Lenovo ThinkPad 560E TP 560Z Technical Reference Manual - Page 47

System Control Port A (X'0092'), Bits 7-3, Bit 2, ThinkPad 560Z System Board

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System Control Port A (X'0092') Bit Function 7-3 Reserved 2 Reserved (must be set to 0) 1 Alternate gate A20 0 Alternate hot reset Figure 2-32. System Control Port A (X'0092') Bits 7-3 Bit 2 Bit 1 Bit 0 These bits are reserved. This bit is reserved. This bit is used to enable the 'address 20' signal (A20) when the microprocessor is in the real address mode. If this bit is set to 0, A20 cannot be used in real mode addressing. This bit is set to 0 during a system reset. 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. If 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. If 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. ThinkPad 560Z System Board 2-29

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System Control Port A (X
'
0092
'
)
Figure
2-32.
System Control Port A (X
'
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.
If
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.
If 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.
If 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.
ThinkPad 560Z System Board
2-29