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Chapter 10

System Management Mode (SMM)

201

20695H/0—March 1998

AMD-K6® Processor Data Sheet

Preliminary Information

The I/O trap dword is related to the I/O trap restart slot (see “I/O

Trap Restart Slot”). If bit 1 of the I/O trap dword is set by the

processor, it m eans that SMI# was asserted during the

execution of an I/O instruction. The SMI handler tests bit 1 to

see if there is a valid I/O instruction trapped. If the I/O

instruction is valid, the SMI handler is required to ensure the

I/O trap restart slot is set properly. The I/O trap restart slot

info rms the CPU whe the r it sh ou ld re-ex ecu te th e I/O

instruction after the RSM or execute the instruction following

the trapped I/O instruction.

Note: If SMI# is sampled asserted during an I/O bus cycle a

minimum of three clock edges before BRDY# is sampled

asserted, the associated I/O instruction is guaranteed to be

trapped by the SMI handler.

10.8

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 would 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.

The fields of the I/O trap restart slot are defined as follows:

Bits 31–16—Reserved

Bits 15–0—I/O instruction restart on return from SMM:

0000h

= execute the next instruction after the trapped

I/O instruction

00FFh = re-execute the trapped I/O instruction

Table 37 shows the format of the I/O trap restart slot.

Table 37. I/O Trap Restart Slot

31–16

15–0

Reserved

I/O Instruction restart on return from SMM:

0000h = execute the next instruction after the trapped I/O

00FFh = re-execute the trapped I/O instruction

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