diff --git a/src/cpu.c b/src/cpu.c
index a78c94c..79d52c6 100644
--- a/src/cpu.c
+++ b/src/cpu.c
@@ -32,7 +32,7 @@
 #include "instr.h"
 
 // Registers r0, r15
-int16_t reg[16] = { 0 };
+int16_t reg[CPU_NUM_REGS] = { 0 };
 
 /* Program */
 unsigned char *instr_mem = NULL;
diff --git a/src/cpu.h b/src/cpu.h
index c9eef2e..a9b1473 100644
--- a/src/cpu.h
+++ b/src/cpu.h
@@ -23,6 +23,8 @@
 #include <stdint.h>
 #include "instr.h"
 
+#define CPU_NUM_REGS 16
+
 int cpu_instr_load(void *ptr, unsigned len);
 
 void cpu_instr_unload();