Emulated hardware is managed by writing to memory-mapped ports beginning at address $ff80.

const SIbaseAddress=0xff80, SIrefreshOff=0, SIrefreshOn=1, SIgraphicsMode=2, SIkeyInterface=3;

SIrefreshOff             Write <any> to force OFF register refresh and display animation
SIrefreshOn              Write <any> to re-enable register refresh and display animation
SIgraphicsMode           Write <number of colours> to select graphics mode (2, 4, 16 allowed values)
SIkeyInterface           Write 0 then read an ASCII code (if<128), write 255 to clear the keyboard buffer

This is a simple way of switching on or off the “Refresh” checkbox from within a running program. For example, it might be a good idea not to have animation during an intensive routine such as clearing the screen. Simply write any value to SIbaseAddress+SIrefreshOff to turn Refresh off, or to SIbaseAddress+SIrefreshOn to turn ot on again.

init:
  clr SIbaseAddress+SIrefreshOff
  jsr ClrScrr
  clr SIbaseAddress+SIrefreshOn