Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: x86 setup rewrite tree ready for flamage^W review | Date | Sat, 12 May 2007 08:20:52 -0600 |
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Alexander van Heukelum <heukelum@mailshack.com> writes:
> On Thu, May 10, 2007 at 03:48:08PM -0700, H. Peter Anvin wrote:
> I can confirm that it works for at least one computer over here (a six > months old x86_64 machine with ATI ES1000-based on-board graphics). Some > non-vesa modes including a nice 100x30 one with 8x16 font are found by > the 'scan' option. No 100x60, however, but that is not a regression. > >> > I thought the 32-bit jump was required to come before the segment loads. >> > Does this code load values from the gdt, or are they just loaded as real >> > mode segments? As long as it does not crash it does not matter, because >> > head.S reloads them again. >> >> Once CR0.PE is set, segments are loaded from the GDT. > > I believe you :).
HPA is both right and wrong on this. The safe sequence for entering protected mode requires a jump immediately after setting PE in %cr0. To serialize the instruction stream and to be on an execution that is tested and guaranteed to work in cpus.
On a lot of processors you can get away with more then that, but Intel at least explicitly states in their "Software Developers Manual Volume 3: System Programming" in 9.9.1 "Switching to Protected Mode" that you need the jump immediately following enabling PE in cr0.
So while I was debugging and instrumenting code I would happily place an instruction before the jump so I could get output. However in production grade code I would not place anything between the setting of PE in cr0 and the first jump instruction.
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