.syntax unified

.set IO_BANK0_BASE, 0x40014000
.set SIO_BASE, 0xd0000000

.set GPIO16_CTRL, IO_BANK0_BASE + 0x84
.set GPIO_OE_SET, SIO_BASE + 0x24 
.set GPIO_OUT_XOR, SIO_BASE + 0x1c

.set PPB_BASE, 0xe0000000

.set SYST_CSR, PPB_BASE + 0xe010
.set SYST_RVR, PPB_BASE + 0xe014
.set SYST_CVR, PPB_BASE + 0xe018

.global main
main:
  // Enable SysTick and its interrupt
  ldr r0, =SYST_CSR
  movs r1, 0b11
  str r1, [r0]

  // Reload SysTick to 2^26
  ldr r0, =SYST_RVR
  movs r1, 1
  lsls r1, 26
  str r1, [r0]

  // Set GPIO to function 5 - SIO
  ldr r0, =GPIO16_CTRL
  movs r1, 5
  str r1, [r0]

  // Pin mask in r1
  movs r1, 1
  lsls r1, 16

  // Set GPIO to OUTPUT
  ldr r0, =GPIO_OE_SET
  str r1, [r0]

  ldr r0, =GPIO_OUT_XOR
  ldr r2, =SYST_CVR
  
blink:
  // gpio0_state ^= 1
  str r1, [r0]

  // Reset SysTick (written value ignored)
  str r0, [r2]
 
  wfi
  b blink