diff --git a/Crc32.cpp b/Crc32.cpp
index 69e501a..e57dc77 100644
--- a/Crc32.cpp
+++ b/Crc32.cpp
@@ -587,12 +587,12 @@ uint32_t crc32_combine(uint32_t crcA, uint32_t crcB, size_t lengthB)
 
   // put operator for one zero bit in odd
   odd[0] = Polynomial;    // CRC-32 polynomial
-  for (int i = 1; i < CrcBits; i++)
+  for (uint32_t i = 1; i < CrcBits; i++)
     odd[i] = 1 << (i - 1);
 
   // put operator for two zero bits in even
   // same as gf2_matrix_square(even, odd);
-  for (int i = 0; i < CrcBits; i++)
+  for (uint32_t i = 0; i < CrcBits; i++)
   {
     uint32_t vec = odd[i];
     even[i] = 0;
@@ -602,11 +602,11 @@ uint32_t crc32_combine(uint32_t crcA, uint32_t crcB, size_t lengthB)
   }
   // put operator for four zero bits in odd
   // same as gf2_matrix_square(odd, even);
-  for (int i = 0; i < CrcBits; i++)
+  for (uint32_t i = 0; i < CrcBits; i++)
   {
     uint32_t vec = even[i];
     odd[i] = 0;
-    for (int j = 0; vec != 0; j++, vec >>= 1)
+    for (uint32_t j = 0; vec != 0; j++, vec >>= 1)
       if (vec & 1)
         odd[i] ^= even[j];
   }
@@ -618,7 +618,7 @@ uint32_t crc32_combine(uint32_t crcA, uint32_t crcB, size_t lengthB)
   for (; lengthB > 0; lengthB >>= 1)
   {
     // same as gf2_matrix_square(a, b);
-    for (int i = 0; i < CrcBits; i++)
+    for (uint32_t i = 0; i < CrcBits; i++)
     {
       uint32_t vec = b[i];
       a[i] = 0;
diff --git a/Crc32TestMultithreaded.cpp b/Crc32TestMultithreaded.cpp
index 0878801..559b0eb 100644
--- a/Crc32TestMultithreaded.cpp
+++ b/Crc32TestMultithreaded.cpp
@@ -103,7 +103,7 @@ bool testCombine(const char* data, size_t maxBytes = 1024)
     // check results
     if (crcAtOnce != crcSequential || crcAtOnce != crcCombined)
     {
-      printf("FAILED @ %d: %08X %08X %08X %08X %08X\n", lengthA, crcA, crcB, crcAtOnce, crcSequential, crcCombined);
+      printf("FAILED @ %zu: %08X %08X %08X %08X %08X\n", lengthA, crcA, crcB, crcAtOnce, crcSequential, crcCombined);
       ok = false;
     }
   }
diff --git a/Crc32cl.cpp b/Crc32cl.cpp
new file mode 100644
index 0000000..e551097
--- /dev/null
+++ b/Crc32cl.cpp
@@ -0,0 +1,177 @@
+// //////////////////////////////////////////////////////////
+// Crc32cl.cpp
+// Author: Sven Gothel <sgothel@jausoft.com>
+// Copyright (c) 2021 Gothel Software e.K. (same zlib license from Stephan Brumme)
+// Copyright (c) 2016-2019 Stephan Brumme.
+// see http://create.stephan-brumme.com/disclaimer.html
+//
+
+#include "Crc32.h"
+#include <cstdlib>
+#include <cinttypes>
+#include <cstdio>
+#include <cstring>
+#include <vector>
+#include <algorithm>
+
+#include <fstream>
+#include <iostream>
+
+#include <ctime>
+
+#ifdef ENABLE_MT
+    // Multithreading is incomplete
+    // - CRC differs, combine works but depending on thread count differs :)
+    // - Should use fixed thread pool and async ringbuffer for data
+    //   so threads can wait until being signaled EOD
+    //
+    // C++11 multithreading
+    #include <thread>
+    #include <future>
+
+    // //////////////////////////////////////////////////////////
+    // run a CRC32 algorithm on multiple threads
+    typedef uint32_t (*Crc32Algorithm)(const void* data, size_t length, uint32_t previousCrc32);
+    // compute CRC32 of up to maxBlockSize bytes and start recursively a new thread for excess data
+    static uint32_t asyncCrc32(Crc32Algorithm myCrc32, const void* data, size_t numBytes, size_t maxBlockSize)
+    {
+      // last block ?
+      if (numBytes <= maxBlockSize) {
+        return myCrc32(data, numBytes, 0); // we're done
+      }
+
+      // compute CRC of the remaining bytes in a separate thread
+      auto dataLeft  = (const char*)data + maxBlockSize;
+      auto bytesLeft =          numBytes - maxBlockSize;
+      auto remainder = std::async(std::launch::async, asyncCrc32, myCrc32, dataLeft, bytesLeft, maxBlockSize);
+
+      // compute CRC of the current block
+      auto currentCrc   = myCrc32(data, maxBlockSize, 0);
+      // get CRC of the remainder
+      auto remainderCrc = remainder.get();
+      // and merge both
+      return crc32_combine(currentCrc, remainderCrc, bytesLeft);
+    }
+    // call: run(crc32_8bytes, data, NumBytes, 8) if you have an octocore CPU
