PRU assembler example taken from https://jumpnowtek.com/beaglebone/Working-with-the-BeagleBone-PRUs.html

loop.p

; Run a simple delay loop on the PRU
; Original source https://jumpnowtek.com/beaglebone/Working-with-the-BeagleBone-PRUs.html

.setcallreg r29.w0
.origin 0
.entrypoint start

#define PRU0_ARM_INTERRUPT 19

#define DELAY_COUNT 50000000
#define LOOP_ITERATIONS 20

start:
  mov r1, LOOP_ITERATIONS

main:
  call delay
  sub r1, r1, 1
  qbne main, r1, 0
  mov r31.b0, PRU0_ARM_INTERRUPT + 16 ; notify ARM we are done
  halt

delay:
 mov r0, DELAY_COUNT

delay_loop:
  sub r0, r0, 1
  qbne delay_loop, r0, 0
  ret