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From: gt8134b@prism.gatech.EDU (Howlin' Bob)
Newsgroups: comp.os.386bsd.development
Subject: Re: V86 mode & the BIOS (was Need advice: Which OS to port to?)
Message-ID: <109310@hydra.gatech.EDU>
Date: 18 Aug 93 17:20:33 GMT
References: <107181@hydra.gatech.EDU> <1993Aug4.073826.24956@fcom.cc.utah.edu> <107725@hydra.gatech.EDU> <1993Aug9.224939.19834@fcom.cc.utah.edu> <108137@hydra.gatech.EDU> <BLYMN.93Aug12205126@mallee.awadi.com.au> <109151@hydra.gatech.EDU> <1993Aug18.071548.5342@cn
Sender: news@prism.gatech.EDU
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rooij@bashful.isp.cft.philips.nl (Guido van Rooij) writes:
>gt8134b@prism.gatech.EDU (Howlin' Bob) writes:
>>Hmm. All you have to do to enter V86 mode is to set the VM bit in
>>the EFLAGS saved register. Are you saying that the kernel cannot
>>modify the process's saved EFLAGS?
>And then what happens when you want to do unpriviledged IO in vm86 mode?
The kernel receives a fault when a privileged instruction is executed in
V86 mode with IOPL < 3; it simply masks off the VM bit in the process's
saved EFLAGS and delivers the appropriate signal to the task (SIGSEGV);
other signals are also handled normally (SIGFPE,SIGILL).
>I guess it isn't that simple.
I was talking about *entering* V86 mode, mind you, but leaving it
isn't that difficult, either. There may be some stack fiddling
involved along the way.
--
Robert Sanders
Georgia Institute of Technology, Atlanta Georgia, 30332
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