$DC05/56325/CIA1+5:   Timer A High-Byte (Kernal-IRQ, Tape)

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

   Default Value: $40/64 on PAL
                  $42/66 on NTSC

Kernal-Reference:

 STA $DC05   : $F90E $FDF3