ST486DX - SMM SOFTWARE CONSIDERATIONS

31

and restore instruction should be used. This can be accomplished by using the following code

example:

; Save the FPU state

mov

eax,CR0

or

eax,00000001h

mov

CR0,eax

;set the PE bit in CR0

jmp

$+2

;clear the prefetch que

db

66h

;do 32bit version of fnsave

fnsave [save_fpu]

;saves fpu state to

;the address DS:[save_fpu]

mov

eax,CR0

and

eax, 0FFFFFFFEh

;clear PE bit in CR0

mov

CR0,eax

;return to real mode

;now the SMM routine can do any FPU instruction.

;Restore the FPU state before executing a RSM

mov

eax,CR0

or

eax,00000001h

mov

CR0,eax

;set the PE bit in CR0

jmp

$+2

;clear the prefetch que

db

66h

;do 32bit version of fnsave

frstor [save_fpu]

;restore the FPU state

;Some assemblers may require

;use of the fnrstor instruction

mov

eax,CR0

and

eax, 0FFFFFFFEh

;clear PE bit in CR0

mov

CR0,eax

;return to real mode

Be sure that all interrupts are disabled before using this method for entering protected mode. Any

attempt to load a selector register while in protected mode will shutdown the CPU since no GDT is

set up. Setting up a GDT and doing a long jump to enter protected mode will also work correctly.

3.4

Initializing the SMM Environment

After entering SMM and saving the CPU registers that will be used by the SMM routine, a few

registers need to be initialized.

Segment registers need to be initialized if the CPU was operating in protected mode when the SMI

interrupt occurred. Segment registers that will be used by the SMM routine need to be loaded with

known limits before they are used. The protected mode application could have set a segment limit

to less than 64K. To avoid a protection error, all segment registers can be given limits of 4 GBytes.

This can be done with the SGS Thomson RSDC instruction and will allow access to the