diff --git a/Makefile.mak b/Makefile.mak
index ce7577582..bacd235da 100644
--- a/Makefile.mak
+++ b/Makefile.mak
@@ -184,7 +184,7 @@ ifeq ($(OPTION_WERROR),enable)
 endif
 
 ifeq ($(OPTION_MCU),disable)
- DEFINES_JERRY += -D__HOST
+ DEFINES_JERRY += -D__HOST -DJERRY_SOURCE_BUFFER_SIZE=1048576
 else
  CFLAGS_COMMON += -ffunction-sections -fdata-sections -nostdlib
  DEFINES_JERRY += -D__TARGET_MCU
diff --git a/src/libjsparser/lexer.c b/src/libjsparser/lexer.c
index ca912f8c5..672601f77 100644
--- a/src/libjsparser/lexer.c
+++ b/src/libjsparser/lexer.c
@@ -104,79 +104,15 @@ is_empty (token tok)
   return tok.type == TOK_EMPTY;
 }
 
-#ifdef __HOST
-FILE *lexer_debug_log;
-#endif
+/* Represents the contents of a script.  */
+static const char *buffer_start = NULL;
+static const char *buffer = NULL;
+static const char *token_start;
+
+#define LA(I)       (*(buffer + I))
 
 #ifdef __HOST
-static FILE *file;
-static char *buffer_start;
-
-/* Represents the contents of a file.  */
-static char *buffer = NULL;
-static char *token_start;
-
-#define BUFFER_SIZE 1024
-
-static char
-get_char (size_t i)
-{
-  size_t error;
-  JERRY_ASSERT (buffer >= buffer_start);
-  const size_t tail_size = BUFFER_SIZE - (size_t) (buffer - buffer_start);
-
-  JERRY_ASSERT (file);
-
-  if (buffer == NULL)
-    {
-      buffer = (char *) mem_heap_alloc_block (BUFFER_SIZE, MEM_HEAP_ALLOC_SHORT_TERM);
-      error = __fread (buffer, 1, BUFFER_SIZE, file);
-      if (error == 0)
-        return '\0';
-      if (error < BUFFER_SIZE)
-        __memset (buffer + error, '\0', BUFFER_SIZE - error);
-      buffer_start = buffer;
-    }
-
-  if (tail_size <= i)
-    {
-    /* We are almost at the end of the buffer.  */
-      if (token_start)
-        {
-          JERRY_ASSERT (buffer >= token_start);
-          const size_t token_size = (size_t) (buffer - token_start);
-          /* Whole buffer contains single token.  */
-          if (token_start == buffer_start)
-            parser_fatal (ERR_BUFFER_SIZE);
-          /* Move parsed token and tail of buffer to head.  */
-          __memmove (buffer_start, token_start, tail_size + token_size);
-          /* Adjust pointers.  */
-          token_start = buffer_start;
-          buffer = buffer_start + token_size;
-          /* Read more characters form input file.  */
-          error = __fread (buffer + tail_size, 1, BUFFER_SIZE - tail_size - token_size, file);
-          if (error == 0)
-            return '\0';
-          if (error < BUFFER_SIZE - tail_size - token_size)
-            __memset (buffer + tail_size + error, '\0',
-                    BUFFER_SIZE - tail_size - token_size - error);
-        }
-      else
-        {
-          __memmove (buffer_start, buffer, tail_size);
-          buffer = buffer_start;
-          error = __fread (buffer + tail_size, 1, BUFFER_SIZE - tail_size, file);
-          if (error == 0)
-            return '\0';
-          if (error < BUFFER_SIZE - tail_size)
-            __memset (buffer + tail_size + error, '\0', BUFFER_SIZE - tail_size - error);
-        }
-    }
-
-  return *(buffer + i);
-}
-
-#define LA(I) 			(get_char (I))
+_FILE *lexer_debug_log;
 
 static void
 dump_current_line (void)
@@ -187,16 +123,7 @@ dump_current_line (void)
     __putchar (*i);
   __putchar ('\n');
 }
-
-#else
-
-/* Represents the contents of a file.  */
-static const char *buffer = NULL;
-static const char *token_start;
-
-#define LA(I)       (*(buffer + I))
-
-#endif // __HOST
+#endif
 
