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.
Does anyone have ideas for why this won't work? Could the Qemu environment restrict this?
go_to_32bit_protected_mode: cli cld
movl %cr0, %eax orl $1, %eax movl %eax, %cr0
ljmp $MODE32_CS_SEL, $pmode
.code32 pmode: movl $MODE32_DS_SEL, %eax movl $16, %eax movw %ax, %ds movw %ax, %es movw %ax, %ss movw %ax, %fs movw %ax, %gs
// 32 bit protected mode GDT descriptors opRom_gdt_32_descs_start: .quad 0x0 .quad 0x00CF9B000000FFFF .quad 0x00CF93000000FFFF .quad 0x00CFFA000000FFFF .quad 0x00CFF2000000FFFF opRom_gdt_32_descs_end:
opRom_gdt_32: .word (opRom_gdt_32_descs_end - opRom_gdt_32_descs_start) - 1 .long opRom_gdt_32_descs_start Western Digital Corporation (and its subsidiaries) E-mail Confidentiality Notice & Disclaimer:
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