mirror of
https://github.com/subhobhai943/SUB-OS.git
synced 2026-10-25 16:52:29 +00:00
273 lines
6.3 KiB
NASM
273 lines
6.3 KiB
NASM
BITS 16
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ORG 0x7E00
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start2:
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cli
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; Ensure segment registers are zero
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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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sti
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; Save boot drive number passed from stage 1 in DL
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mov [boot_drive], dl
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; Print Stage 2 startup message
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mov si, msg_stage2_start
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call print_string_16
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; -------------------------------------------------------------
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; Step 1: Enable A20 Line
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; -------------------------------------------------------------
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mov si, msg_a20
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call print_string_16
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in al, 0x92
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test al, 2
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jnz .a20_done
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or al, 2
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and al, 0xFE
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out 0x92, al
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.a20_done:
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; -------------------------------------------------------------
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; Step 2: Detect Memory via INT 15h, EAX=0xE820
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; Destination: 0x9000 (count at 0x9000, entries at 0x9008)
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; -------------------------------------------------------------
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mov si, msg_mmap
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call print_string_16
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; Zero out count at 0x9000
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mov dword [0x9000], 0
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mov dword [0x9004], 0
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mov di, 0x9008 ; Memory map entries destination
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xor ebx, ebx ; Continuation value
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xor bp, bp ; Entry counter
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mov edx, 0x534D4150 ; 'SMAP' signature
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mov eax, 0xE820
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mov ecx, 24 ; Buffer size
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int 0x15
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jc .mmap_error
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.mmap_loop:
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inc bp
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add di, 24
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test ebx, ebx
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jz .mmap_finished
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mov edx, 0x534D4150
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mov eax, 0xE820
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mov ecx, 24
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int 0x15
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jc .mmap_finished
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jmp .mmap_loop
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.mmap_error:
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; If E820 failed, create 1 fallback entry (128MB usable)
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mov dword [0x9000], 1
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mov dword [0x9004], 0
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mov dword [0x9008], 0x100000 ; base = 1MB
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mov dword [0x900C], 0
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mov dword [0x9010], 0x07F00000 ; length = 127MB
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mov dword [0x9014], 0
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mov dword [0x9018], 1 ; type = 1 (USABLE)
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mov dword [0x901C], 1
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jmp .mmap_store_done
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.mmap_finished:
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mov word [0x9000], bp
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mov word [0x9002], 0
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mov dword [0x9004], 0
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.mmap_store_done:
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; -------------------------------------------------------------
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; Step 3: Load Kernel from Disk to 0x10000 (Conventional RAM)
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; 12 chunks x 64 sectors = 768 sectors (384 KB)
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; -------------------------------------------------------------
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mov si, msg_kernel_load
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call print_string_16
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mov word [kernel_dap_off], 0x0000
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mov word [kernel_dap_seg], 0x1000
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mov dword [kernel_dap_lba], 16
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mov dword [kernel_dap_lba + 4], 0
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mov cx, 6 ; 6 chunks * 64 sectors = 384 sectors (192 KB)
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.kernel_chunk_loop:
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push cx
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mov ah, 0x42
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mov dl, [boot_drive]
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mov si, kernel_dap
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int 0x13
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jc .kernel_disk_error
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; Advance LBA by 64 sectors
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add dword [kernel_dap_lba], 64
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; Advance Segment by 0x800 (64 sectors * 512 bytes = 32768 bytes = 0x800 paragraphs)
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add word [kernel_dap_seg], 0x0800
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pop cx
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loop .kernel_chunk_loop
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.kernel_loaded_ok:
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; -------------------------------------------------------------
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; Step 4: Set up Identity Paging for Long Mode (All 4 GB)
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; -------------------------------------------------------------
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; Clear 24 KB for 6 page tables (0x1000 - 0x6FFF)
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mov ax, 0x0000
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mov es, ax
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mov di, 0x1000
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mov cx, 6144
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xor eax, eax
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rep stosd
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; PML4[0] -> PDPT at 0x2000
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mov dword [0x1000], 0x2003
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mov dword [0x1004], 0
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; PDPT -> 4 PDs
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mov dword [0x2000], 0x3003
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mov dword [0x2004], 0
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mov dword [0x2008], 0x4003
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mov dword [0x200C], 0
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mov dword [0x2010], 0x5003
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mov dword [0x2014], 0
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mov dword [0x2018], 0x6003
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mov dword [0x201C], 0
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; Map 4GB (2048 x 2MB pages)
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mov di, 0x3000
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mov eax, 0x00000083
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mov cx, 2048
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.map_4gb_loop:
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mov [di], eax
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mov dword [di + 4], 0
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add eax, 0x200000
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add di, 8
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loop .map_4gb_loop
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; -------------------------------------------------------------
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; Step 5: Switch to 32-bit Protected Mode
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; -------------------------------------------------------------
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mov si, msg_entering_pm
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call print_string_16
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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, 1
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mov cr0, eax
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jmp 0x08:pm_entry
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.kernel_disk_error:
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mov si, msg_kernel_err
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call print_string_16
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.halt16:
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cli
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hlt
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jmp .halt16
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print_string_16:
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pusha
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mov ah, 0x0E
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.p_loop:
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lodsb
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test al, al
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jz .p_done
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int 0x10
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jmp .p_loop
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.p_done:
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popa
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ret
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; -----------------------------------------------------------------
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; 32-Bit Protected Mode Entry Point
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; -----------------------------------------------------------------
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BITS 32
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pm_entry:
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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 esp, 0x7C00
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; Copy kernel from 0x10000 to 0x100000 (1MB mark) - 192 KB (384 sectors)
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mov esi, 0x10000
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mov edi, 0x100000
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mov ecx, (384 * 512) / 4
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rep movsd
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; Enable PAE in CR4
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mov eax, cr4
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or eax, (1 << 5)
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mov cr4, eax
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; Load CR3
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mov eax, 0x1000
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mov cr3, eax
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; Enable Long Mode in EFER MSR
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mov ecx, 0xC0000080
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rdmsr
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or eax, (1 << 8)
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wrmsr
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; Enable Paging and Protected Mode
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mov eax, cr0
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or eax, (1 << 31) | (1 << 0)
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mov cr0, eax
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jmp 0x18:lm_entry
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; -----------------------------------------------------------------
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; 64-Bit Long Mode Entry Point
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; -----------------------------------------------------------------
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BITS 64
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lm_entry:
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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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; Arguments: RDI = 0x9008 (entries), RSI = count at 0x9000
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mov rdi, 0x9008
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mov rsi, [0x9000]
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mov rax, 0x100000
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jmp rax
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; -----------------------------------------------------------------
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; Data Section
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; -----------------------------------------------------------------
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BITS 16
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align 4
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boot_drive: db 0
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msg_stage2_start: db "Stage 2: Loaded", 13, 10, 0
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msg_a20: db "Stage 2: A20 enabled", 13, 10, 0
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msg_mmap: db "Stage 2: Memory map detected", 13, 10, 0
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msg_kernel_load: db "Stage 2: Loading kernel...", 13, 10, 0
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msg_entering_pm: db "Stage 2: Jumping to 64-bit mode...", 13, 10, 0
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msg_kernel_err: db "Stage 2: Kernel Disk Read Error!", 13, 10, 0
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align 4
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kernel_dap:
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db 0x10
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db 0
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dw 64 ; 64 sectors per chunk (32 KB)
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kernel_dap_off:
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dw 0x0000
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kernel_dap_seg:
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dw 0x1000
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kernel_dap_lba:
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dq 16
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%include "gdt.asm"
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