 /* If TOKEN represents a keyword, return decoded keyword,
    if TOKEN represents a Future Reserved Word, return KW_RESERVED,
@@ -708,25 +635,20 @@ grobble_whitespaces (void)
     }
 }
 
-#ifdef __HOST
-void
-lexer_set_file (FILE *ex_file)
-{
-  JERRY_ASSERT (ex_file);
-  file = ex_file;
-  lexer_debug_log = __fopen ("lexer.log", "w");
-  saved_token = empty_token;
-  buffer = buffer_start = token_start = NULL;
-}
-#else
-void
+static void
 lexer_set_source (const char * source)
 {
-  buffer = source;
-  saved_token = empty_token;
+  buffer_start = source;
+  buffer = buffer_start;
 }
 
-#endif
+static void
+lexer_rewind( void)
+{
+  JERRY_ASSERT( buffer_start != NULL );
+
+  buffer = buffer_start;
+}
 
 static bool
 replace_comment_by_newline (void)
@@ -765,13 +687,8 @@ replace_comment_by_newline (void)
     }
 }
 
-#ifdef __HOST
 static token
 lexer_next_token_private (void)
-#else
-token
-lexer_next_token (void)
-#endif
 {
   char c = LA (0);
 
@@ -805,12 +722,7 @@ lexer_next_token (void)
   if (__isspace (c))
     {
       grobble_whitespaces ();
-      return 
-#ifdef __HOST
-        lexer_next_token_private ();
-#else
-        lexer_next_token ();
-#endif
+      return lexer_next_token_private ();
     }
 
   if (c == '/' && LA (1) == '*')
@@ -818,23 +730,13 @@ lexer_next_token (void)
       if (replace_comment_by_newline ())
         return (token) { .type = TOK_NEWLINE, .data.uid = 0 };
       else
-        return 
-#ifdef __HOST
-          lexer_next_token_private ();
-#else
-          lexer_next_token ();
-#endif
+        return lexer_next_token_private ();
     }
 
   if (c == '/' && LA (1) == '/')
     {
       replace_comment_by_newline ();;
-      return 
-#ifdef __HOST
-        lexer_next_token_private ();
-#else
-        lexer_next_token ();
-#endif
+      return lexer_next_token_private ();
     }
 
   switch (c)
@@ -904,16 +806,19 @@ lexer_next_token (void)
 
 #ifdef __HOST
 static int i = 0;
+#endif /* __HOST */
 
 token
 lexer_next_token (void)
 {
-  LA (0); // Init buffers
-  
+#ifdef __HOST
   if (buffer == buffer_start)
     dump_current_line ();
+#endif /* __HOST */
 
   token tok = lexer_next_token_private ();
+
+#ifdef __HOST
   if (tok.type == TOK_NEWLINE)
     {
       dump_current_line ();
@@ -925,9 +830,9 @@ lexer_next_token (void)
         __fprintf (lexer_debug_log, "lexer_next_token(%d): type=%d, data=%d\n", i, tok.type, tok.data.uid);
         i++;
     }
+#endif /* __HOST */
   return tok;
 }
-#endif
 
 void
 lexer_save_token (token tok)
@@ -935,7 +840,7 @@ lexer_save_token (token tok)
 #ifdef __HOST
   if (tok.type == TOK_CLOSE_BRACE)
     __fprintf (lexer_debug_log, "lexer_save_token(%d): type=%d, data=%d\n", i, tok.type, tok.data.uid);
-#endif
+#endif /* __HOST */
   saved_token = tok;
 }
 
