
14
AMD-K5 Processor
AMD K86 Family BIOS and Software Tools Developers Guide
21062E/0—June 1997
Preliminary Information
I/O Trap Dword
If the assertion of SMI is recognized on the boundary of an I/O
instruction, the I/O trap dword at offset FFA4h in the SMM
state-save area contains information about the instruction. The
fields of the I/O trap dword, shown in Table 6, are configured as
follows:
I
Bits 31–16
—I/O port address
Bit 15
—I/O string operation (1 = string, 0 = non-string)
Bits 14–2
—
reserved
Bit 1
—Valid I/O instruction (1 = valid, 0 = invalid)
Bit 0
—Input or output instruction (1 = INx, 0 = OUTx)
I
I
I
I
The I/O trap dword is related to the I/O trap restart slot,
described below. Bit 1 of the I/O trap dword (the valid bit)
should be tested if the I/O trap restart slot is to be changed.
I/O Trap Restart Slot
The I/O trap restart slot at offset FF00h in the SMM state-save
area specifies whether the trapped I/O instruction should be
re-executed on return from SMM. This slot in the state-save
area is called the I/O instruction restart function. Re-executing
a trapped I/O instruction is useful, for example, if an I/O write
occurs to a disk that is powered down. The system logic
monitoring such an access can assert SMI#. Then the SMM
service routine can query the system logic, detect a failed I/O
write, take action to power-up the I/O device, enable the I/O
trap restart slot feature, and return from SMM.
Table 6.
I/O Trap Dword Fields
Bits 31–16
I/O Port Address
Bit 15
Bit 14–2
Reserved
Bit 1
Bit 0
I/O String Operation
Valid I/O Instruction
Input or Output