diff --git a/src/main/java/de/neemann/digital/core/IntFormat.java b/src/main/java/de/neemann/digital/core/IntFormat.java
index 0eb419c2e..7a8a2eaa6 100644
--- a/src/main/java/de/neemann/digital/core/IntFormat.java
+++ b/src/main/java/de/neemann/digital/core/IntFormat.java
@@ -99,9 +99,9 @@ public enum IntFormat {
     public int strLen(int bits) {
         switch (this) {
             case dec:
-                return (int) Math.ceil(Math.log10(1L << bits));
+                return decStrLen(bits);
             case decSigned:
-                return (int) Math.ceil(Math.log10(1L << (bits - 1))) + 1;
+                return decStrLen(bits - 1) + 1;
             case hex:
                 return (bits - 1) / 4 + 3;
             case bin:
@@ -115,6 +115,15 @@ public enum IntFormat {
         }
     }
 
+    private int decStrLen(int bits) {
+        if (bits == 64)
+            return 20;
+        else if (bits == 63) {
+            return 19;
+        } else
+            return (int) Math.ceil(Math.log10(1L << bits));
+    }
+
     private static final char[] DIGITS = new char[]{'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
 
     private static String toHex(Value inValue) {
diff --git a/src/test/java/de/neemann/digital/core/IntFormatTest.java b/src/test/java/de/neemann/digital/core/IntFormatTest.java
index f5e1bb428..e3111a4da 100644
--- a/src/test/java/de/neemann/digital/core/IntFormatTest.java
+++ b/src/test/java/de/neemann/digital/core/IntFormatTest.java
@@ -101,11 +101,19 @@ public class IntFormatTest extends TestCase {
         assertEquals(3,IntFormat.dec.strLen(8));
         assertEquals(3,IntFormat.dec.strLen(9));
         assertEquals(4,IntFormat.dec.strLen(10));
+        assertEquals(19, IntFormat.dec.strLen(60));
+        assertEquals(19, IntFormat.dec.strLen(61));
+        assertEquals(19, IntFormat.dec.strLen(62));
+        assertEquals(19, IntFormat.dec.strLen(63));
+        assertEquals(20, IntFormat.dec.strLen(64));
 
         assertEquals(4,IntFormat.decSigned.strLen(8));
         assertEquals(4,IntFormat.decSigned.strLen(9));
         assertEquals(4,IntFormat.decSigned.strLen(10));
         assertEquals(5,IntFormat.decSigned.strLen(11));
+        assertEquals(20, IntFormat.decSigned.strLen(62));
+        assertEquals(20, IntFormat.decSigned.strLen(63));
+        assertEquals(20, IntFormat.decSigned.strLen(64));
 
         assertEquals(4,IntFormat.oct.strLen(4));
         assertEquals(4,IntFormat.oct.strLen(5));