On Sun, 14 Mar 2021, Liam Proven via cctalk wrote: > > I should also note, that the other way to get back to real mode from > > protected mode is via a triple-fault. What gets me (and I railed on > > Intel when I worked there for a time) that it still existing in the > > architecture even though they have a machine check architecture now > > (which while at IBM pushed Intel to implement for the '386!). > > (!)
Well, software exists that relies on the triple-fault feature for reboots including current versions of Linux (you can trigger a triple-fault in the real mode too). These days it is implemented by the southbridge catching the shutdown special cycle on PCI and asserting the reset pin to the CPU (the details might be slightly different for PCIe or HyperTransport). Back in the day I experimented with that stuff myself and all the weird ways to switch between modes with the x86, setting the IDTR in the real mode for interesting effects which would impress fellow students, etc. I ended up writing this: <http://www.rcollins.org/articles/pvi1/pvi1.html> as a result. I wrote a simple resident VM86 monitor for DOS too, just to fiddle with processor features. Also resetting the CPU with the shutdown code of 0xa put at the location 0xf of the RTC/NVRAM chip was the only way to get the family, model, and stepping ID in the EDX register for old processors that did not have the CPUID instruction (i.e. all 80386 and many 80486 implementations), unless the system BIOS clobbered it for no good reason in the short bypass code involved (sadly sometimes that did happen). I just double-checked my old DOS assembly code to see if I got the details right! NB I didn't know LOADALL would not work for switching from the protected to the real mode and did not find out about the instruction until after I already lost access to any 80286 hardware, so I never experimented with it myself. Maciej
