Lab 9 Ex5 SysTick solution

.type SysTick_Handler, %function
@ --parameters--
@ none
SysTick_Handler:
  push {lr}

  bl red_led_toggle

  @ Read state of red LED
  ldr r0, =ADR_GPIOB
  ldr r1, =OFS_GPIOx_ODR
  ldr r2, =2
  bl read_bit
  beq SysTick_Handler_disable_EXIT0

SysTick_Handler_enable_EXIT0:
  @ First clear both EXTI and NVIC interrupt pending.
  @ EXTI_PR_clear_pending 0
  ldr r0, =(ADR_EXTI + 0x14)
  ldr r1, =(1 << 0)
  str r1, [r0]
  @ NVIC_set ICPR 6
  ldr r0, =ADR_NVIC
  ldr r1, =OFS_NVIC_ICPR0
  ldr r2, =(1 << 6)
  str r2, [r0, r1]

  @ Then enable EXTI0 interrupt
  @ NVIC_set ISER 6
  ldr r0, =ADR_NVIC
  ldr r1, =OFS_NVIC_ISER0
  ldr r2, =(1 << 6)
  str r2, [r0, r1]

  b SysTick_Handler_end

SysTick_Handler_disable_EXIT0:
  @ Simply disables the interrupt
  @ NVIC_set ICER 6
  ldr r0, =ADR_NVIC
  ldr r1, =OFS_NVIC_ICER0
  ldr r2, =(1 << 6)
  str r2, [r0, r1]

SysTick_Handler_end:
  pop {lr}
  bx lr
.size SysTick_Handler, .-SysTick_Handler