
SMSC DS – FDC37N958FR
Page 178
Rev. 09/01/99
Port 92
The FDC37N958FR supports ISA I/O writes to
port 92h as a quick alternate mechanism
for
controlling the state of GATEA20.
generating
a
CPU_RESET
pulse
or
PORT 92 REGISTER DESCRIPTION
D7-D2
R/W
0
Reserved
D1
R/W
D0
R/W
Host R/W
Bit Def
ALT_GATEA20
ALT_CPU_RESET
The Port92h register resides at ISA address
0x92 and is used to support the alternate reset
(nALT_RST) and alternate GATEA20 (ALT_A20)
functions. This register defaults to 0x00 on
assertion of RESET_OUT or on VCC2 Power On
Reset.
The Port92h Register is enabled by setting the
Port 92 Enable bit (bit 0 of Logical Device 7
Configuration Register 0xF0). When Port92 is
disabled, by clearing the Port 92 Enable bit, then
access to this register is completely disabled
(I/O writes to ISA 92h are ignored and I/O reads
float the system data bus SD[7:0]).
When Port92h is enabled the bits have the
following meaning:
D7-D2 Reserved
Writes are ignored and reads return 0.
D1 - ALT_GATEA20
This bit provides an alternate means for system
control of the FDC37N958FR GATEA20 pin.
= 0: ALT_A20 is driven low
= 1: ALT_A20 is driven high
When Port 92 is enabled, writing a 0 to bit 1 of
the Port92 Register forces ALT_A20 low.
ALT_A20 low drivesGATEA20 low, if A20 from
the keyboard controller is also low. When Port
92 is enabled, writing a 1 to bit 1 of the Port92
register forces ALT_A20 high. ALT_A20 high
drives GATEA20 high regardless of the state of
A20 from the keyboard controller.
D0 - ALT_CPU_RESET
This bit provides an alternate means to generate
a CPU_RESET pulse. The CPU_RESET output
provides a means to reset the system CPU to
effect a mode switch from Protected Virtual
Address Mode to the Real Address Mode. This
provides a faster means of reset than is provided
through the 8051 keyboard controller. Writing a
“1” to this bit will cause the nALT_RST internal
signal to pulse (active low) for a minimum of 6
"
s
after a delay of 14
"
s. Before another
nALT_RST pulse can be generated, this bit must
be written back to “0”.