IBM Personal System/2 60 Technical Reference page 50

Bit 0
This bit provides an alternate method of resetting the
system microprocessor. This alternate method supports
operating systems requiring faster operation than was
provided on the IBM Personal Computer AT®. Resetting
the system microprocessor is used to switch the
microprocessor from the protected mode to the real
address mode. The alternate reset takes 13.4
microseconds.
This bit is set to 0 either by a system reset or a Write
operation. When a Write operation changes this bit from 0
to 1, the alternate reset pin is pulsed high for 100 to 125
nanoseconds. The reset occurs after a minimum delay of
6.72 microseconds. While the reset is occurring, the latch
remains set so that POST can read this bit. If the bit is 0,
POST assumes the system was just powered on. If the bit
is 1, POST assumes a switch from the protected mode to
the real mode has taken place.
IBM Personal Computer AT is a registered trademark of the International
Business Machines Corporation.
3-24 Model 60 System Board