Feat: Finnaly in 32-bits
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AfonsoCMSousa
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af903b190a
181
boot.asm
181
boot.asm
@ -1,101 +1,122 @@
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; File: boot.asm
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; File: boot.asm
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global _start
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[org 0x7c00] ; BIOS loads us at 0x7C00
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extern kmain
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[bits 16] ; We start in 16-bit Real Mode
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section .multiboot
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start:
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; Multiboot header
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mov bp, 0x7c00
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dd 0x1BADB002 ; magic number
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mov sp, bp
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dd 0x00000003 ; flags
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dd -(0x1BADB002 + 0x00000003) ; checksum
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call clear_screen
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section .text
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mov si, real_msg
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[bits 32]
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call print_string
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_start:
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mov esp, kernel_stack_top
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cli ; Clear interrupts
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; Load the gdt_ptr
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cli
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lgdt [gdt_ptr]
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mov eax, pml4_table
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lgdt [gdt_descriptor]
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mov cr3, eax ; Load the PML4 table into CR3
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mov eax, cr0
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; Link PML4 -> PDPT
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or eax, 0x1
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mov dword [pml4_table], pdpt_table + 0x3
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mov dword [pml4_table + 4], 0x0
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; Link PDPT -> PD
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mov dword [pdpt_table], pd_table + 0x3
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mov dword [pdpt_table + 4], 0x0
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; Link PD -> 2MB Physical Page 0
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mov dword [pd_table], 0x83
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mov dword [pd_table + 4], 0x0
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mov eax, cr4
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or eax, 0x20 ; Enable PAE
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mov cr4, eax
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mov ecx, 0xC0000080 ; IA32_EFER MSR
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rdmsr
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or eax, 0x00000100 ; Set LME (Long Mode Enable) bit
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wrmsr
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; Enable long mode
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mov eax, cr0
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or eax, 0x80000000 ; Set the Long Mode Enable (LME) bit
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mov cr0, eax
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mov cr0, eax
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; Jump to 64-bit code Segment
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; Far jump into 32-bit mode.
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jmp 0x08:long_mode_start
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jmp 0x08:init_pm
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section .bss
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; --- Helper: Print String (BIOS INT 0x10) ---
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; We define the stack size and labels in the uninitialized data section (.bss)
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print_string:
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kernel_stack_bottom:
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pusha
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; Reserve 16 KB for the stack
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mov ah, 0x0e ; INT 0x10 "Teletype Output" function
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resb 16 * 1024
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.loop:
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kernel_stack_top:
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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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pml4_table:
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real_msg: db '[REAL] - Loaded successfully.', 0x0D, 0x0A, 0
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; Define a simple PML4 table here (identity mapping for simplicity)
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; In a real kernel, you would set up proper paging structures
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resb 4096 ; Reserve 4 KB for the PML4 table
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pml4_end:
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pdpt_table:
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gdt_start:
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resb 4096 ; Reserve 4 KB for the PDPT table
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pdpt_end:
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pd_table:
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gdt_null: ; 8 bytes of zeros
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resb 4096 ; Reserve 4 KB for the PD table
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dd 0x0
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pd_end:
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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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section .data
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gdt_ptr:
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dw (gdt_end - gdt_start - 1) ; Limit (set by linker/code later)
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dq gdt_start ; Base Address (set by linker/code later)
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gdt_start: ; Start of the actual GDT entries
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; Entry 0: Null Descriptor
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dq 0x0000000000000000
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dq 0x00AF9A000000FFFF
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dq 0x00AF92000000FFFF
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gdt_end:
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gdt_end:
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section .text
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gdt_descriptor:
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[bits 64] ; Tell NASM to assemble in 64-bit mode
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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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long_mode_start:
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; ==================================================
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; 1. Pass GRUB's Multiboot info (in ebx) as the first C argument (in rdi)
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; 32-BIT PROTECTED MODE
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; We must use the 64-bit register RDI, but the data is in the 32-bit EBX.
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; ==================================================
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; Moving ebx to rdi automatically clears the upper 32 bits of rdi.
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[bits 32]
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mov rdi, rbx
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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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; 2. Call the C kernel's main function
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mov ebp, 0x90000 ; Update stack to a safe 32-bit area
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call kmain
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mov esp, ebp
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; 3. If kmain ever returns, halt the CPU
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; 6. Print "SoraOS Protected" directly to Video Memory
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hlt
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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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16
build.sh
16
build.sh
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echo ">>> Compiling bootloader"
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#!/bin/sh
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nasm boot.asm -o boot.o -f elf32
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export PATH=/usr/local/cross/bin:$PATH
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echo ">>> Compiling kernel"
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# 1. Assemble raw binary
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gcc kernel.c -o kernel.o -c -m32 -ffreestanding -nostdlib -no-pie
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echo ">>> Assembling boot.asm to boot.bin..."
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nasm -f bin boot.asm -o boot.bin
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echo ">>> Linking kernel"
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# 2. Run QEMU as a raw disk drive
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gcc -m32 -nostdlib -no-pie -T linker.ld -o SoraOS boot.o kernel.o
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echo ">>> Running QEMU with boot.bin..."
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qemu-system-x86_64 -drive format=raw,file=boot.bin
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echo "Done!"
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@ -1,4 +1,4 @@
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OUTPUT_FORMAT(elf32-i386)
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OUTPUT_FORMAT(binary)
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ENTRY(_start)
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ENTRY(_start)
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SECTIONS
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SECTIONS
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@ -21,7 +21,7 @@ SECTIONS
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/* Define the .data section immediately after .text */
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/* Define the .data section immediately after .text */
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.data :
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.data :
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{
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{
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/* Put all .data sections from all input files here */
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/* Put all .data sections §from all input files here */
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*(.data)
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*(.data)
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}
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}
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