AMD-K6 Datasheet, PDF (219/346 Pages) Advanced Micro Devices – AMD-K6 Processor

20695H/0—March 1998
Preliminary Information
AMD-K6^® Processor Data Sheet
10.8
Chapter 10
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 means 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
informs the CPU whether it should re-execute the 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.
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:
s Bits 31–16—Reserved
s 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
Reserved
15–0
I/O Instruction restart on return from SMM:
s 0000h = execute the next instruction after the trapped I/O
s 00FFh = re-execute the trapped I/O instruction
System Management Mode (SMM)
201