mirror of
https://github.com/gbowne1/ClassicOS.git
synced 2025-05-16 01:11:28 -07:00
68 lines
1.2 KiB
NASM
68 lines
1.2 KiB
NASM
[BITS 16]
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[ORG 0x7C00]
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start:
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cli
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xor ax, ax
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mov ds, ax
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mov es, ax
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mov ss, ax
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mov sp, 0x7C00
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call enable_a20
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call setup_gdt
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call switch_to_pm
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; ----------------------
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; A20 Gate Enable (Fast method)
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enable_a20:
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in al, 0x92
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or al, 0x02
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out 0x92, al
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ret
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; ----------------------
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; Set up a minimal GDT
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gdt_start:
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dq 0x0000000000000000 ; null descriptor
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dq 0x00CF9A000000FFFF ; code segment descriptor
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dq 0x00CF92000000FFFF ; data segment descriptor
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gdt_descriptor:
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dw gdt_end - gdt_start - 1 ; size of GDT
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dd gdt_start ; address of GDT
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gdt_end:
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setup_gdt:
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lgdt [gdt_descriptor]
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ret
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; ----------------------
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; Switch to Protected Mode
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switch_to_pm:
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cli
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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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jmp 0x08:protected_mode_entry
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; ----------------------
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; 32-bit Protected Mode Entry Point
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[BITS 32]
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protected_mode_entry:
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mov ax, 0x10 ; data segment selector
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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 esp, 0x90000
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; Kernel is assumed to be loaded at 0x100000
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jmp 0x08:0x100000
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; ----------------------
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; Boot signature
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times 510 - ($ - $$) db 0
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dw 0xAA55
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