[SeaBIOS] Entering protected mode (and back) in option ROM
Robert.Moore at hgst.com
Tue Mar 1 00:05:44 CET 2016
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?
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
.word (opRom_gdt_32_descs_end - opRom_gdt_32_descs_start) - 1
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