AMD AMD-K6-2/500AFX Data Sheet - Page 239
I/O Trap Restart Slot, Table 46.
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21850J/0-February 2000 Preliminary Information AMD-K6®-2 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 processor 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 46 shows the format of the I/O trap restart slot. Table 46. 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) 219