NextBASIC High Resolution Timer

#program asmstpwtch
  10 RUN AT 3
  20 SAVE "asmstpwtch.bas"
  30 CLEAR $beff
  40 .asm
  50 ;  org $bf00
  60 ;ctc2   equ $1a3b
  70 ;regsel equ $243b
  80 ;  jp init
  90 ;  jp start
 100 ;  jp read
 110 ;  jp stop
 120 ;init ; initialize timer
 130 ;  di
 140 ;  nreg $c0,ivt & %11100000 | 1
 150 ;  nreg $c4,%00000001 ;ULA interrupt enabled
 160 ;  nreg $c5,%00000000 ;Timers disabled
 170 ;  nreg $c6,%00000000
 180 ;  ld bc,regsel  ;Register select port
 190 ;  ld a,$11      ;Video timing register
 200 ;  out (c),a     ;  select the register
 210 ;  inc b         ;Register read port
 220 ;  in a,(c)      ;Read register
 230 ;  and %00000111 ;Mask video mode
 240 ;  ld hl,ms1     ;1ms Prescalar table
 250 ;  add hl,a      ;Table offset for video mode
 260 ;  ld a,(hl)     ;Read prescalar
 270 ;  ld (prescl),a ;Save prescalar
 280 ;  ld a,$bf      ;Load IVT high byte
 290 ;  ld i,a        ;  into I
 300 ;  ld a, 1       ;Flag value
 310 ;  ld (initialized),a
 320 ;  ld (stopped),a
 330 ;  im 2          ;Set interrupt mode 2
 340 ;  ei            ;Enable interrupts
 350 ;  ret
 360 ;initialized db 0
 370 ;stopped     db 0
 380 ;;
 390 ;; Timer prescalars
 400 ;;
 410 ;ms1 db 109,112,115,117,121,125,129,105
 420 ;prescl db 0
 430 ;;
 440 ;; Start timer at 0
 450 ;;
 460 ;start
 470 ;  ld a,(initialized)
 480 ;  or a
 490 ;  call z,init   ;Initialize IM 2
 500 ;  di
 510 ;  ld a,(stopped)
 520 ;  or a
 530 ;  jr z, reset   ;Not stopped, just reset counter
 540 ;  xor a
 550 ;  ld (stopped),a;Flag not stopped
 560 ;  ld bc,regsel
 570 ;  ld a,$c5
 580 ;  out (c),a     ;Select timer interrupt register
 590 ;  inc b
 600 ;  in a,(c)      ;Read CTC0..7 interrupt state
 610 ;  or %00000100  ;  enable CTC2 interrupts
 620 ;  out (c),a     ;Write new interrupt state
 630 ;  ld bc,ctc2
 640 ;  ld a,1
 650 ;  out (c),a     ;Disable CTC2
 660 ;  ld a,%10100101;Timer config flags
 670 ;  out (c),a     ;Send config flags to CTC2
 680 ;  ld a,(prescl) ;Load prescalar
 690 ;  out (c),a     ;  into CTC2
 700 ;reset
 710 ;  ld hl,0
 720 ;  ld (ctr),hl   ;Zero counter
 730 ;  ei            ;Enable interrupts, start counter
 740 ;  ret
 750 ;;
 760 ;; Read current timer value
 770 ;;
 780 ;read
 790 ;  di
 800 ;  ld bc,(ctr)   ;Load counter into BC for BASIC
 810 ;  ei
 820 ;  ret
 830 ;;
 840 ;; Stop timer
 850 ;;
 860 ;stop
 870 ;  di
 880 ;  ld bc,ctc2
 890 ;  ld a,%00000011 ;Reset and stop timer flags
 900 ;  out (c),a      ;Stop CTC2
 910 ;  ld bc,regsel
 920 ;  ld a,$c5
 930 ;  out (c),a      ;Select timer interrupt register
 940 ;  inc b
 950 ;  in a,(c)       ;Read timer interrupt register
 960 ;  and %11111011  ;Disable CTC2 interrupts
 970 ;  out (c),a      ;Update timer interrupt register
 980 ;  ld a,1
 990 ;  ld (stopped),a ;Set stopped flag
1000 ;  ei
1010 ;  ret
1020 ;;
1030 ;; Timer interrupt handler
1040 ;;
1050 ;ctchandler
1060 ;  push hl
1070 ;   ld hl,(ctr)   ;Get current counter value
1080 ;   inc hl        ;Increment value
1090 ;   ld (ctr),hl   ;Store new counter value
1100 ;  pop hl
1110 ;  ei
1120 ;  reti
1130 ;ctr dw 0
1140 ;;
1150 ;; ULA interrupt handler
1160 ;;
1170 ;ulahandler
1180 ;  call $38      ;Call ROM ULA Interrupt
1190 ;  reti          ;ROM does RET so we do RETI
1200 ;;
1210 ;; Default interrupt handler
1220 ;;
1230 ;inthandler
1240 ;  ei
1250 ;  reti
1260 ;;
1270 ;;Interrupt Vector Table
1280 ;;
1290 ;  align 32
1300 ;ivt
1310 ;  dw inthandler ; 0-line interrupt
1320 ;  dw inthandler ; 1-uart0 rx
1330 ;  dw inthandler ; 2-uart1 rx
1340 ;  dw inthandler ; 3-ctc0
1350 ;  dw inthandler ; 4-ctc1
1360 ;  dw ctchandler ; 5-ctc2
1370 ;  dw inthandler ; 6-ctc3
1380 ;  dw inthandler ; 7-ctc4
1390 ;  dw inthandler ; 8-ctc5
1400 ;  dw inthandler ; 9-ctc6
1410 ;  dw inthandler ;10-ctc7
1420 ;  dw ulahandler ;11-ula
1430 ;  dw inthandler ;12-uart0 tx
1440 ;  dw inthandler ;13-uart1 tx
1450 ;  dw inthandler ;14-unused
1460 ;  dw inthandler ;15-unused
1470 REM -------------------------------
1480 REM BASIC TEST PROGRAM
1490 REM -------------------------------
1500 PROC StartTimer()
1510 FOR %r=0 TO 3
1520 RUN AT %r
1530 PRINT "RUN AT ";%r
1540 %s = FN ReadTimer()
1550 f= TIME
1560 FOR i=1 TO 10
1570 PRINT i;", "; % INT { FN ReadTimer()}-s;"ms, "; TIME -f;" frames"
1580 f= TIME
1590 %s= FN ReadTimer()
1600 NEXT i
1610 PRINT "Press ANY Key": PAUSE 0: CLS
1620 NEXT %r
1630 PROC StopTimer()
1640 STOP
1650 REM -------------------------------
1660 REM BASIC 2 Machine Code Interface
1670 REM -------------------------------
1680 DEFPROC InitTimer()
1690 RANDOMIZE % USR $bf00
1700 ENDPROC
1710 DEFPROC StartTimer()
1720 RANDOMIZE % USR $bf03
1730 ENDPROC
1740 DEF FN ReadTimer() = % USR $bf06
1750 DEFPROC StopTimer()
1760 RANDOMIZE % USR $bf09
1770 ENDPROC