; A beat a program sets for itself.
;
; The only regular thing this machine had was the screen finishing a frame, sixty times a
; second and not negotiable. That is a clock a program BORROWS: every duration becomes a
; multiple of 16.67 ms, so a sixteenth note at 120 beats a minute - 125,000 cycles, which is
; seven and a half frames - cannot be asked for at all.
;
; Counted in cycles, because that is what everything else on this machine is counted in.
;
; Written by Anachronaut

#Program

start:
  ; ---- Repeating, and polled ----
  ;
  ; 100,000 cycles, a tenth of a second. Four of them, so the whole thing is 400,000 and the
  ; recorded cycle count is the check that the period is what it says.
  INIA 0x01
  OUTA 0x52
  INIA 0x86
  OUTA 0x53
  INIA 0xA0
  OUTA 0x54                 ; 0x0186A0

  INIA 0x03
  OUTA 0x51                 ; Run, repeat, no interrupt.

  INIB 0d4
everyTick:
  INA 0x50
  PSHB
  INIB 0x01
  AND
  POPB
  BRQ everyTick             ; Not yet.
  INIA 0d46
  OUTA 0x00
  DECB
  BNB everyTick

  ; ---- And looking is what answered it ----
  ;
  ; The bit came down when it was read, so asking again immediately says nothing has happened
  ; since. A timer whose flag stayed up would look like a beat every time round the loop.
  INA 0x50
  INIB 0x01
  AND
  BRQ tickCleared
  INIA 0d78                 ; 'N'
  OUTA 0x00
  BRI oneShot
tickCleared:
  INIA 0d89                 ; 'Y'
  OUTA 0x00

oneShot:
  ; ---- Once, and then stopped ----
  ;
  ; Without the repeat bit it runs its period out and turns itself off, which the status port
  ; says: bit 1 is whether it is running.
  RSTA
  OUTA 0x51                 ; Stop first, so starting below is a start.
  INIA 0x01
  OUTA 0x51                 ; Run, no repeat.

waitOnce:
  INA 0x50
  INIB 0x01
  AND
  BRQ waitOnce
  INIA 0d46
  OUTA 0x00

  INA 0x50
  INIB 0x02                 ; RUNNING
  AND
  BRQ stopped
  INIA 0d78
  OUTA 0x00
  BRI done
stopped:
  INIA 0d89
  OUTA 0x00

done:
  INIA 0d10
  OUTA 0x00
  HALT

#Vectors

  Boot start