ST486DX - SMM SOFTWARE CONSIDERATIONS

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full 4-GBytes of possible system memory without entering protected mode. Once the limits of a

segment register are set, the base can be changed by use of the MOV instruction.

An Interrupt Descriptor Table (IDT) needs to be set up in SMM memory before any interrupts or

exceptions occur. Once initialized, the SMI handler can execute calls, jumps, and other changes of

flow and will generate software interrupts and faults. The Interrupt Descriptor Table Register can

be loaded with an LIDT instruction to point to a small IDT in SMM memory that can handle the

possible interrupts and exceptions that might occur while in the SMM routine.

A stack should always be set up in SMM memory so that stack operations done within SMM do not

affect the system memory.

; SMM environment initialization example

rsdc

ds,cs:,seg4G

;DS is a 4GByte segment, base=0

rsdc

es,cs:,seg4G

;ES is a 4GByte segment, base=0

rsdc

fs,cs:,seg4G

;FS is a 4GByte segment, base=0

rsdc

gs,cs:,seg4G

;GS is a 4GByte segment, base=0

rsdc

ss,cs:,seg4G

;SS is a 4GByte segment, base=0

lidt

cs:smm_idt

;load IDT base and limit for

;SMM’s IDT

mov

esp, smm_stack

jmp

continue_smm_code

;

;descriptor of 4GByte data segment for use by rsdc

seg4G

dw

0ffffh

; limit 4G

dw

0

; base = 0

db

0

; base = 0

db

10010011B

; data segment, DPL=0,P=1

db

8fh

; limit = 4G,

db

0h

; base = 0

dw

0

; segment register = 0

smm_idt

dw

smm_idt_limit

dd

smm_idt_base

3.5

Accessing Main Memory Overlapped by SMM Memory

When in SMM mode there are instances where the program needs access to the system memory

that is overlapping with SMM memory. This need most commonly occurs when the SMM routine

is trying to save the entire memory image to disk before powering down the system. To access

main memory overlapping the SMM space (i.e., generate ADS# for memory MOV instructions

rather than SMADS#) set the MMAC bit in CCR1. The following code will enable and then disable

MMAC: