$DC04/56324/CIA1+4:   Timer A Low-Byte (Kernal-IRQ, Tape)

   Read : Get current value
   Write: Set Low-Byte of value to count down from

   Default Value: $25/37 on PAL
                  $95/149 on NTSC

Kernal-Reference:

 STA $DC04   : $F907 $FDE4 $FDEE