It uses ports fbe0-fbe3. The exact decoding of these ports is unknown.
fbe0 This port is read only it seems.
bits 7..4 seem to be related to the dip switches on the CSD
with bit 7 seems to be fixed to GND/0v.
bit 6,5 is the timer value. It is the time allowed between choosing next cartridge to play.
bit 6, bit 5
bit 6, bit 5 off off = 8 8s 1 1 off on = 16 16s 1 0 on off = 24 24s 0 1 on on = 32 32s 0 0
bit 4 seems to be timer on/off for time limited play.
fbe0 bits 3..0 are defined by the dial
bit 3 is ignored.