82371AB Datasheet, PDF (83/284 Pages) Intel Corporation – PCI-TO-ISA / IDE XCELERATOR PIIX4

E
82371AB (PIIX4)
4.2.4. NMI REGISTERS
The NMI logic incorporates two different 8-bit registers. The CPU reads the NMISC Register to determine the
NMI source (bits set to a 1). After the NMI interrupt routine processes the interrupt, software clears the NMI
status bits by setting the corresponding enable/disable bit to a 1. The NMI Enable and Real-Time Clock Register
can mask the NMI signal and disable/enable all NMI sources.
To ensure that all NMI requests are serviced, the NMI service routine software flow should be as follows:
1. NMI is detected by the processor on the rising edge of the NMI input.
2. The processor will read the status stored in port 061h to determine what sources caused the NMI. The
processor may then set to 0 the register bits controlling the sources that it has determined to be active.
Between the time the processor reads the NMI sources and sets them to a 0, an NMI may have been
generated by another source. The level of NMI will then remain active. This new NMI source will not be
recognized by the processor because there was no edge on NMI.
3. The processor must then disable all NMIs by setting bit 7 of port 070H to a 1 and then enable all NMIs by
setting bit 7 of port 070H to a 0. This will cause the NMI output to transition low then high if there are any
pending NMI sources. The CPU’s NMI input logic will then register a new NMI.
4.2.4.1.
NMISC—NMI Status and Control Register (IO)
I/O Address:
Default Value:
Attribute:
061h
00h
Read/Write
This register reports the status of different system components, controls the output of the speaker counter
(Counter 2), and gates the counter output that drives the SPKR signal.
Bit
Description
7 SERR# NMI Source Status—RO. Bit 7 is set if a system board agent (PCI devices or main
memory) detects a system board error and pulses the PCI SERR# line. This interrupt source is
enabled by setting bit 2 to 0. To reset the interrupt, set bit 2 to 0 and then set it to 1. When writing to
port 061h, bit 7 must be 0.
6 IOCHK# NMI Source Status—RO. Bit 6 is set if an expansion board asserts IOCHK# on the ISA
Bus. This interrupt source is enabled by setting bit 3 to 0. To reset the interrupt, set bit 3 to 0 and
then set it to 1. When writing to port 061h, bit 6 must be a 0.
5 Timer Counter 2 OUT Status—RO. The Counter 2 OUT signal state is reflected in bit 5. The value
on this bit following a read is the current state of the Counter 2 OUT signal. Counter 2 must be
programmed following a CPURST for this bit to have a determinate value. When writing to port 061h,
bit 5 must be a 0.
4 Refresh Cycle Toggle—RO. The Refresh Cycle Toggle signal toggles from either 0 to 1 or 1 to 0
following every refresh cycle. When writing to port 061h, bit 4 must be a 0.
3 IOCHK# NMI Enable—R/W. 1=Clear and disable; 0=Enable IOCHK# NMIs.
2 PCI SERR# Enable—R/W. 1=Clear and Disable; 0=Enable. For PIIX4, the SERR# signal can be for
a special protocol between the host-to-PCI bridge and PIIX4 (see MSTAT Register description, 6B−
6Ah, function 0).
1 Speaker Data Enable—R/W. 0=SPKR output is 0; 1=the SPKR output is the Counter 2 OUT signal
value.
0 Timer Counter 2 Enable—R/W. 0=Disable; 1=Enable.
PRELIMINARY
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4/9/97 2:23 PM PIIX4aDS
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