Bit 0 of Port 92, which generates the nALT_RST
signal, is used to reset the CPU under program
pulse can be generated, bit 0 must be set to 0
either by a system reset of a write to Port 92.
Upon reset, this signal is driven inactive high (bit
0 in the Port 92 Register is set to 0).
control.
This signal is AND’ed together
externally with the reset signal (nKBDRST) from
the keyboard controller to provide a software
means of resetting the CPU. This provides a
faster means of reset than is provided by the
keyboard controller. Writing a 1 to bit 0 in the
Port 92 Register causes this signal to pulse low
for a minimum of 6µs, after a delay of a
minimum of 14µs. Before another nALT_RST
If Port 92 is enabled, i.e., bit 2 of KRST_GA20 is
set to 1, then a pulse is generated by writing a 1
to bit 0 of the Port 92 Register and this pulse is
AND’ed with the pulse generated from the 8042.
This pulse is output on pin KRESET and its
polarity is controlled by the GPI/O polarity
configuration.
14us
6us
8042
P20
KRST
KBDRST
KRST_GA20
P92
Bit 2
nALT_RST
Bit 0
Pulse
Gen
14us
Note: When Port 92 is disabled,
writes are ignored and reads
return undefined values.
6us
KRESET Generation
119
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