Hello,

 

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?

 

Thanks.

 

Bob

 

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go_to_32bit_protected_mode:

        cli

        cld

 

        lgdtw %cs:opRom_gdt_32

 

        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

 

        sti

        ret

 

// 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