Detect TV system

The following code detects the TV system. A game can use it to compensate for differences in clock rate among various NES models. It has been tested on an emulator in both NTSC and PAL mode.

;
; NES TV system detection code
; Copyright 2011 Damian Yerrick
;
; Copying and distribution of this file, with or without
; modification, are permitted in any medium without royalty
; provided the copyright notice and this notice are preserved.
; This file is offered as-is, without any warranty.
;
.export getTVSystem
.importzp nmis

.align 32  ; ensure that branches do not cross a page boundary

;;
; Detects which of NTSC, PAL, or Dendy is in use by counting cycles
; between NMIs.
;
; NTSC NES produces 262 scanlines, with 341/3 CPU cycles per line.
; PAL NES produces 312 scanlines, with 341/3.2 CPU cycles per line.
; Its vblank is longer than NTSC, and its CPU is slower.
; Dendy is a Russian famiclone distributed by Steepler that uses the
; PAL signal with a CPU as fast as the NTSC CPU.  Its vblank is as
; long as PAL's, but its NMI occurs toward the end of vblank (line
; 291 instead of 241) so that cycle offsets from NMI remain the same
; as NTSC, keeping Balloon Fight and any game using a CPU cycle-
; counting mapper (e.g. FDS, Konami VRC) working.
;
; nmis is a variable that the NMI handler modifies every frame.
; Make sure your NMI handler finishes within 1500 or so cycles (not
; taking the whole NMI or waiting for sprite 0) while calling this,
; or the result in A will be wrong.
;
; @return A: TV system (0: NTSC, 1: PAL, 2: Dendy; 3: unknown
;         Y: high byte of iterations used (1 iteration = 11 cycles)
;         X: low byte of iterations used
.proc getTVSystem
  ldx #0
  ldy #0
  lda nmis
nmiwait1:
  cmp nmis
  beq nmiwait1
  lda nmis

nmiwait2:
  ; Each iteration takes 11 cycles.
  ; NTSC NES: 29780 cycles or 2707 = $A93 iterations
  ; PAL NES:  33247 cycles or 3022 = $BCE iterations
  ; Dendy:    35464 cycles or 3224 = $C98 iterations
  ; so we can divide by $100 (rounding down), subtract ten,
  ; and end up with 0=ntsc, 1=pal, 2=dendy, 3=unknown
  inx
  bne :+
  iny
:
  cmp nmis
  beq nmiwait2
  tya
  sec
  sbc #10
  cmp #3
  bcc notAbove3
  lda #3
notAbove3:
  rts
.endproc

What to change

PAL needs a different music pitch from NTSC and Dendy, but NTSC needs different music speed from PAL and Dendy. PAL needs different raster effects from NTSC and Dendy, but NTSC needs different sprite speed from PAL and Dendy. If you want to take advantage of PAL's longer vertical blank to load more tiles into CHR RAM, Dendy needs different detection of the end of rendering.