55 lines
1.4 KiB
NASM
55 lines
1.4 KiB
NASM
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SECTION .text
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USE16
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; switch to unreal mode; ds and es can address up to 4GiB
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unreal:
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cli
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lgdt [unreal_gdtr]
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push es
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push ds
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mov eax, cr0 ; switch to pmode by
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or al,1 ; set pmode bit
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mov cr0, eax
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jmp $+2
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; http://wiki.osdev.org/Babystep7
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; When this register given a "selector", a "segment descriptor cache register"
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; is filled with the descriptor values, including the size (or limit). After
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; the switch back to real mode, these values are not modified, regardless of
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; what value is in the 16-bit segment register. So the 64k limit is no longer
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; valid and 32-bit offsets can be used with the real-mode addressing rules
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mov bx, unreal_gdt.data
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mov es, bx
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mov ds, bx
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and al,0xFE ; back to realmode
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mov cr0, eax ; by toggling bit again
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pop ds
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pop es
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sti
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ret
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unreal_gdtr:
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dw unreal_gdt.end + 1 ; size
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dd unreal_gdt ; offset
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unreal_gdt:
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.null equ $ - unreal_gdt
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dq 0
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.data equ $ - unreal_gdt
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istruc GDTEntry
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at GDTEntry.limitl, dw 0xFFFF
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at GDTEntry.basel, dw 0x0
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at GDTEntry.basem, db 0x0
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at GDTEntry.attribute, db attrib.present | attrib.user | attrib.writable
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at GDTEntry.flags__limith, db 0xFF | flags.granularity | flags.default_operand_size
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at GDTEntry.baseh, db 0x0
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iend
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.end equ $ - unreal_gdt
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