.text
    .data
    .include "bh_tm4c123gh6pm.h"
    .thumb
    .global main

main:
    bl GPIO_Init
    ldr R0, GPIO_PORTF_DATA_R
    ldr R1, GPIO_PORTE_DATA_R

flash:
    mov R5, #0
    mov R8, #0x200000
    mov R2, #0x3E
    str R2, [R0]
    str R2, [R1]
    bl delay

    mov R5, #0
    mov R8, #0x200000
    mov R3, #0x00
    str R3, [R1]
    str R3, [R0]
    bl delay

    B flash

delay:
    add R5, R5, #1
    cmp R5, R8
    bne delay
    bx lr

loop:
    nop
    B loop
.end

GPIO_Init:
        ; 1) activate clock
        ldr R1, SYSCTL_RCGCGPIO_R
        ldr R0, [R1]
        ldr R2, SYSCTL_RCGC2_GPIOF
        orr R0, R0, R2 ; clock
        str R0, [R1]
        ldr R2, SYSCTL_RCGC2_GPIOE
        orr R0, R0, R2 ; clock
        str R0, [R1]
        nop  ; allow time extra time
        nop
        ; 2) unlock
        ldr R0, GPIO_LOCK_KEY
        ldr R1, GPIO_PORTF_LOCK_R
        str R0, [R1]
        ldr R0, GPIO_PORTF_CR_R
        mov R1, #0x1F
        str R1, [R0]
        ldr R0, GPIO_LOCK_LOCKED
        ldr R1, GPIO_PORTF_LOCK_R
        str R0, [R1]
        ; 3) disable analog functionality
        ldr R1, GPIO_PORTF_AMSEL_R
        ldr R0, [R1]
        bic R0, R0, #0x1F
        str R0, [R1]
        ldr R1, GPIO_PORTE_AMSEL_R
        ldr R0, [R1]
        bic R0, R0, #0x3E
        str R0, [R1]
        ; 4) configure as GPIO
        ldr R1, GPIO_PORTF_PCTL_R
        ldr R0, [R1]
        mov R3, #0x000F
        lsl R2, R3, #0x10
        mov R3, #0xFFFF
        add R2, R2, R3
        bic R0, R0, R2
        str R0, [R1]
        ldr R1, GPIO_PORTE_PCTL_R
        ldr R0, [R1]
        mov R3, #0x000E
        lsl R2, R3, #0x30
        mov R3, #0xFFFF
        add R2, R2, R3
        bic R0, R0, R2
        str R0, [R1]
        ; 5) set direction register
        ldr R1, GPIO_PORTF_DIR_R
        ldr R0, [R1]
        orr R0, R0, #0x0E
        str R0, [R1]
        ldr R1, GPIO_PORTE_DIR_R
        ldr R0, [R1]
        orr R0, R0, #0x3E
        str R0, [R1]
        ; 6) port function
        ldr R1, GPIO_PORTF_AFSEL_R
        ldr R0, [R1]
        bic R0, R0, #0x1F
        str R0, [R1]
        ldr R1, GPIO_PORTE_AFSEL_R
        ldr R0, [R1]
        bic R0, R0, #0x3E
        str R0, [R1]
        ; pull-up resistors
        ldr R1, GPIO_PORTF_PUR_R
        ldr R0, [R1]
        orr R0, R0, #0x11
        str R0, [R1]
        ; enable 8mA drive
        ldr R1, GPIO_PORTF_DR8R_R
        ldr R0, [R1]
        orr R0, R0, #0x1F
        str R0, [R1]
        ; 7) enable digital port
        ldr R1, GPIO_PORTF_DEN_R
        ldr R0, [R1]
        orr R0, R0, #0x1F
        str R0, [R1]
        ldr R1, GPIO_PORTE_DEN_R
        ldr R0, [R1]
        orr R0, R0, #0x3E
        str R0, [R1]
        bx  LR