This register is not used in CPC computers. In detail, a AY-3-8910 sound chip would have external connectors for this register, so that it could be used as a further IO port, but the CPC's sound chip (AY-3-8912, in 28 pin package) doesn't have such connectors, even though the register still does exist internally.
The [[Aleste 520EX]] (russian CPC clone) is a special case: does have a 8910 chip, with PSG Port B being used as 8bit printer port data.
<br>
== Mono and Stereo Output ==