ST486DX - SMM SOFTWARE CONSIDERATIONS

35

3.8

Return to HLT Instruction

To make an SMI interrupt truly transparent to the system, an SMI interrupt from a HLT instruction

should return to the HLT instruction. There are known cases with DOS software where returning

from an SMI handler to the instruction following the HLT will cause a system error. To determine

if a HLT instruction was interrupted by the SMI, the opcode from memory needs to be interrogated.

This code example describes how to determine if the current instruction is a HLT and how to

restart it.

;This is the start of specific code to check if the SMI

;occurred while in a HLT instruction. If it did, then

;return back to the HLT instruction when SMI is finished.

rsdc

fs,cs:,[seg4G]

;FS is base=0 limit=4G data

;segment to be used to check if

;HLT instruction was executing

;on a SGS Thomson part, if the SMI occurred while in a HLT

;instruction, the CURRENT IP and the NEXT IP will both

;point to the instruction following the HLT.

mov

eax,cs:dword ptr[SMI_CURRENTIP]

cmp

eax,cs:dword ptr[SMI_NEXTIP]

jne

not_hlt

;can’t be a HLT but could be

;a LOOP or REP

;load EAX with CS base from the SMM header

mov

ax,cs:word ptr [SMI_CSSELH+2]

mov

al,cs:byte ptr [SMI_CSSELH]

shl

eax,10h

mov

ax,cs:word ptr[SMI_CSSELL+2]

;calculate linear address

add

eax,cs:dword ptr [SMI_CURRENTIP]

dec

eax

;decrement to HLT instruction

mov

edx,eax

;save lin addr in edx

mov

eax,cs:dword ptr [SMI_CR0] ;check if paging on

test

eax,80000000h

je

no_paging

;if no paging then linear

;address = physical address

;set MMAC to get access to Main memory

mov

al,0c1h

out

22h,al

in

al,23h

mov

cl,al

;save old CCR1 value in cl

mov

al,0c1h

out

22h,al