diff --git a/src/sim65/main.c b/src/sim65/main.c
index f2daf9295..d92d52ef6 100644
--- a/src/sim65/main.c
+++ b/src/sim65/main.c
@@ -36,6 +36,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <limits.h>
 
 /* common */
 #include "abend.h"
@@ -63,6 +64,12 @@ const char* ProgramFile;
 /* exit simulator after MaxCycles Cycles */
 unsigned long MaxCycles;
 
+/* maximum number of cycles that can be tested,
+** requires overhead for longest possible instruction,
+** which should be 7, using 16 for safety.
+*/
+#define MAXCYCLES_LIMIT (ULONG_MAX-16)
+
 /* Header signature 'sim65' */
 static const unsigned char HeaderSignature[] = {
     0x73, 0x69, 0x6D, 0x36, 0x35
@@ -72,7 +79,6 @@ static const unsigned char HeaderSignature[] = {
 static const unsigned char HeaderVersion = 2;
 
 
-
 /*****************************************************************************/
 /*                                   Code                                    */
 /*****************************************************************************/
@@ -140,6 +146,10 @@ static void OptQuitXIns (const char* Opt attribute ((unused)),
 /* quit after MaxCycles cycles */
 {
     MaxCycles = strtoul(Arg, NULL, 0);
+    /* Guard against overflow. */
+    if (MaxCycles >= MAXCYCLES_LIMIT) {
+        Error("'-x parameter out of range. Max: %lu",MAXCYCLES_LIMIT);
+    }
 }
 
 static unsigned char ReadProgramFile (void)
@@ -184,6 +194,7 @@ static unsigned char ReadProgramFile (void)
     }
 
     /* Get load address */
+    Val2 = 0; /* suppress uninitialized variable warning */
     if (((Val = fgetc(F)) == EOF) ||
         ((Val2 = fgetc(F)) == EOF)) {
         Error ("'%s': Header missing load address", ProgramFile);
diff --git a/src/sim65/paravirt.c b/src/sim65/paravirt.c
index 9e5c28432..0b16f89e9 100644
--- a/src/sim65/paravirt.c
+++ b/src/sim65/paravirt.c
@@ -242,7 +242,15 @@ static void PVClose (CPURegs* Regs)
 
     Print (stderr, 2, "PVClose ($%04X)\n", FD);
 
-    RetVal = close (FD);
+    if (FD != 0xFFFF) {
+        RetVal = close (FD);
+    } else {
+        /* test/val/constexpr.c "abuses" close, expecting close(-1) to return -1.
+        ** This behaviour is not the same on all target platforms.
+        ** MSVC's close treats it as a fatal error instead and terminates.
+        */
+        RetVal = 0xFFFF;
+    }
 
     SetAX (Regs, RetVal);
 }
diff --git a/test/asm/val/Makefile b/test/asm/val/Makefile
index 91dae9afd..09a6b91bc 100644
--- a/test/asm/val/Makefile
+++ b/test/asm/val/Makefile
@@ -22,7 +22,8 @@ ifdef QUIET
   NULLERR = 2>$(NULLDEV)
 endif
 
-SIM65FLAGS = -x 5000000000
+# sim65 can support 64-bit cycle counts on some platforms, but not all. This must fit in 32-bit.
+SIM65FLAGS = -x 4000000000
 
 CA65 := $(if $(wildcard ../../../bin/ca65*),..$S..$S..$Sbin$Sca65,ca65)
 LD65 := $(if $(wildcard ../../../bin/ld65*),..$S..$S..$Sbin$Sld65,ld65)
diff --git a/test/standard/Makefile b/test/standard/Makefile
index 054623b79..40299c1bf 100644
--- a/test/standard/Makefile
+++ b/test/standard/Makefile
@@ -22,7 +22,8 @@ ifdef QUIET
   NULLERR = 2>$(NULLDEV)
 endif
 
-SIM65FLAGS = -x 5000000000 -c
+# sim65 can support 64-bit cycle counts on some platforms, but not all. This must fit in 32-bit.
+SIM65FLAGS = -x 4000000000 -c
 
 CC65 := $(if $(wildcard ../../bin/cc65*),..$S..$Sbin$Scc65,cc65)
 CA65 := $(if $(wildcard ../../bin/ca65*),..$S..$Sbin$Sca65,ca65)
diff --git a/test/val/Makefile b/test/val/Makefile
index a3722f7bf..158967f9e 100644
--- a/test/val/Makefile
+++ b/test/val/Makefile
@@ -24,7 +24,8 @@ ifdef QUIET
   NULLERR = 2>$(NULLDEV)
 endif
 
-SIM65FLAGS = -x 5000000000 -c
+# sim65 can support 64-bit cycle counts on some platforms, but not all. This must fit in 32-bit.
+SIM65FLAGS = -x 4000000000 -c
 
 CC65 := $(if $(wildcard ../../bin/cc65*),..$S..$Sbin$Scc65,cc65)
 CA65 := $(if $(wildcard ../../bin/ca65*),..$S..$Sbin$Sca65,ca65)