What does "zsN: silo overflow" mean?

        The CPU serial ports - both ordinary serial ports A and B, and
        the port for the keyboard and mouse - use the Zilog Z8530 SCC
        chip.  That chip has a 3-character on-board buffer called the
        "silo".  If a character arrives in the silo, the chip interrupts
        the CPU at a high priority, and the interrupt service routine
        reads the character out of the silo.

        If the interrupt isn't serviced in time, more than 3 characters
        can be placed in the silo by the chip; if so, the chip notes
        that the silo "overflowed", and the interrupt service routine,
        when called, will note that a "silo overflow" occurred.

        If the machine is printing a message from the kernel, interrupts
        from the chip will be held off; if the message takes long enough
        to print, and characters are coming in quickly enough on the
        serial port, more than 3 can arrive, and a "silo overflow" will
        occur.

        It is possible that a machine that's sufficiently busy in other
        code that runs with interrupts held off could get a silo
        overflow as well.

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