+    static uint32_t run(Crc32Algorithm myCrc32, const void* data, size_t numBytes, size_t numThreads = 0)
+    {
+      // run on all cores
+      if (numThreads == 0) {
+        numThreads = std::thread::hardware_concurrency();
+      }
+
+      // split data evenly, rounding up
+      auto defaultBlocksize = (numBytes + numThreads - 1) / numThreads;
+
+      return asyncCrc32(myCrc32, data, numBytes, defaultBlocksize);
+    }
+#endif // ENABLE_MT
+
+
+// timing
+static double seconds()
+{
+  timespec now;
+  clock_gettime(CLOCK_REALTIME, &now);
+  return now.tv_sec + now.tv_nsec / 1000000000.0;
+}
+
+void print_usage(const char*progname) {
+#ifdef ENABLE_MT
+    fprintf(stderr, "Usage %s [-v] [-bsz <buffer-size>] [-threads <thread-count>] <input-file>\n", progname);
+#else // ENABLE_MT
+    fprintf(stderr, "Usage %s [-v] [-bsz <buffer-size>] <input-file>\n", progname);
+#endif // ENABLE_MT
+}
+
+int main(int argc, char**argv)
+{
+    int argi=0;
+
+    bool verbose = false;
+    int thread_count = 1; // minimum
+    size_t buffer_size = 20*4096; // minimum 81920 (empiric most efficient on arm64, raspi4 single thread)
+
+    for(int i=1; i<argc; i++) {
+        if( 0 == strcmp("-v", argv[i]) ) {
+            verbose=true;
+        } else if( 0 == strcmp("-threads", argv[i]) && i+1<argc ) {
+#ifdef ENABLE_MT
+            const int v = atoi(argv[++i]);
+            thread_count = std::max(0, v);
+#else // ENABLE_MT
+            print_usage(argv[0]);
+            return -1;
+#endif // ENABLE_MT
+        } else if( 0 == strcmp("-bsz", argv[i]) && i+1<argc ) {
+            const size_t v = atol(argv[++i]);
+            buffer_size = std::max(buffer_size, v);
+        } else if( 0 != strncmp("-", argv[i], 1) ) {
+            break; // end of options
+        }
+        argi = i;
+    }
+
+    if( 1+argi >= argc ) {
+        print_usage(argv[0]);
+        return -1;
+    }
+    std::string input_file(argv[++argi]);
+    std::ifstream in(input_file, std::ios::binary);
+    if( !in ) {
+        fprintf(stderr, "Error reading file %s\n", input_file.c_str());
+        return -1;
+    }
+    std::vector<uint8_t> buffer;
+
+    buffer.reserve(buffer_size);
+
+    double startTime;
+    if( verbose ) {
+        startTime = seconds();
+        fprintf(stderr, "  Using: buffer_size %zu, threads %d, ", buffer_size, thread_count);
+        #ifdef CRC32_USE_LOOKUP_TABLE_SLICING_BY_16
+            fprintf(stderr, "LOOKUP_TABLE_SLICING_BY_16\n");
+        #elif defined(CRC32_USE_LOOKUP_TABLE_SLICING_BY_8)
+            fprintf(stderr, "LOOKUP_TABLE_SLICING_BY_8\n");
+        #elif defined(CRC32_USE_LOOKUP_TABLE_SLICING_BY_4)
+            fprintf(stderr, "LOOKUP_TABLE_SLICING_BY_4\n");
+        #elif defined(CRC32_USE_LOOKUP_TABLE_BYTE)
+            fprintf(stderr, "LOOKUP_TABLE_BYTE\n");
+        #else
+            fprintf(stderr, "HALFBYTE\n");
+        #endif
+    } else {
+        startTime = 0;
+    }
+
+    uint32_t res = 0;
+    uint64_t total = 0;
+    bool has_more = in.good();
+    uint64_t loops = 0;
+    while( has_more ) {
+        buffer.resize(buffer.capacity());
+
+        in.read(reinterpret_cast<char*>(buffer.data()), buffer.capacity());
+        const size_t got = static_cast<size_t>(in.gcount());
+        buffer.resize(got);
+        total += got;
+        has_more = in.good();
+#ifdef ENABLE_MT
+        if( 1 >= thread_count ) {
+            res = crc32_fast(reinterpret_cast<char*>(buffer.data()), got, res);
+        } else {
+            res = run(crc32_fast, reinterpret_cast<char*>(buffer.data()), got, thread_count);
+        }
+#else // ENABLE_MT
+        res = crc32_fast(reinterpret_cast<char*>(buffer.data()), got, res);
+#endif // ENABLE_MT
+        loops++;
+    }
+    if ( verbose ) {
+        double duration  = seconds() - startTime;
+        fprintf(stderr, "  Duration %.3fs, %.3f MiB/s, %" PRIu64 " KiB/loop, %" PRIu64 " loops\n", duration, (total / (1024*1024)) / duration, (total / 1024) / loops, loops);
+    }
+    printf("%08x\n", res);
+    return 0;
+}
diff --git a/Makefile b/Makefile
index e72d1ea..94b4285 100644
--- a/Makefile
+++ b/Makefile
@@ -3,24 +3,35 @@ CPP       = g++
 