@@ -944,3 +849,25 @@ lexer_dump_buffer_state (void)
 {
   __printf ("%s\n", buffer);
 }
+
+void
+lexer_init( const char *source)
+{
+  saved_token = empty_token;
+  lexer_set_source( source);
+
+#ifdef __HOST
+  lexer_debug_log = __fopen ("lexer.log", "w");
+#endif /* __HOST */
+}
+
+void
+lexer_run_first_pass( void)
+{
+  token tok = lexer_next_token ();
+  while (tok.type != TOK_EOF)
+    tok = lexer_next_token (); 
+
+  lexer_rewind();
+}
+
diff --git a/src/libjsparser/lexer.h b/src/libjsparser/lexer.h
index bbc10b9c7..3d9988299 100644
--- a/src/libjsparser/lexer.h
+++ b/src/libjsparser/lexer.h
@@ -147,11 +147,8 @@ typedef struct
 } __packed
 token;
 
-#ifdef __HOST
-void lexer_set_file (FILE *);
-#else
-void lexer_set_source (const char *);
-#endif
+void lexer_init(const char *);
+void lexer_run_first_pass( void);
 token lexer_next_token (void);
 void lexer_save_token (token);
 void lexer_dump_buffer_state (void);
diff --git a/src/libjsparser/parser.c b/src/libjsparser/parser.c
index 78f039ff3..9549b0fdc 100644
--- a/src/libjsparser/parser.c
+++ b/src/libjsparser/parser.c
@@ -27,7 +27,7 @@ static uint8_t current_opcode_in_buffer = 0;
 static uint8_t opcode_counter = 0;
 
 #ifdef __HOST
-FILE *debug_file;
+_FILE *debug_file;
 #endif
 
 static T_IDX parse_expression (void);
diff --git a/src/libruntime/jerry-libc.h b/src/libruntime/jerry-libc.h
index 9a3013781..ee812c0a8 100644
--- a/src/libruntime/jerry-libc.h
+++ b/src/libruntime/jerry-libc.h
@@ -21,7 +21,7 @@
 
 #include "globals.h"
 
-typedef void FILE;
+typedef void _FILE;
 
 extern void* __memset (void *s, int c, size_t n);
 extern int __memcmp (const void *s1, const void *s2, size_t n);
@@ -44,12 +44,26 @@ extern int __isalpha (int);
 extern int __isdigit (int);
 extern int __isxdigit (int);
 
-extern FILE* __fopen(const char *, const char *);
-extern int __fclose(FILE *);
-extern void __rewind(FILE *);
-extern size_t __fread(void *, size_t, size_t, FILE *);
-extern size_t __fwrite(const void *, size_t, size_t, FILE *);
-extern int __fprintf(FILE *, const char *, ...);
+/**
+ * 'whence' argument of __fseek that identifies position
+ * the 'offset' argument is added to.
+ */
+typedef enum
+{
+  __SEEK_SET, /**< relative to begin of file */
+  __SEEK_CUR, /**< relative to current position */
+  __SEEK_END /**< relative to end of file */
+} _whence_t;
+
+extern _FILE* __fopen(const char *, const char *);
+extern int __fclose(_FILE *);
+extern int __fseek(_FILE *, long offset, _whence_t);
+extern long __ftell(_FILE *);
+extern void __rewind(_FILE *);
+extern size_t __fread(void *, size_t, size_t, _FILE *);
+extern size_t __fwrite(const void *, size_t, size_t, _FILE *);
+extern int __ferror(_FILE *);
+extern int __fprintf(_FILE *, const char *, ...);
 
 #define DBL_MANT_DIG    (  52)
 #define DBL_DIG         (  10)
diff --git a/src/libruntime/serializer.h b/src/libruntime/serializer.h
index 4802c139d..1bcb4fcfc 100644
--- a/src/libruntime/serializer.h
+++ b/src/libruntime/serializer.h
@@ -28,4 +28,4 @@ void serializer_dump_opcode (const void *);
 
 void serializer_rewrite_opcode (const int8_t, const void *);
 
