ST486DX - SMM SOFTWARE CONSIDERATIONS

34

bt

ax,2

;rep instruction?

;(result to Carry)

adc

ecx,0

;if so, increment ecx

test

al,1 shl 1

;test bit 1 to see

;if an OUTS or INS

jnz

out_instr

; A port read (INS or IN) instruction caused the

; chipset to generate an SMI instruction.

; Restore EDI from SMM header.

mov

edi, dword ptr cs:[SMI_ESIEDI]

jmp

common1

; A port write (OUTS or OUT) instruction caused the

; chipset to generate an SMI instruction.

; Restore ESI from SMM header.

out_instr:

mov

esi, dword ptr cs:[SMI_ESIEDI]

common1:

3.7

I/O Port Shadowing and Emulation

Some system peripherals contain write-only ports. In a system that does power management, these

peripherals need to be powered off and then reinitialized when their functions are needed later. The

SGS Thomson SMM implementation makes it very easy to monitor the last value written to spe-

cific I/O ports. This process is known as shadowing. If the system can generate an SMI whenever

specific I/O addresses get accessed, the SMM routine can, transparently to the system, monitor the

port activity. The SMM header contains the address of the I/O write as well as the data. In addi-

tion, information is saved which indicates whether it is a byte, word or dword write. With this in-

formation, shadowing system write-only ports becomes trivial.

Some peripheral components contain registers that must be programmed in a specific order. If an

SMI interrupt occurs while an application is accessing this type of peripheral, the SMI routine must

be sure to reload the peripheral registers to the same stage before returning to normal mode. If the

SMM routine needs to access such a peripheral, the previous normal mode state must be restored.

The previous accesses that were shadowed by previous SMM calls can be used to reload the periph-

eral registers back to the stage where the application was interrupted. The application can then con-

tinue where it left off accessing the peripheral.

In a similar way, the SGS-Thomson SMM implementation allows the SMM routine to emulate the

function of peripheral components in software.