ST486DX ST Microelectronics, ST486DX Datasheet - Page 13

• Manufacturer Part Number: ST486DX
• Description: PROGRAMMING MANUAL
• Manufacturer: ST Microelectronics

3.
This section provides an overview of SGS-Thomson SMM coding and information helpful in devel-
oping SMM code.
3.1
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
SMI
SMAC = 1 shl 2
MMAC = 1 shl 3
SMM SOFTWARE CONSIDERATIONS
Enabling SMM
= 1 shl 1
mov
out
mov
out
mov
out
mov
out
mov
al, 0cdh
22h, al
al, 00h
23h, al
al, 0ceh
22h, al
al, 06h
23h, al
al, 0cfh
;SMM SIZE is 16K
;index SMAR, SMM baseA31-A24
;select
;set high SMM address to 00
;write value
;index SMAR,SMM baseA23-A16
;select
;set mid SMM address to 06h
;write value
;SMAR,SMM baseA15-A12 & SIZE
ST486DX - SMM SOFTWARE CONSIDERATIONS
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