diff --git a/pcm-io.s b/pcm-io.s
new file mode 100644
index 0000000..e5887aa
--- /dev/null
+++ b/pcm-io.s
@@ -0,0 +1,171 @@
+
+    /* PCM channel increment */
+    .equ    PCM_FDL, 0xFF0005
+    .equ    PCM_FDH, 0xFF0007
+
+    /* General use timer */
+    .equ    TIMER,    0x8030
+    .equ    INT_MASK, 0x8032
+    .equ    _LEVEL3,  0x5F82        /* TIMER INTERRUPT jump vector */
+
+
+    .text
+    .align  2
+
+
+| uint8_t pcm_lcf(uint8_t pan);
+| Convert pan setting to simple linear crossfade volume settings
+    .global pcm_lcf
+pcm_lcf:
+    move.l  4(sp),d0                /* pan */
+    addq.b  #8,d0
+    bcc.b   0f
+    move.l  4(sp),d0
+0:
+    andi.w  #0x00F0,d0              /* right pan volume nibble */
+
+    move.l  4(sp),d1
+    not.b   d1
+    addq.b  #8,d1
+    bcc.b   1f
+    move.l  4(sp),d1
+    not.b   d1
+1:
+    lsr.b   #4,d1                   /* left pan volume nibble */
+    or.b    d1,d0
+    rts
+
+
+| void pcm_delay(void);
+| Delay after writing PCM register
+    .global pcm_delay
+pcm_delay:
+    moveq   #20,d1
+0:
+    dbra    d1,0b
+    rts
+
+
+| void pcm_set_period(uint32_t period);
+    .global pcm_set_period
+pcm_set_period:
+    move.l  4(sp),d1
+    cmpi.l  #4,d1
+    blo.b   0f                      /* saturate increment to 65535 */
+    addq.l  #1,d1
+    move.l  #446304,d0
+    divu.w  d1,d0
+    lsr.w   #1,d0                   /* incr = (446304 / Period + 1) >> 1 */
+    bra.b   1f
+0:
+    move.w  #65535,d0
+1:
+    move.b  d0,PCM_FDL.l
+    bsr.b   pcm_delay
+    lsr.w   #8,d0
+    move.b  d0,PCM_FDH.l
+    bra.b   pcm_delay
+
+
+| void pcm_set_freq(uint32_t freq);
+    .global pcm_set_freq
+pcm_set_freq:
+    move.l  4(sp),d0
+    cmpi.l  #1041648,d0
+    bhs.b   0f                      /* saturate increment to 65535 */
+    lsl.l   #8,d0
+    lsl.l   #3,d0                   /* shift freq for fixed point result */
+    move.w  #32552,d1
+    divu.w  d1,d0                   /* incr = (freq << 11) / 32552 */
+    bra.b   1f
+0:
+    move.w  #65535,d0
+1:
+    move.b  d0,PCM_FDL.l
+    bsr.b   pcm_delay
+    lsr.w   #8,d0
+    move.b  d0,PCM_FDH.l
+    bra.b   pcm_delay
+
+
+| void pcm_set_timer(uint16_t bpm);
+| TIMER = 32552 / (bpm * 2) - 1
+|   should be 32552 * 5 / (bpm * 2) - 1, but that's too big for TIMER, so
+|   we leave out the * 5 and compensate in the int handler
+    .global pcm_set_timer
+pcm_set_timer:
+    move.l  #32552,d0
+    move.l  4(sp),d1
+    add.w   d1,d1
+    beq.b   0f                      /* safety check... passing 0 will just exit */
+    divu.w  d1,d0
+    subq.w  #1,d0
+    ble.b   0f                      /* safety check */
+    move.w  d0,TIMER.w
+0:
+    rts
+
+
+| void pcm_stop_timer(void);
+    .global pcm_stop_timer
+pcm_stop_timer:
+    move    #0x2700,sr              /* disable interrupts */
+
+    move.w  #0,TIMER.w              /* stop General Timer */
+    move.w  INT_MASK.w,d0
+    andi.w  #0xFFF7,d0
+    move.w  d0,INT_MASK.w           /* disable General Timer interrupt */
+
+    move    #0x2000,sr              /* enable interrupts */
+    rts
+
+
+| void pcm_start_timer(void (*callback)(void));
+    .global pcm_start_timer
+pcm_start_timer:
+    move    #0x2700,sr              /* disable interrupts */
+
+    move.l  4(sp),int3_callback     /* set callback vector */
+    move.w  #0,int3_cntr            /* clear int counter */
+
+    move.w  #0x4EF9,_LEVEL3.w
+    move.l  #timer_int,_LEVEL3+2.w  /* set level 3 int vector for timer */
+
+    move.w  #129,TIMER.w            /* 125 BPM */
+    move.w  INT_MASK.w,d0
+    ori.w   #0x0008,d0
+    move.w  d0,INT_MASK.w           /* enable General Timer interrupt */
+
+    move    #0x2000,sr              /* enable interrupts */
+    rts
+
+
+timer_int:
+    move.l  d0,-(sp)
+    move.w  int3_cntr,d0
+    addq.w  #1,d0
+    cmpi.w  #5,d0
+    blo.b   0f                      /* once every 5 ints for actual beats per minute rate */
+
+    movem.l d1/a0-a1,-(sp)
+    movea.l int3_callback,a1
+    jsr     (a1)
+    movem.l (sp)+,d1/a0-a1
+    moveq   #0,d0
+0:
+    move.w  d0,int3_cntr
+    move.l  (sp)+,d0
+    rte
+
+
+    .data
+
+
+    .align  4
+
+int3_callback:
+    .long   0
+
+int3_cntr:
+    .word   0
+