Assembly code for flashing the Stellaris Launchpad LED

blinky.s

.text

.word 0x2000000 // Initial stack pointer
.word start + 1 // Address of the reset vector '+ 1' denotes THUMB instructions vs ARM instructions.

start:

  mov r0, #0x00 // Loads 0x00 into register r0
  mov r1, #0xFF // Loads 0xFF into register r1

  //RCGCGPIO -> Turns the clock on for each of the GPIO ports
  ldr r2, =0x400FE608 // Load the register address into r2
  str r1, [r2]        // Copy 0xFF into the address contained in r2

  //GPIODEN -> Enables all digital input/output on the ARM
  ldr r2, =0x4002551C
  str r1, [r2]

  //GPIODIR -> Sets the data direction register to all Port F output
  ldr r2, =0x40025400
  str r1, [r2]

  //GPIOAFSEL -> Makes sure that digital is being used as opposed to other peripheral
  ldr r2, =0x40025420
  str r0, [r2]

  // Default all pins have 2ma source, but that can be configured (4, or 8ma).
  // Pullup not needed because of output pin (always being actively driven, doesn't need to be tied to something).

  //GPIODATA
  // Bits [9:2] in this address are actually working as a bitmask to see
  // what actually gets updated. The mask (0xFF << 2) + 0x4002.5000 == 0x400253FC
  // which is why the address is 0x400253FC not 0x40025000. I don't get
  // why the implementation is like that, but whatever.
  ldr r2, =0x400253FC

loop: 

  str r1, [r2] // Set the pins on Port F to high
  str r0, [r2] // Set the pins on Port F to low

  b loop // GOTO loop