diff --git a/doc/c128.sgml b/doc/c128.sgml
index f133d8d4c..14c874998 100644
--- a/doc/c128.sgml
+++ b/doc/c128.sgml
@@ -87,6 +87,7 @@ url="funcref.html" name="function reference"> for declaration and usage.
 <item>c64mode
 <item>fast
 <item>slow
+<item>isfast
 </itemize>
 
 
diff --git a/doc/funcref.sgml b/doc/funcref.sgml
index 235a0d40f..193a10c52 100644
--- a/doc/funcref.sgml
+++ b/doc/funcref.sgml
@@ -131,6 +131,7 @@ function.
 <itemize>
 <item><ref id="c64mode" name="c64mode">
 <item><ref id="fast" name="fast">
+<item><ref id="isfast" name="isfast">
 <item><ref id="slow" name="slow">
 <item><ref id="toggle_videomode" name="toggle_videomode">
 <item><ref id="videomode" name="videomode">
@@ -3302,6 +3303,7 @@ will nearly double the speed compared to slow mode.
 </itemize>
 <tag/Availability/C128
 <tag/See also/
+<ref id="isfast" name="isfast">,
 <ref id="slow" name="slow">,
 <ref id="toggle_videomode" name="toggle_videomode">,
 <ref id="videomode" name="videomode">
@@ -3954,6 +3956,28 @@ fastcall function, so it may only be used in presence of a prototype.
 </quote>
 
 
+<sect1>isfast<label id="isfast"><p>
+
+<quote>
+<descrip>
+<tag/Function/Check if the C128 is in 2MHz mode.
+<tag/Header/<tt/<ref id="c128.h" name="c128.h">/
+<tag/Declaration/<tt/unsigned char isfast (void);/
+<tag/Description/The function returns a 1 if the C128 is in 2MHz mode.
+<tag/Notes/<itemize>
+<item>The function is specific to the C128.
+</itemize>
+<tag/Availability/C128
+<tag/See also/
+<ref id="fast" name="fast">,
+<ref id="slow" name="slow">,
+<ref id="toggle_videomode" name="toggle_videomode">,
+<ref id="videomode" name="videomode">
+<tag/Example/None.
+</descrip>
+</quote>
+
+
 <sect1>isgraph<label id="isgraph"><p>
 
 <quote>
@@ -6073,6 +6097,7 @@ will halve the speed compared to fast mode.
 <tag/Availability/C128
 <tag/See also/
 <ref id="fast" name="fast">,
+<ref id="isfast" name="isfast">,
 <ref id="toggle_videomode" name="toggle_videomode">,
 <ref id="videomode" name="videomode">
 <tag/Example/None.
@@ -6864,6 +6889,7 @@ name="videomode"> instead!
 <tag/Availability/C128
 <tag/See also/
 <ref id="fast" name="fast">,
+<ref id="isfast" name="isfast">,
 <ref id="slow" name="slow">,
 <ref id="videomode" name="videomode">
 <tag/Example/None.
@@ -7064,6 +7090,7 @@ used in presence of a prototype.
 <tag/Availability/C128 and enhanced Apple //e
 <tag/See also/
 <ref id="fast" name="fast">,
+<ref id="isfast" name="isfast">,
 <ref id="slow" name="slow">,
 <ref id="toggle_videomode" name="toggle_videomode">
 <tag/Example/None.
diff --git a/include/c128.h b/include/c128.h
index 356140d41..99e050a9b 100644
--- a/include/c128.h
+++ b/include/c128.h
@@ -167,7 +167,8 @@ void fast (void);
 void slow (void);
 /* Switch the CPU into 1MHz mode. */
 
-
+unsigned char isfast (void);
+/* Returns 1 if the CPU is in 2MHz mode. */
 
 /* End of c128.h */
 #endif
diff --git a/libsrc/c128/isfast.s b/libsrc/c128/isfast.s
new file mode 100644
index 000000000..d40e2c7e9
--- /dev/null
+++ b/libsrc/c128/isfast.s
@@ -0,0 +1,21 @@
+;
+; Marco van den Heuvel, 2018-03-19
+;
+; unsigned char isfast (void);
+; /* Returns 1 if the CPU is in 2MHz mode. */
+;
+
+        .export         _isfast
+
+        .include        "c128.inc"
+
+
+.proc   _isfast
+
+        lda     VIC_CLK_128
+        and     #$01
+        ldx     #$00
+        rts
+
+.endproc
+