DS17885/DS17887

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SYSTEM MAINTENANCE INTERRUPT (SMI) RECOVERY STACK

An SMI recovery register stack is located in the extended register bank, locations 4Eh and 4Fh. This

register stack, shown below, can be used by the BIOS to recover from an SMI occurring during an RTC

read or write.

The RTC address is latched on the falling edge of the ALE signal. Each time an RTC address is latched,

the register address stack is pushed. The stack is only four registers deep, holding the three previous RTC

addresses in addition to the current RTC address being accessed. The following waveform illustrates how

the BIOS could recover the RTC address when an SMI occurs.

1) The RTC address is latched.

2) An SMI is generated before an RTC read or write occurs.

3) RTC address 0Ah is latched and the address from 1 is pushed to the “RTC Address–1” stack location.

This step is necessary to change the bank select bit, DV0 = 1.

4) RTC address 4Eh is latched and the address from 1 is pushed to location 4Eh, “RTC Address–2”

while 0Ah is pushed to the “RTC Address–1” location. The data in this register, 4Eh, is the RTC

address lost due to the SMI.