Register DIn real tapes, bits are encoded serially along the tape. The drive must decode them and group them into bytes using some kind of shift register. Once a byte have been decoded, it is transferred in parallel to register D and an interrupt is asserted to the EXT-BUS to let LC-81 know that a byte is ready for reading. Software running at LC-81 reads register D in response to the interrupt.
The time needed to decode a byte, and consequently the time between interrupts, varies depending of the drive, but is expected to be in the order of milisecons.
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