47 lines
974 B
NASM
47 lines
974 B
NASM
SECTION .text
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USE16
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protected_mode:
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.func: dd 0
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.entry:
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; disable interrupts
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cli
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; load protected mode GDT
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lgdt [gdtr]
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; set protected mode bit of cr0
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mov eax, cr0
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or eax, 1
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mov cr0, eax
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; far jump to load CS with 32 bit segment
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; (we are in 32-bit mode, but instruction pipeline
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; has 16-bit instructions.
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jmp gdt.pm32_code:.inner
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; gdt.pm32_code is a multiple of 8, so it always ends with three zero bits.
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; The GDT spec abuses this fact, and uses these last three bits to store other
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; data (table type and privilege). In this case, 000 is what we need anyway.
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;
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; Also note that CS isn't an address in protected mode---it's a GDT descriptor.
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USE32
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.inner:
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; load all the other segments with 32 bit data segments
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mov eax, gdt.pm32_data
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mov ds, eax
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mov es, eax
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mov fs, eax
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mov gs, eax
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mov ss, eax
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; jump to specified function
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mov eax, [.func]
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jmp eax
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