DetectTVType:
; Returns a=0 for NTSC, a=1 for PAL
; uses a, hl, de
    di             ; disable interrupts
    ld a,%01100000 ; set VDP such that the screen is on
    out ($bf),a    ; with VBlank interrupts enabled
    ld a,$81
    out ($bf),a
    ld hl,$0000    ; init counter
-:  in a,($bf)     ; get VDP status
    or a           ; inspect
    jp p,-         ; loop until frame interrupt flag is set
-:  in a,($bf)     ; do the same again, in case we were unlucky and came in just
    or a           ;   before the start of the VBlank with the flag already set
    jp p,-
    ; the VDP must now be at the start of the VBlank
-:  inc hl         ; (6 cycles) increment counter until interrupt flag comes on again
    in a,($bf)     ; (11 cycles)
    or a           ; (4 cycles)
    jp p,-         ; (10 cycles)
    xor a          ; reset carry flag, also set a=0
    ld de,2048     ; see if hl is more or less than 2048
    sbc hl,de
    ret c          ; if less, return a=0
    ld a,1
    ret            ; if more or equal, return a=1