123 lines
2.8 KiB
NASM
123 lines
2.8 KiB
NASM
; File: boot.asm
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[org 0x7c00] ; BIOS loads us at 0x7C00
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[bits 16] ; We start in 16-bit Real Mode
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start:
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mov bp, 0x7c00
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mov sp, bp
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call clear_screen
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mov si, real_msg
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call print_string
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cli
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lgdt [gdt_descriptor]
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mov eax, cr0
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or eax, 0x1
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mov cr0, eax
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; Far jump into 32-bit mode.
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jmp 0x08:init_pm
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; --- Helper: Print String (BIOS INT 0x10) ---
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print_string:
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pusha
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mov ah, 0x0e ; INT 0x10 "Teletype Output" function
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.loop:
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lodsb ; Load byte at [SI] into AL, increment SI
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cmp al, 0 ; Check for null terminator
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je .done
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int 0x10 ; Call BIOS video interrupt
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jmp .loop
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.done:
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popa
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ret
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; --- Helper: Clear Screen ---
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clear_screen:
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pusha
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mov ah, 0x00 ; Set Video Mode function
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mov al, 0x03 ; 80x25 Text Mode
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int 0x10
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popa
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ret
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real_msg: db '[REAL] - Loaded successfully.', 0x0D, 0x0A, 0
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gdt_start:
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gdt_null: ; 8 bytes of zeros
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dd 0x0
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dd 0x0
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gdt_code: ; Code Segment (0x08)
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; Base=0, Limit=0xFFFFF, Access=0x9A, Flags=0xC
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dw 0xffff ; Limit (bits 0-15)
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dw 0x0 ; Base (bits 0-15)
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db 0x0 ; Base (bits 16-23)
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db 10011010b ; Access Byte (0x9A)
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db 11001111b ; Flags (0xC) + Limit (bits 16-19)
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db 0x0 ; Base (bits 24-31)
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gdt_data: ; Data Segment (0x10)
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; Base=0, Limit=0xFFFFF, Access=0x92, Flags=0xC
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dw 0xffff
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dw 0x0
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db 0x0
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db 10010010b ; Access Byte (0x92)
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db 11001111b ; Flags (0xC) + Limit
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db 0x0
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gdt_end:
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gdt_descriptor:
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dw gdt_end - gdt_start - 1 ; Size (Limit)
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dd gdt_start ; Start Address
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; ==================================================
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; 32-BIT PROTECTED MODE
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; ==================================================
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[bits 32]
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init_pm:
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; 5. Update Segment Registers
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; Now that we are in 32-bit, we must point all segment registers
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; to our new Data Segment (0x10)
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mov ax, 0x10
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mov ds, ax
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mov ss, 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 ebp, 0x90000 ; Update stack to a safe 32-bit area
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mov esp, ebp
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; 6. Print "SoraOS Protected" directly to Video Memory
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; We can't use BIOS interrupts anymore! We must write to 0xB8000.
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mov ebx, 0xb8000
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mov byte [ebx], 'S'
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mov byte [ebx+1], 0x0f ; White on Black
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mov byte [ebx+2], 'O'
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mov byte [ebx+3], 0x0f
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mov byte [ebx+4], 'R'
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mov byte [ebx+5], 0x0f
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mov byte [ebx+6], 'A'
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mov byte [ebx+7], 0x0f
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mov byte [ebx+8], ' '
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mov byte [ebx+9], 0x0f
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mov byte [ebx+10], 'P'
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mov byte [ebx+11], 0x0f
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mov byte [ebx+12], 'M'
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mov byte [ebx+13], 0x0f
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jmp $
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; --- The Magic Boot Sector Footer ---
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; Fill the rest of the 512 bytes with zeros
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times 510-($-$$) db 0
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; The BIOS signature (must be at the very end)
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dw 0xAA55
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