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?












        lgdtw %cs:opRom_gdt_32


        movl %cr0, %eax

        orl $1, %eax

        movl %eax, %cr0


        ljmp $MODE32_CS_SEL, $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


.quad 0x0

.quad 0x00CF9B000000FFFF

.quad 0x00CF93000000FFFF

.quad 0x00CFFA000000FFFF

.quad 0x00CFF2000000FFFF




.word  (opRom_gdt_32_descs_end - opRom_gdt_32_descs_start) - 1

.long  opRom_gdt_32_descs_start

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