$4A/74:   Step counter for head transport

   Values 0 - 127 move the head outside and values over 128 move towards
   the centre.

ROM-Reference:

 ASL $4A    : $F313
 DEC $4A    : $FA63
 INC $4A    : $FA32
 LDA $4A    : $FA05 $FA2E $FA3B
 STA $4A    : $F311 $F38A $F9C1 $FAD9

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