-#endif // SERIALIZER_H
\ No newline at end of file
+#endif // SERIALIZER_H
diff --git a/src/libruntime/target/linux/jerry-libc.c b/src/libruntime/target/linux/jerry-libc.c
index 699bcda72..abfb38682 100644
--- a/src/libruntime/target/linux/jerry-libc.c
+++ b/src/libruntime/target/linux/jerry-libc.c
@@ -21,16 +21,8 @@
 #include "jerry-libc.h"
 
 #include <stdarg.h>
-
-
-extern void __noreturn exit(int status);
-extern FILE* fopen(const char *path, const char *mode);
-extern int fclose(FILE *fp);
-extern int rewind (FILE *);
-extern size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
-extern size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
-extern int vprintf (__const char *__restrict __format, __builtin_va_list __arg);
-extern int vfprintf (FILE *stream, __const char *__restrict __format, __builtin_va_list __arg);
+#include <stdio.h>
+#include <stdlib.h>
 
 /**
  * printf
@@ -69,10 +61,10 @@ __exit (int status)
 /**
  * fopen
  *
- * @return FILE pointer - upon successful completion,
+ * @return _FILE pointer - upon successful completion,
  *         NULL - otherwise
  */
-FILE*
+_FILE*
 __fopen(const char *path, /**< file path */
         const char *mode) /**< file open mode */
 {
@@ -82,7 +74,7 @@ __fopen(const char *path, /**< file path */
 /** The rewind() function sets the file position 
   indicator for the stream pointed to by STREAM to the beginning of the file.  */
 void
-__rewind (FILE *stream)
+__rewind (_FILE *stream)
 {
   rewind (stream);
 }
@@ -94,11 +86,46 @@ __rewind (FILE *stream)
  *         non-zero value - otherwise.
  */
 int
-__fclose(FILE *fp) /**< stream pointer */
+__fclose(_FILE *fp) /**< stream pointer */
 {
   return fclose( fp);
 } /* __fclose */
 
+/**
+ * fseek
+ */
+int
+__fseek(_FILE * fp, /**< stream pointer */
+        long offset, /**< offset */
+        _whence_t whence) /**< specifies position type
+                               to add offset to */
+{
+  int whence_real;
+  switch ( whence )
+  {
+    case __SEEK_SET:
+      whence_real = SEEK_SET;
+      break;
+    case __SEEK_CUR:
+      whence_real = SEEK_CUR;
+      break;
+    case __SEEK_END:
+      whence_real = SEEK_END;
+      break;
+  }
+
+  return fseek( fp, offset, whence_real);
+} /* __fseek */
+
+/**
+ * ftell
+ */
+long
+__ftell(_FILE * fp) /**< stream pointer */
+{
+  return ftell( fp);
+} /* __ftell */
+
 /**
  * fread
  *
@@ -108,7 +135,7 @@ size_t
 __fread(void *ptr, /**< address of buffer to read to */
         size_t size, /**< size of elements to read */
         size_t nmemb, /**< number of elements to read */
-        FILE *stream) /**< stream pointer */
+        _FILE *stream) /**< stream pointer */
 {
   return fread(ptr, size, nmemb, stream);
 } /* __fread */
@@ -122,18 +149,27 @@ size_t
 __fwrite(const void *ptr, /**< data to write */
          size_t size, /**< size of elements to write */
          size_t nmemb, /**< number of elements */
-         FILE *stream) /**< stream pointer */
+         _FILE *stream) /**< stream pointer */
 {
   return fwrite(ptr, size, nmemb, stream);
 } /* __fwrite */
 
+/**
+ * ferror
+ */
+int
+__ferror(_FILE * fp) /**< stream pointer */
+{
+  return ferror( fp);
+} /* __ferror */
+
 /**
  * fprintf
  * 
  * @return number of characters printed
  */
 int
-__fprintf(FILE *stream, /**< stream pointer */
+__fprintf(_FILE *stream, /**< stream pointer */
           const char *format, /**< format string */
          ...)                /**< parameters' values */
 {
diff --git a/src/libruntime/target/linux/serializer.c b/src/libruntime/target/linux/serializer.c
index 65ffbd23f..f3b440166 100644
--- a/src/libruntime/target/linux/serializer.c
+++ b/src/libruntime/target/linux/serializer.c
@@ -17,7 +17,7 @@
 #include "jerry-libc.h"
 #include "opcodes.h"
 
