This message means that the Zilog 8530 character input silo (or serial port FIFO) overflowed before it could be serviced. The zs(7D) driver, which talks to a Zilog Z8530 chip, is reporting that the FIFO (holding about two characters) has been overrun. The number after zs(7D) shows which serial port experienced an overflow:
zs0 - tty serial port 0 (/dev/ttya) zs1 - tty serial port 1 (/dev/ttyb) zs2 - keyboard port (/dev/kbd) zs3 - mouse port (/dev/mouse)
Silo overflows indicate that data in the respective serial port FIFO have been lost. However, the consequences of silo overflows might be negligible if the overflows occur infrequently, if data loss is not catastrophic, or if data can be recovered or reproduced. For example, although a silo overflow on the mouse driver (zs3) indicates that the system could not process mouse events quickly enough, the user can perform mouse motions again. Similarly, lost data from a silo overflow on a serial port with a modem connection transferring data using uucp(1C) is recovered when uucp(1C) discovers the loss of data and requests retransmission of the corrupted packet.
Frequent silo overflow messages can indicate a zs(7D) hardware FIFO problem, a serial driver software problem, or abnormal data or system activity. For example, the system ignores interrupts during system panics, so mouse and keyboard activity result in silo overflows.
If the serial ports experiencing silo overflows are not being used, a silo overflow could indicate the onset of a hardware problem.
Another type of silo overflow is one that occurs during reboot, when an HDLC line is connected to any of the terminal ports. For example, an X.25 network could be sending frames before the kernel has been told to expect them. Such overflow messages can be ignored.