if (addr < (MemSize-3u))
+        host_writed(MemBase+addr,val);
 }
 
 static INLINE uint8_t phys_readb(const PhysPt addr) {
-    return host_readb(MemBase+addr);
+    if (addr < MemSize)
+        return host_readb(MemBase+addr);
+    else
+        return 0xFF;
 }
 static INLINE uint16_t phys_readw(const PhysPt addr) {
-    return host_readw(MemBase+addr);
+    if (addr < (MemSize-1u))
+        return host_readw(MemBase+addr);
+    else
+        return 0xFFFFu;
 }
 static INLINE uint32_t phys_readd(const PhysPt addr) {
-    return host_readd(MemBase+addr);
+    if (addr < (MemSize-3u))
+        return host_readd(MemBase+addr);
+    else
+        return 0xFFFFFFFFu;
 }
 
 /* These don't check for alignment, better be sure it's correct */

src/cpu/paging.cpp

@@ -797,14 +797,8 @@ initpage_retry:
 			PhysPt dirEntryAddr = GetPageDirectoryEntryAddr(lin_addr);
 			// Range check to avoid emulator segfault: phys_readd() reads from MemBase+addr and does NOT range check.
 			// Needed to avoid segfault when running 1999 demo "Void Main" in a bootable Windows 95 image in pure DOS mode.
-			if ((dirEntryAddr+4) <= (MEM_TotalPages()<<12u)) {
-				dir_entry.load=phys_readd(dirEntryAddr);
-			}
-			else {
-				LOG(LOG_CPU,LOG_WARN)("Page directory access beyond end of memory, page %08x >= %08x",
-					(unsigned int)(dirEntryAddr>>12u),(unsigned int)MEM_TotalPages());
-				dir_entry.load=0xFFFFFFFF;
-			}
+			// 2024/12/22: phys_readx() does range checking now
+			dir_entry.load=phys_readd(dirEntryAddr);
 
 			if (!dir_entry.dirblock.p) {
 				// table pointer is not present, do a page fault
@@ -851,14 +845,8 @@ initpage_retry:
 			else {
 				PhysPt tableEntryAddr = GetPageTableEntryAddr(lin_addr, dir_entry);
 				// Range check to avoid emulator segfault: phys_readd() reads from MemBase+addr and does NOT range check.
-				if ((tableEntryAddr+4) <= (MEM_TotalPages()<<12u)) {
-					table_entry.load=phys_readd(tableEntryAddr);
-				}
-				else {
-					LOG(LOG_CPU,LOG_WARN)("Page table entry access beyond end of memory, page %08x >= %08x",
-						(unsigned int)(tableEntryAddr>>12u),(unsigned int)MEM_TotalPages());
-					table_entry.load=0xFFFFFFFF;
-				}
+				// 2024/12/22: phys_readx() does range checking now
+				table_entry.load=phys_readd(tableEntryAddr);
 
 				// set page table accessed (IA manual: A is set whenever the entry is 
 				// used in a page translation)

src/hardware/memory.cpp

@@ -215,7 +215,14 @@ uint32_t MEM_get_address_bits4GB() { /* some code cannot yet handle values large
         return memory.address_bits;
 }
 
+/* WARNING: When DOSBox-X enables emulation of more than 4GB of RAM, this MemBase and MemSize will only reflect the memory below 4GB.
+ *          Which means phys_readx/writex(), which are limited to the first 4GB anyway (32-bit addresses), cannot be used to poke at
+ *          memory above 4GB. Instead of extending MemBase and the singular allocation block to 4GB or larger, the memory above 4GB
+ *          is a different block. The reason for this is that the gap that needs to be left open for PCI devices and the ROM BIOS is
+ *          large enough that such an arrangement would lead to the waste of about 64MB of emulator memory, which is significant, while
+ *          the 384KB wasted at the 8086 1MB limit is too small to worry about. */
 HostPt MemBase = NULL;
+size_t MemSize = 0;
 
 class UnmappedPageHandler : public PageHandler {
 public:
@@ -1333,7 +1340,7 @@ void mem_writed(PhysPt address,uint32_t val) {
 }
 
 void phys_writes(PhysPt addr, const char* string, Bitu length) {
-    for(Bitu i = 0; i < length; i++) host_writeb(MemBase+addr+i,(uint8_t)string[i]);
+    for(Bitu i = 0; i < length && (addr+i) < MemSize; i++) host_writeb(MemBase+addr+i,(uint8_t)string[i]);
 }
 
 #include "control.h"
@@ -1882,6 +1889,7 @@ void ShutDownRAM(Section * sec) {
 #endif
         MemBase = NULL;
     }
+    MemSize = 0;
     ACPI_free();
 }
 
@@ -2014,6 +2022,7 @@ void Init_RAM() {
 #else // C_GAMELINK
     MemBase = new(std::nothrow) uint8_t[memory.pages*4096];
 #endif // C_GAMELINK
+    MemSize = size_t(memory.pages*4096);
     if (!MemBase) E_Exit("Can't allocate main memory of %d KB",(int)memsizekb);
     /* Clear the memory, as new doesn't always give zeroed memory
      * (Visual C debug mode). We want zeroed memory though. */