-FILE *dump;
+_FILE *dump;
 
 #define OPCODE_STR(op) \
   #op,
diff --git a/src/libruntime/target/stm32f4/serializer.c b/src/libruntime/target/stm32f4/serializer.c
index 3e7072876..816979ef6 100644
--- a/src/libruntime/target/stm32f4/serializer.c
+++ b/src/libruntime/target/stm32f4/serializer.c
@@ -19,27 +19,32 @@
 void
 serializer_init (void)
 {
+  JERRY_UNIMPLEMENTED();
 }
 
 uint8_t
-serializer_dump_strings (const char *strings[] __unused, uint8_t size __unused)
+serializer_dump_strings (const char *strings[], uint8_t size)
 {
+  JERRY_UNIMPLEMENTED_REF_UNUSED_VARS( strings, size);
 }
 
 void 
-serializer_dump_nums (const int nums[] __unused, uint8_t size __unused, uint8_t offset __unused, uint8_t strings_num __unused)
+serializer_dump_nums (const int nums[], uint8_t size, uint8_t offset, uint8_t strings_num)
 {
+  JERRY_UNIMPLEMENTED_REF_UNUSED_VARS( nums, size, offset, strings_num);
 }
 
 void
-serializer_dump_opcode (const void *opcode __unused)
+serializer_dump_opcode (const void *opcode)
 {
+  JERRY_UNIMPLEMENTED_REF_UNUSED_VARS( opcode);
 }
 
 void
-serializer_rewrite_opcode (const int8_t offset __unused, const void *opcode __unused)
+serializer_rewrite_opcode (const int8_t offset, const void *opcode)
 {
+  JERRY_UNIMPLEMENTED_REF_UNUSED_VARS( offset, opcode);
 }
 
 
-TODO (Dump memory)
\ No newline at end of file
+TODO (Dump memory)
diff --git a/src/main.c b/src/main.c
index c1f240fe7..b7c848a4d 100644
--- a/src/main.c
+++ b/src/main.c
@@ -40,12 +40,120 @@
 #define MAX_STRINGS 100
 #define MAX_NUMS 25
 
-void fake_exit (void);
+static void
+jerry_run( const char *script_source,
+           size_t script_source_size __unused)
+{
+  const char *strings[MAX_STRINGS];
+  int nums[MAX_NUMS];
+  uint8_t strings_num, nums_count;
+  uint8_t offset;
+
+  mem_init();
+
+  TODO( Consider using script_source_size in lexer to check buffer boundaries );
+
+  lexer_init( script_source);
+
+  lexer_run_first_pass();
+
+  strings_num = lexer_get_strings (strings);
+  nums_count = lexer_get_nums (nums);
+  lexer_adjust_num_ids ();
+
+  offset = serializer_dump_strings (strings, strings_num);
+  serializer_dump_nums (nums, nums_count, offset, strings_num);
+
+  parser_init ();
+  parser_parse_program ();
+
+  //gen_bytecode (generated_source);
+  //gen_bytecode ();
+  //run_int ();
+} /* jerry_run */
+
+#ifdef __HOST
+static uint8_t source_buffer[ JERRY_SOURCE_BUFFER_SIZE ];
+
+static const char*
+read_source( const char *script_file_name,
+             size_t *out_source_size_p)
+{
+  _FILE *file = __fopen (script_file_name, "r");
+
+  if (file == NULL)
+    {
+      jerry_exit (ERR_IO);
+    }
+
+  int fseek_status = __fseek( file, 0, __SEEK_END);
+
+  if ( fseek_status != 0 )
+    {
+      jerry_exit (ERR_IO);
+    }
+
+  long script_len = __ftell( file);
+
+  if ( script_len < 0 )
+    {
+      jerry_exit (ERR_IO);
+    }
+
+  __rewind( file);
+
+  const size_t source_size = (size_t)script_len;
+  size_t bytes_read = 0;
+ 
+  while ( bytes_read < source_size )
+    {
+      bytes_read += __fread( source_buffer, 1, source_size, file);
+
+      if ( __ferror( file) != 0 )
+        {
+          jerry_exit (ERR_IO);
+        }
+    }
+
+  __fclose( file);
+
+  *out_source_size_p = source_size;
+  return (const char*)source_buffer;
+}
+
+int
+main (int argc __unused,
+      char **argv __unused)
+{
+  const char *file_name = NULL;
+
+  if (argc > 2)
+    {
+      jerry_exit (ERR_SEVERAL_FILES);
+    }
+  else if (argc == 2)
+    {
+      file_name = argv[1];
+    }
+  else
+    {
+      jerry_exit (ERR_NO_FILES);
+    }
+ 
+  size_t source_size;
+  const char *source_p = read_source( file_name, &source_size);
+
+  jerry_run( source_p,
+             source_size);
+
+  return 0;
+}
+#elif !defined(__HOST) && defined(__TARGET_MCU)
+void fake_exit(void);
 
