GYM Player by Vladikcomper (extract)

; Memory consts

vdp_data    equ $C00000
vdp_ctrl    equ $C00004
z80_ram     equ $A00000

wait_for_vblank equ $FFFF8000

ym      equ $4000

wait_ym macro
@0\@    move.b  ym(a1),d2
    btst    #7,d2
    bne.s   @0\@
    endm

; ----------------------------------------------------------
    lea music_file,a0
    lea z80_ram,a1

; ----------------------------------------------------------
mainloop
    moveq   #0,d0
    move.b  (a0)+,d0
    bmi.s   error
    andi.b  #%11,d0
    add.w   d0,d0
    jmp cmd_lst(pc,d0.w)
; ----------------------------------------------------------
error   bra.s   *

; ----------------------------------------------------------
cmd_lst bra.s   cmd0    ; +$00
    bra.s   cmd1    ; +$02
    bra.s   cmd2    ; +$04
    bra.w   cmd3    ; +$06

; ----------------------------------------------------------
; cmd - skip a frame
cmd0    move    #$2300,sr
    st.b    wait_for_vblank
@0  tst.b   wait_for_vblank
    bne.s   @0
    bra.s   mainloop

; ----------------------------------------------------------
; cmd - set ym reg 0
cmd1    wait_ym
    move.b  (a0)+,ym(a1)
    wait_ym
    move.b  (a0)+,ym+1(a1)
    bra.w   mainloop

; ----------------------------------------------------------
; cmd - set ym reg 1
cmd2    wait_ym
    move.b  (a0)+,ym+2(a1)
    wait_ym
    move.b  (a0)+,ym+3(a1)
    bra.w   mainloop

cmd3    move.b  (a0)+,$C00011
    bra.w   mainloop

; ----------------------------------------------------------
; vblank interrupt
vblank  sf.b    wait_for_vblank
    move    #$2700,sr

; ----------------------------------------------------------
; hblank interrupt
hblank  rte

; ----------------------------------------------------------
music_file
    incbin  test.gym
    dc.b    -1      ; eof flag