diff --git a/Makefile b/Makefile
index 60bd00b..ac7e7ab 100644
--- a/Makefile
+++ b/Makefile
@@ -4,5 +4,5 @@ CFLAGS = -Wall -Werror -pedantic -O3
 .PHONY: all
 all: um
 
-um: um.c
-	$(CC) $< -o $@ $(CFLAGS)
+um: src/um.c
+	$(CC) $^ -o $@ $(CFLAGS)
diff --git a/README.org b/README.org
index a96c5e9..400881a 100644
--- a/README.org
+++ b/README.org
@@ -6,8 +6,8 @@ Solutions to the [[http://www.boundvariable.org/task.shtml][2006 ICFP Programmin
 
 ** Universal Machine
 
-The Universal Machine is implemented in [[um.c]]. Compile it with =make=
-and run it on a UM program:
+The Universal Machine is implemented in [[src/um.c]]. Compile it with
+=make= and run it on a UM program:
 
 #+begin_src sh
   ./um sandmark.umz # benchmark
diff --git a/um.c b/src/um.c
similarity index 84%
rename from um.c
rename to src/um.c
index 17a76bf..f43f675 100644
--- a/um.c
+++ b/src/um.c
@@ -1,3 +1,4 @@
+#include "um.h"
 #include "uthash.h"
 #include <stdint.h>
 #include <stdio.h>
@@ -5,13 +6,6 @@
 #include <string.h>
 #include <sys/stat.h>
 
-struct array {
-  uint32_t id;
-  uint32_t *arr;
-  uint32_t size;
-  UT_hash_handle hh;
-};
-
 void allocate_array(struct array **arrays, uint32_t next_id, uint32_t size) {
   struct array *a;
   a = malloc(sizeof(struct array));
@@ -35,22 +29,17 @@ void free_array(struct array **arrays, uint32_t id) {
   free(a);
 }
 
-int main(int argc, char **argv) {
-  if (argc < 2) {
-    fprintf(stderr, "Usage: %s <program>\n", argv[0]);
-    return EXIT_SUCCESS;
-  }
-
+uint32_t *read_program(char *filename) {
   struct stat file_stat;
-  if (stat(argv[1], &file_stat) == -1) {
-    fprintf(stderr, "Could not stat file %s\n", argv[1]);
-    return EXIT_FAILURE;
+  if (stat(filename, &file_stat) == -1) {
+    fprintf(stderr, "Could not stat file %s\n", filename);
+    exit(EXIT_FAILURE);
   }
 
-  FILE *fp = fopen(argv[1], "rb");
+  FILE *fp = fopen(filename, "rb");
   if (fp == NULL) {
-    fprintf(stderr, "Could not open file %s\n", argv[1]);
-    return EXIT_FAILURE;
+    fprintf(stderr, "Could not open file %s\n", filename);
+    exit(EXIT_FAILURE);
   }
 
   uint32_t *program = malloc(file_stat.st_size);
@@ -69,6 +58,10 @@ int main(int argc, char **argv) {
   }
   fclose(fp);
 
+  return program;
+}
+
+void run(uint32_t *program) {
   uint32_t reg[8] = {0};       // registers
   struct array *arrays = NULL; // arrays of platters
   uint32_t next_id = 1;        // next free index available for a new array
@@ -79,8 +72,6 @@ int main(int argc, char **argv) {
     uint32_t c = program[pc] & 7;
     uint32_t b = (program[pc] >> 3) & 7;
     uint32_t a = (program[pc] >> 6) & 7;
-    /* printf("\n%zu: %u (op=%u, a=%u, b=%u, c=%u)\n", pc, program[pc], opcode, a, b, c); */
-    /* printf("reg[a] = %u, reg[b] = %u, reg[c] = %u\n", reg[a], reg[b], reg[c]); */
     pc++;
     switch (opcode) {
     case 0:
@@ -111,7 +102,7 @@ int main(int argc, char **argv) {
       reg[a] = (~reg[b]) | (~reg[c]);
       break;
     case 7:
-      return EXIT_SUCCESS;
+      return;
     case 8:
       allocate_array(&arrays, next_id, reg[c]);
       reg[b] = next_id;
@@ -144,6 +135,16 @@ int main(int argc, char **argv) {
   }
   free(program);
   free(arrays);
+}
+
+int main(int argc, char **argv) {
+  if (argc < 2) {
+    fprintf(stderr, "Usage: %s <program>\n", argv[0]);
+    return EXIT_SUCCESS;
+  }
+
+  uint32_t *program = read_program(argv[1]);
+  run(program);
 
   return EXIT_SUCCESS;
 }
diff --git a/src/um.h b/src/um.h
new file mode 100644
index 0000000..04ace6a
--- /dev/null
+++ b/src/um.h
@@ -0,0 +1,15 @@
+#include "uthash.h"
+#include <stdint.h>
+
+struct array {
+  uint32_t id;
+  uint32_t *arr;
+  uint32_t size;
+  UT_hash_handle hh;
+};
+
+void allocate_array(struct array **arrays, uint32_t next_id, uint32_t size);
+struct array *get_array(struct array *arrays, uint32_t id);
+void free_array(struct array **arrays, uint32_t id);
+uint32_t *read_program(char *filename);
+void run(uint32_t *program);
diff --git a/uthash.h b/src/uthash.h
similarity index 100%
rename from uthash.h
rename to src/uthash.h