Figure 1 - 1.

Typical SMM Routine

SMM Entry

Save State

Initialize SMM

Environment

Trap?

Service

Non-Trap SMI

HALT?

Decrement

EIP

Device

OFF?

Shadow

Service

Trap SMI

Modify State

For I/O Restart

Restore

State

Resume

I/O

SMM Exit

N

Y

Y

Y

N

N

or Emulate

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ST486DX - SMM OVERVIEW

10

1.3 Typical SMM Routines

A typical SMM routine is illustrated in the flowchart shown in Figure 1-1. Upon entry to SMM,

the CPU registers that will be used by the SMM routine, must be saved. The SMM environment is

initialized by setting up an Interrupt Descriptor Table, initializing segment limits and setting up a

stack. If the SMI was a result of an I/O bus cycle, the SMM routine can monitor peripheral activ-

ity, shadow read-only ports ,and/or emulate peripherals in software. If a peripheral was powered

down, the SMM routine can power up a peripheral and reissue the I/O instruction. If the SMI was

not caused by an I/O bus cycle, non-trap SMI functions can be serviced. If the instruction execut-

ing, when an SMI occurred, was a HLT instruction, the HLT instruction it should be restarted when

the SMM routine is complete. Before normal operation is resumed, any CPU registers modified

during the SMM routine must be restored to their previous state.