wait.S

.section .text

.equ MTIME_REG, 0x200bff8 # memory address of MTIME register
.equ RTC_FREQ, 33

.global wait_ms
#
# Arguments:
#   - a0: number of milliseconds to busy wait
#
wait_ms:
  li s0, MTIME_REG # li: load the constant MTIME_REG into s0 to
  lw s1, 0(s0)     # lw: load the value at offset 0 of MTIME_REG
                   # get the number of cycles counted by the RTC
              
  li s2, RTC_FREQ  # Load the constant RTC_FREQ into S2
  mul s2, s2, a0   # Multiply milliseconds with the frequency
                   # to get cycles to wait
                       
  add s2, s1, s2   # Add cycles to wait to cycles counted
cmp:
  lw s1, 0(s0)     # Load current number of cycles into s1
  blt s1, s2, cmp  # If current number of cycles are lower
                   # than target, keep looping
  ret