systick initialization

main.c

@@ -1,3 +1,12 @@
 #include<stdint.h>
 #include<stdbool.h>
 #include"tm4c123gh6pm.h"
+
+
+/* Systick memory mapped registers */
+#define STCTRL *((volatile long *) 0xE000E010)      // control and status
+#define STRELOAD *((volatile long *) 0xE000E014)    // reload value
+#define STCURRENT *((volatile long *) 0xE000E018)   // current value
+#define ENABLE      (1 << 0)    // bit 0 of CSR to enable the timer
+#define CLKINT      (1 << 2)    // bit 2 of CSR to specify CPU clock
+#define SYS_INTEN      (1 << 1)    // enable systick interrupt