diff --git a/HACKING b/HACKING
index 740390fa..84eb5f7d 100644
--- a/HACKING
+++ b/HACKING
@@ -151,6 +151,13 @@ actual: -./,),(-*,(
 ** mes+mescc: parse tcc.c->tcc.E works, compile tcc.E -> tcc.M1 segfaults.
   time GUILE_LOAD_PATH=/home/janneke/src/nyacc/module:$GUILE_LOAD_PATH ../mes/scripts/mescc -E -o tcc.E -I . -I ../mes/lib -I ../mes/include -D 'CONFIG_TCCDIR="usr/lib/tcc"' -D 'CONFIG_TCC_CRTPREFIX="usr/lib:{B}/lib:."' -D 'CONFIG_TCC_ELFINTERP="/gnu/store/70jxsnpffkl7fdb7qv398n8yi1a3w5nx-glibc-2.26.105-g0890d5379c/lib/ld-linux.so.2"' -D 'CONFIG_TCC_LIBPATHS="/home/janneke/src/tinycc/usr/lib:{B}/lib:."' -D 'CONFIG_TCC_SYSINCLUDEPATHS="../mes/include:usr/include:{B}/include"' -D CONFIG_USE_LIBGCC=1 -D 'TCC_LIBGCC="/home/janneke/src/tinycc/usr/lib/libc+tcc-gcc.mlibc-o"' -D CONFIG_TCC_STATIC=1 -D ONE_SOURCE=yes -D TCC_TARGET_I386=1 -D BOOTSTRAP=1 tcc.c
   time GUILE_LOAD_PATH=/home/janneke/src/nyacc/module:$GUILE_LOAD_PATH MES_ARENA=200000000 ../mes/scripts/mescc -c -o tcc.M1 tcc.E
+** mescc: 64 bit compiled mes loses top 4 bytes
+*** 64 bit mescc-compiled mes:
+  #x100000000 => 0
+  (modulo 1 #x100000000) => divide-by-zero
+*** 64 bit gcc-compiled mes:
+  #x100000000 => 0
+  (modulo 1 #x100000000) => 1
 ** mescc: 7n-struct-struct-array.c:  struct file f = {"first.h"};
 ** test/match.test ("nyacc-simple"): hygiene problem in match
 * OLD: Booting from LISP-1.5 into Mes
diff --git a/module/mes/guile.scm b/module/mes/guile.scm
index 9c60077d..451843b4 100644
--- a/module/mes/guile.scm
+++ b/module/mes/guile.scm
@@ -45,6 +45,7 @@
             core:write-error
             core:write-port
             core:type
+            %compiler
             equal2?
             pmatch-car
             pmatch-cdr
@@ -83,6 +84,8 @@
   (define <cell:symbol> 11)
   (define <cell:vector> 15)
 
+  (define %compiler "gnuc")
+
   (define (core:type x)
     (cond ((guile:keyword? x) <cell:keyword>)
           ((guile:number? x) <cell:number>)
diff --git a/module/mescc/M1.scm b/module/mescc/M1.scm
index ef53ad74..8dbbc304 100644
--- a/module/mescc/M1.scm
+++ b/module/mescc/M1.scm
@@ -81,13 +81,16 @@
   (if hex? (string-append "%0x" (dec->hex o))
       (string-append "%" (number->string o))))
 
+(define mesc? (string=? %compiler "mesc"))
+
 (define (hex2:immediate8 o)
-  (if hex? (string-append "%0x" (dec->hex (modulo o #x100000000))
+  ;; FIXME: #x100000000 => 0 divide-by-zero when compiled with 64 bit mesc
+  (if hex? (string-append "%0x" (dec->hex (if mesc? 0 (modulo o #x100000000)))
                           " %0x" (if (< o 0) "-1"
-                                     (dec->hex (quotient o #x100000000))))
-      (string-append "%" (number->string (dec->hex (modulo o #x100000000)))
+                                     (dec->hex (if mesc? o (quotient o #x100000000)))))
+      (string-append "%" (number->string (dec->hex (if mesc? 0 (modulo o #x100000000))))
                      " %" (if (< o 0) "-1"
-                              (number->string (dec->hex (quotient o #x100000000)))))))
+                              (number->string (dec->hex (if mesc? o (quotient o #x100000000))))))))
 
 (define* (display-join o #:optional (sep ""))
   (let loop ((o o))
diff --git a/src/math.c b/src/math.c
index 3a1c67ce..1241a28e 100644
--- a/src/math.c
+++ b/src/math.c
@@ -128,9 +128,12 @@ divide (SCM x)                  ///((name . "/") (arity . n))
   while (x != cell_nil)
     {
       assert_number ("divide", CAR (x));
+      long y = VALUE (CAR (x));
+      if (y == 0)
+        error (cstring_to_symbol ("divide-by-zero"), x);
       if (!n)
         break;
-      n /= VALUE (car (x));
+      n /= y;
       x = cdr (x);
     }
   return MAKE_NUMBER (n);
@@ -142,9 +145,12 @@ modulo (SCM a, SCM b)
   assert_number ("modulo", a);
   assert_number ("modulo", b);
   long x = VALUE (a);
+  long y = VALUE (b);
+  if (y == 0)
+    error (cstring_to_symbol ("divide-by-zero"), a);
   while (x < 0)
-    x += VALUE (b);
-  x = x ? x % VALUE (b) : 0;
+    x += y;
+  x = x ? x % y : 0;
   return MAKE_NUMBER (x);
 }