GATEA20 AND KEYBOARD RESET
PORT 92 FAST GATEA20 AND KEYBOARD
RESET
The FDC37B78x provides two options for
GateA20 and Keyboard Reset: 8042 Software
Generated GateA20 and KRESET and Port 92
Fast GateA20 and KRESET.
Port 92 Register
This port can only be read or written if Port 92
has been enabled via bit 2 of the KRST_GA20
Register (Logical Device 7, 0xF0) set to 1.
This register is used to support the alternate
reset (nALT_RST) and alternate A20 (ALT_A20)
functions.
Name
Port 92
92h
24h
Read/Write
8 bits
Location
Default Value
Attribute
Size
Port 92 Register
Bit
7:6
5
4
3
Function
Reserved. Returns 00 when read
Reserved. Returns a 1 when read
Reserved. Returns a 0 when read
Reserved. Returns a 0 when read
Reserved. Returns a 1 when read
2
1
ALT_A20 Signal control. Writing a 0 to this bit causes the ALT_A20 signal to be
driven low. Writing a 1 to this bit causes the ALT_A20 signal to be driven high.
Alternate System Reset. This read/write bit provides an alternate system reset
function. This function provides an alternate 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 by the Keyboard
controller. This bit is set to a 0 by a system reset. Writing a 1 to this bit will cause
the nALT_RST signal to pulse active (low) for a minimum of 1 µs after a delay of
500 ns. Before another nALT_RST pulse can be generated, this bit must be written
back to a 0.
0
134
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