Alexander van Heukelum <[EMAIL PROTECTED]> 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. Eric - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
