IBM 560 - Manual (Page 48)

Miscellaneous System Functions and Ports
This section provides information about nonmaskable interrupts
(NMIs), the power-on password, and hardware compatibility.
Nonmaskable Interrupt (NMI)
The NMI signals the system microprocessor that a parity error or a
channel check timeout has occurred. This situation can cause lost
data or an overrun error on some I/O devices. The NMI masks all
other interrupts. The interrupt return (IRET) instruction restores the
interrupt flag to the state it was in before the interrupt occurred. A
system reset causes a reset of the NMI.
The NMI requests from system board parity and channel check are
subject to mask control with the NMI mask bit in the RT/CMOS
Address register. See “RT/CMOS Address and NMI Mask Register
(Hex 0070)” on page 2-17. The power-on default of the NMI mask is
1 (NMI disabled). Before the NMI is enabled after a power-on reset,
the parity-check states are initialized by POST.
Attention
The operation following a write to hex 0070 should access hex
0071; otherwise, intermittent failures of the RT/CMOS RAM can
occur.
2-26 ThinkPad 560/560E System Board