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<p class="MsoNormal">Hello,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I have a need to access 32 bit address space from within a 16 bit option ROM I’m writing. As a test I added code to load the gdt as required to set up 32 bit protected mode but I’ve encountered a problem. Below are my code and gdt structures.
My issue is that when the ljmp is executed the model reboots (this is running on seabios under Qemu). I suspect a GP violation but can’t seem to put my finger on it. I tried defining an idt containing a handler that hangs (i.e. jmp .) but was never successful
in seeing that work.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Does anyone have ideas for why this won’t work? Could the Qemu environment restrict this?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Bob<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">--------------------------------------------<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">go_to_32bit_protected_mode:<o:p></o:p></p>
<p class="MsoNormal"> cli<o:p></o:p></p>
<p class="MsoNormal"> cld<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> lgdtw %cs:opRom_gdt_32<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> movl %cr0, %eax<o:p></o:p></p>
<p class="MsoNormal"> orl $1, %eax<o:p></o:p></p>
<p class="MsoNormal"> movl %eax, %cr0<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> ljmp $MODE32_CS_SEL, $pmode<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> .code32<o:p></o:p></p>
<p class="MsoNormal">pmode:<o:p></o:p></p>
<p class="MsoNormal"> movl $MODE32_DS_SEL, %eax<o:p></o:p></p>
<p class="MsoNormal"> movl $16, %eax<o:p></o:p></p>
<p class="MsoNormal"> movw %ax, %ds<o:p></o:p></p>
<p class="MsoNormal"> movw %ax, %es<o:p></o:p></p>
<p class="MsoNormal"> movw %ax, %ss<o:p></o:p></p>
<p class="MsoNormal"> movw %ax, %fs<o:p></o:p></p>
<p class="MsoNormal"> movw %ax, %gs<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> sti<o:p></o:p></p>
<p class="MsoNormal"> ret<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">// 32 bit protected mode GDT descriptors<o:p></o:p></p>
<p class="MsoNormal">opRom_gdt_32_descs_start:<o:p></o:p></p>
<p class="MsoNormal">.quad 0x0<o:p></o:p></p>
<p class="MsoNormal">.quad 0x00CF9B000000FFFF<o:p></o:p></p>
<p class="MsoNormal">.quad 0x00CF93000000FFFF<o:p></o:p></p>
<p class="MsoNormal">.quad 0x00CFFA000000FFFF<o:p></o:p></p>
<p class="MsoNormal">.quad 0x00CFF2000000FFFF<o:p></o:p></p>
<p class="MsoNormal">opRom_gdt_32_descs_end:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">opRom_gdt_32:<o:p></o:p></p>
<p class="MsoNormal">.word (opRom_gdt_32_descs_end - opRom_gdt_32_descs_start) - 1<o:p></o:p></p>
<p class="MsoNormal">.long opRom_gdt_32_descs_start<o:p></o:p></p>
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