IBM 560 - Manual (Page 50)

System Control Port A (Hex 0092)
Figure 2-32. System Control Port A (Hex 0092)
Bit Function
7–3 Reserved
2 Reserved (must be set to 0)
1 Alternate gate A20
0 Alternate hot reset
Bits 7–3 These bits are reserved.
Bit 2 This bit is reserved.
Bit 1 This bit is used to enable the ‘address 20’ signal ( A20 )
when the microprocessor is in the real address mode.
When this bit is set to 0,
A20 cannot be used in real
mode addressing. This bit is set to 0 during a system
reset.
Bit 0 This bit provides an alternative method of resetting the
system microprocessor. This alternative method
supports operating systems requiring faster operation
than that provided on the IBM Personal Computer AT.
Resetting the system microprocessor switches the
microprocessor from protected mode to real address
mode.
This bit is set to 0 by either a system reset or a write
operation. When a write operation changes this bit from
0 to 1, the ‘processor reset’ signal is pulsed after the
reset has occurred. While the reset is occurring, the
latch remains set so that POST can read this bit. If the
bit is set to 0, POST assumes that the system was just
powered on. If the bit is set to 1, POST assumes that
the microprocessor has been switched from protected
mode to real mode.
When bit 0 is used to reset the system microprocessor
to the real mode, use the following procedure:
1. Disable all maskable and nonmaskable interrupts.
2. Reset the system microprocessor by writing a 1 to
bit 0.
3. Issue a Halt instruction to the system
microprocessor.
4. Reenable all maskable and nonmaskable interrupts.
2-28 ThinkPad 560/560E System Board