 # files
 PROGRAM   = Crc32Test
-LIBS      = -lrt
+LIBS      =
+LIBS_mt   = -lpthread
 HEADERS   = Crc32.h
 OBJECTS   = Crc32.o Crc32Test.o
 
+OBJECTS2  = Crc32.o Crc32TestMultithreaded.o
+
+OBJECTS3  = Crc32.o Crc32cl.o
+
 # flags
 FLAGS     = -O3 -Wall -pedantic -s
 
-default: $(PROGRAM)
+default: $(PROGRAM) Crc32TestMultithreaded crc32
 all: default
 
 $(PROGRAM): $(OBJECTS) Makefile
 	$(CPP) $(OBJECTS) $(FLAGS) $(LIBS) -o $(PROGRAM)
 
+Crc32TestMultithreaded: $(OBJECTS2) Makefile
+	$(CPP) $(OBJECTS2) $(FLAGS) $(LIBS_mt) -o Crc32TestMultithreaded
+
+crc32: $(OBJECTS3) Makefile
+	$(CPP) $(OBJECTS3) $(FLAGS) $(LIBS) -o crc32
+
 %.o: %.cpp $(HEADERS) Makefile
 	$(CPP) $(FLAGS) -c $< -o $@
 
 clean:
-	-rm -f $(OBJECTS) $(PROGRAM)
+	-rm -f $(OBJECTS) $(OBJECTS2) $(OBJECTS3) $(PROGRAM) Crc32TestMultithreaded crc32
 
 run: $(PROGRAM)
 	./$(PROGRAM)