207 lines
4.5 KiB
NASM
Executable File
207 lines
4.5 KiB
NASM
Executable File
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GDT_MEMORY_AREA equ 0x20000
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GDT_ENTRIES_CAPACITY equ 8
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GDT_ENTRY_SIZE equ 8
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db "gdt_set_segment"
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; [Furthest from EBP]
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; 3. Bool (Lowest Address) Task State Segment (1) or Code/Data (0)
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; Bool (Second-Lowest) Executable (1) or Not (0)
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; Bool (Second-Highest) Stack (1) or Heap (0) Page
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; Bool (Highest Address) Readable AND Writeable (1) or only what's necessary (0)
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; 2. U32 Number of pages
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; 1. Ptr32 Base address
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; 0. U32 Index of Segment (0 is not allowed)
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; [Nearest to EBP]
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gdt_set_segment:
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.prolog:
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push dword esi
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sub esp, 64
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mov esi, esp
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mov [esi + (64 - 4)], eax
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mov [esi + (64 - 8)], ebx
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mov [esi + (64 - 12)], ecx
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mov [esi + (64 - 16)], edx
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mov [esi + (64 - 20)], edi
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.get_work_address:
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mov eax, [ebp - 4]
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mov edx, GDT_ENTRY_SIZE
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mul edx
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mov [esi], eax
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.prepare_flags_and_access_byte:
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; Access Privilege
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; [Highest]
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; 7. Present Bit (Is in RAM: 1, is swapped out: 0)
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; 5+6. Descriptor Privilege Level (Kernel: 00, User; 11)
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; 4. Descriptor Type (0: System Segment, 1: Code or Data Segment)
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; 3. Is Executable? (Yes: 1, No: 0)
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; 2. Grows Up (Heap) / Grows Down (Stack: 0, Heap, 1)
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; 1. Secondary Access Bit (whether Read or Write is possible
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; even when it isn't strictly necessary for the segment type)
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; 0. Dirty Bit (Was Accessed: 1, Still Clean: 0)
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; [Lowest]
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xor al, al
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cmp [ebp - 16], byte 0
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je .after_task_segment_bit_setter
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or al, (1 << 4)
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.after_task_segment_bit_setter:
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cmp [ebp - 15], byte 0
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je .after_executable_bit_setter
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or al, (1 << 3)
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.after_executable_bit_setter:
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cmp [ebp - 13], byte 0
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je .after_direction_bit_setter
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or al, (1 << 2)
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.after_direction_bit_setter:
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cmp [ebp - 12], byte 0
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je .after_read_write_bit_setter
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or al, (1 << 1)
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.after_read_write_bit_setter:
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; Flags
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; [Highest]
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; 3. Address and Size Granularity (Bytes: 0, Pages: 1)
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; 2. 16-Bit or 32-Bit selector (0: 16-Bit, 1: 32-Bit)
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; 1. Long Mode / 64-Bit (
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; 0. Reserved, should be 0
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; [Lowest]
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mov ah, (0b00001100 << 4)
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; OR the uppermost 4 bits of the Limit into the
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; appropriate position in the byte of the flags.
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mov ecx, [ebp - 12]
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shr ecx, 16
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and ecx, 0b00001111
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or al, cl
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; Store the Access Byte and the Flags into the entry.
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mov ebx, [esi]
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mov [ebx + 5], al ; Access Byte
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mov [ebx + 6], ah ; Flags + Limit
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mov eax, [ebp - 8]
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mov [ebx + 2], ax
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shr eax, 16
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mov [ebx + 3], al
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mov [ebx + 7], ah
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mov ax, [ebp - 12]
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mov [ebx], ax
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.epilog:
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mov ebx, [esi + (64 - 8)]
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mov ecx, [esi + (64 - 12)]
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mov edx, [esi + (64 - 16)]
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mov edi, [esi + (64 - 20)]
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add esp, 64
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pop dword esi
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ret
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db "load_flat_gdt"
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; [Furthest from EBP]
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; 0. 4B Return address
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; [Nearest to EBP]
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load_flat_gdt:
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.prolog:
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push dword esi
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sub esp, 64
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mov esi, esp
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mov [esi + (64 - 8)], ebx
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mov [esi + (64 - 12)], ecx
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mov [esi + (64 - 16)], edx
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mov [esi + (64 - 20)], edi
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mov eax, [ebp - 4]
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mov [esi + 20], eax
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.create_gdt:
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push ebp
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mov ebp, esp
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push dword (GDT_MEMORY_AREA)
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push dword (GDT_ENTRIES_CAPACITY * GDT_ENTRY_SIZE)
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push dword 0
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call mem_set
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mov esp, ebp
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pop ebp
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; Code Segment
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push ebp
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mov ebp, esp
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push dword 1
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push dword 0xa00
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push dword 0xfffff
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push byte 1
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push byte 0
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push byte 1
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push byte 0
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call gdt_set_segment
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mov esp, ebp
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pop ebp
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; Stack Segment
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push ebp
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mov ebp, esp
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push dword 2
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push dword 0
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push dword 0xfffff
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push byte 1
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push byte 1
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push byte 0
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push byte 0
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call gdt_set_segment
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mov esp, ebp
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pop ebp
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; Data Segment
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push ebp
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mov ebp, esp
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push dword 3
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push dword 0
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push dword 0xfffff
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push byte 1
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push byte 0
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push byte 1
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push byte 0
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call gdt_set_segment
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mov esp, ebp
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pop ebp
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; Write the GDT Descriptor
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cli
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mov [esi + 10], dword GDT_MEMORY_AREA
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mov [esi + 8], word 31
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mov eax, esi
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add ebx, 8
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lgdt [ebx]
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sti
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mov ax, (3 << 4)
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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 ax, (2 << 4)
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mov ss, ax
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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 dword (1 << 4):.epilog
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.epilog:
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mov ebx, [esi + (64 - 8)]
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mov ecx, [esi + (64 - 12)]
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mov edx, [esi + (64 - 16)]
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mov edi, [esi + (64 - 20)]
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mov esi, [esi + 20]
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jmp esi
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