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Jan 19, 2017 at 19:17 comment added Thomas Let me add a comment: the pokey sound chip also handles the I/O. The serial data, from tape or disk, goes through the pokey. By forcing the volume of the channels to 0, the noise becomes very faint (but you can still hear it). The loading code reset the volume upon starting, so it was a voluntary feature driven by code; however there is also a hardware component to this too, since code is not aware when the tape reading is out of sync, but data stops flowing through the pokey and then the sound stops. Possibly they reuse some shift register for both I/O and sound, but that's speculation.
Jan 19, 2017 at 17:32 vote accept bjb
Sep 24, 2020 at 17:09
Dec 7, 2016 at 23:04 history answered Thomas CC BY-SA 3.0