🚀 go-pugleaf

I was just looking through the HP Vectra 286/386 BIOS Technical Reference
manual (HP Part No. 5959-7745) which does give *some* information about this.
There is nothing plastered over the manual to suggest it is HP confidential
so I guess yu could get a copy. I don't know how generic the information is
though.

To summarise the manual, as the previous reponses indicated, the only way to
get from protected mode to real mode is to do a processor reset. From software
this may be done by getting the 8042 keyboard controller to toggle the 80286
reset line, or by doing a jump to FFFFFFF0h. The question now arises for the 
BIOS "is this a power up, or is it a return from protected mode?"

On the Vectra the BIOS looks at CMOS addres 0Fh, which is the shutdown status
byte. I don't know whether this value is set by the BIOS when protected mode
is entered, or if you must do it yourself. I would *guess* the former.

As far as entering protected mode is concerned, you need INT15h with AH=89h.
From here on it gets beyond my knowledge of 80286. One point though. The
aforementioned manual says that address line A20 must be released prior to
entering protected mode, and it is your program's responsibility to do this
by issuing the approriate command to the 8042 keyboard controller.

I wish you the best of luck.....                    

Steve South.
HP Analytical Response Centre.
Amstelveen, Netherlands.

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