mov

al,ah

out

23h,al

;set MMAC back to normal

got_inst:

cmp

bl,0f4h

;was it a HLT instruction?

jne

not_hlt

;if not a F4 then not a HLT

;set up SMM header to return

;to the HLT instruction

dec

cs:dword ptr [SMI_NEXTIP]

not_hlt:

jmp

continue_SMI_routine

; data within the SMM Space Code Segment

seg4G dw

0ffffh

;limit 15-0

dw

0

;base

db

0

;base

db

10010011B

;data segment, DPL=0, present

db

8Fh

;high limit =f, Gran =4K, 16 bit

db

0

;base

dw

0

3.9

Exiting the SMI Handler

When the RSM instruction is executed at the end of the SMI handler, the EIP is loaded from the

SMM header at the address (SMMbase + SMMsize - 14h) called NEXT_IP. This permits the in-

struction to be restarted if NEXT_IP was modified by the SMM program. The values of ECX, ESI,

and EDI, prior to the execution of the instruction that was interrupted by SMI, can be restored from

information in the header which pertains to the INx and OUTx instructions. See Section 3.6 for an

example program to restart an I/O instruction. The only registers that are restored from the SMM

header are CS, NEXT_IP, EFLAGS, CR0, and DR7. All other registers which were modified by

the SMM program need to be restored before executing the RSM instruction.

ST486DX - SMM SOFTWARE CONSIDERATIONS

37