 void
 fake_exit (void)
 {
-#ifdef __TARGET_MCU
   uint32_t pin = LED_RED;
   uint32_t mode = (uint32_t)GPIO_Mode_OUT << (pin * 2);
   uint32_t speed = (uint32_t)GPIO_Speed_100MHz << (pin * 2);
@@ -90,76 +198,21 @@ fake_exit (void)
     
     for (index = 0; index < dash * 7; index++);
   }
-#else
-  for (;;);
-#endif
 }
 
 int
-main (int argc __unused,
-      char **argv __unused)
+main(void)
 {
-#ifdef __HOST
-  const char *file_name = NULL;
-  FILE *file = NULL;
-#endif
-  const char *strings[MAX_STRINGS];
-  int nums[MAX_NUMS];
-  uint8_t strings_num, nums_count, offset;
+  const char *source_p = generated_source;
+  const size_t source_size = sizeof(generated_source);
 
-  mem_init ();
+  jerry_run( source_p,
+             source_size);
 
-#ifdef __HOST
-  if (argc > 0)
-    {
-      if (file_name == NULL)
-        file_name = argv[1];
-      else
-        jerry_exit (ERR_SEVERAL_FILES);
-    }
-#endif
+  fake_exit();
 
-#ifdef __HOST
-  if (file_name == NULL)
-    jerry_exit (ERR_NO_FILES);
-
-  file = __fopen (file_name, "r");
-
-  if (file == NULL)
-  {
-    jerry_exit (ERR_IO);
-  }
-
-  lexer_set_file (file);
-#else
-  lexer_set_source (generated_source);
-#endif
-
-  // First run parser to fill list of strings
-  token tok = lexer_next_token ();
-  while (tok.type != TOK_EOF)
-    tok = lexer_next_token ();
-
-  strings_num = lexer_get_strings (strings);
-  nums_count = lexer_get_nums (nums);
-  lexer_adjust_num_ids ();
-
-  // Reset lexer
-#ifdef __HOST
-  __rewind (file);
-  lexer_set_file (file);
-#else
-  lexer_set_source (generated_source);
-#endif
-
-  parser_init ();
-  offset = serializer_dump_strings (strings, strings_num);
-  serializer_dump_nums (nums, nums_count, offset, strings_num);
-  parser_parse_program ();
-
-#ifdef __TARGET_MCU
-  fake_exit ();
-#endif 
-
-  return 0;
+  JERRY_UNREACHABLE();
 }
+#else /* !__HOST && !__TARGET_MCU */
+# error "!__HOST && !__TARGET_MCU"
+#endif /* !__HOST && !__TARGET_MCU */