86 lines
1.1 KiB
NASM
86 lines
1.1 KiB
NASM
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section .realmode
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global pxe_call
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global set_pxe_fp
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set_pxe_fp:
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mov eax, [esp + 4]
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mov [pxe_call.pxe_fp], eax
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ret
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pxe_call:
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; Save GDT in case BIOS overwrites it
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sgdt [.gdt]
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; Save non-scratch GPRs
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push ebx
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push esi
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push edi
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push ebp
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mov ebx, eax
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; Jump to real mode
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jmp 0x08:.bits16
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.bits16:
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bits 16
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mov ax, 0x10
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov ss, ax
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mov eax, cr0
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and al, 0xfe
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mov cr0, eax
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jmp 0x00:.cszero
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.cszero:
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xor ax, ax
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mov ss, ax
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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sti
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push dx
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push cx
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push bx
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call far [.pxe_fp]
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add sp, 6
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mov bx, ax
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cli
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; Restore GDT
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lgdt [ss:.gdt]
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; Jump back to pmode
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mov eax, cr0
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or al, 1
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mov cr0, eax
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jmp 0x18:.bits32
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.bits32:
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bits 32
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mov ax, 0x20
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov ss, ax
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mov eax, ebx
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; Restore non-scratch GPRs
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pop ebp
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pop edi
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pop esi
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pop ebx
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; Exit
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ret
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align 16
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.pxe_fp: dd 0
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.esp: dd 0
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.gdt: dq 0
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