ST486DX - SMM SOFTWARE CONSIDERATIONS

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3.

SMM SOFTWARE CONSIDERATIONS

This section provides an overview of SGS-Thomson SMM coding and information helpful in devel-

oping SMM code.

3.1

Enabling SMM

Many systems have memory controllers that aid in the initialization of SMM memory. SGS-Thom-

son SMM features allow the initialization of SMM memory without external hardware memory re-

mapping.

When loading SMM memory with an SMI interrupt handler it is important that the SMI# does not

occur before the handler is loaded. This can be done by not setting SMAC=0 and SMI=1 before

the SMI handler is installed. It is necessary to load SMAR with appropriate values before the

SMM memory is accessible. To load SMM memory with a program it is first necessary to enable

SMM memory without enabling the SMI pins by setting SMAC. Setting SMI=1 will then map the

SMM memory region over main memory. The SMM region is physically mapped by the assertion

of SMADS# to allow memory access within the SMM region. A REP MOV instruction can then

be used to transfer the program to SMM memory. After initializing SMM memory, negate SMAC

to activate potential SMI#s.

SMM space can be located anywhere in the 4-GByte address range. However, if the location of

SMM space is beyond 1 Mbyte, the value in CS will truncate the segment above 16-bits when

stored to the stack. This would prohibit doing calls or interrupts from real mode without restoring

the 32-bit features of the 486 because of the incorrect return address on the stack.

; load SMM memory from system memory

include SMIMAC.INC

SMMBASE = 68000h

SMMSIZE = 4000h

;SMM SIZE is 16K

SMI

= 1 shl 1

SMAC

= 1 shl 2

MMAC

= 1 shl 3

mov

al, 0cdh

;index SMAR, SMM baseA31-A24

out

22h, al

;select

mov

al, 00h

;set high SMM address to 00

out

23h, al

;write value

mov

al, 0ceh

;index SMAR,SMM baseA23-A16

out

22h, al

;select

mov

al, 06h

;set mid SMM address to 06h

out

23h, al

;write value

mov

al, 0cfh

;SMAR,SMM baseA15-A12 & SIZE