i have questions about smm mode.

where does its code is?
ive read thats in other memory, not avaiable for normal physical memory code.

i can enter it by triggering some io port, and cpu will execute handler, and return control.


how do i gain any control over it?

can i have really simple example in fasm?
i want to only set ax to some value, and read/write physical memory from locaiton.

There is some SMM related posts and code already on this board.

Once SMM mode is setup, access to it is locked - so once BIOS initialization has done it's job, you can't mess around with it.

Some mobos allow access to the memory. A few special I/O port writes and it unlocks. IIRC bitRAKE posted some info a few years ago.

f0dder,

I've just looked into SMRAMC register (B:D:F==0:0:0, reg. 9Dh) of my 82865PE MCH, it contains 0Ah, i.e. SMRAMC.D_LCK==0 (bit 4 locks SMM space if set). It means that I probably can use SMRAMC.D_OPEN bit to access SMRAM in 0A0000h…0BFFFFh range (SMRAMC.C_BASE_SEG, bits 2…0 of SMRAMC, are hardwired to 010b) right now (though I'm not ready for this yet ).

baldr: interesting - I thought BIOSes would be locking access to SMM after initialization. I wonder how often it's left unlocked?

f0dder,

Well, it's easy to check: get datasheet for your north bridge and use something like WPCREdit. My boxes (845G, 865PE and P965, all Gigabyte, latest BIOS) have this bit cleared. Plenty of room to experiment.

well, there is the possebility to suppress a access to SMM,
for example to be safe against Rootkits. but it is Bios dependent whether it is set or not...(and maybe a Windows driver sets it.)

Theory says BIOSes should lock access to SMRAM once set up, but I've read no one really did it before 2006 and I wouldn't wonder if many still wouldn't do it now.

to enter SMM, an external pin of the CPU should be activated.
i don't remember wich one, but i am sure of it.
exactlly as A20 is activated via the chipset (the chipset is responsible of all the effects of CPU buses (data bus, address bus and control bus).
the only equipment on the mother board who have access to the pins of the CPU are the chipset and the power supply.

then, the pin for smm is connected to the chipset, and it is the chipset that will fire this pin under some conditions depending on the specs.

edfed: it is possible to trigger SMM by write to an I/O port (good-old OUT instruction)