2025-07-04 03:10:55 +02:00
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// vim: et:sw=12:ts=12:sts=12
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.global _start
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.text
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2025-07-04 04:02:05 +02:00
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.code16
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2025-07-04 03:10:55 +02:00
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2025-07-04 15:29:41 +02:00
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_start: cli
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cld
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2025-07-04 04:02:05 +02:00
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mov %cs, %ax
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mov %ax, %ds
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2025-07-04 15:29:41 +02:00
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mov $message, %si
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call print
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call enable_a20
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lea message, %si
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2025-07-04 04:02:05 +02:00
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call print
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2025-07-04 15:29:41 +02:00
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lgdt gdt_ptr
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call enter_prot
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2025-07-04 04:02:05 +02:00
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1: hlt
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jmp 1b
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2025-07-04 15:29:41 +02:00
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enable_a20: // TODO more thorough implementation
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inb $0x92, %al
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or $2, %al
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outb %al, $0x92
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ret
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enter_prot: mov %cr0, %eax
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or $1, %al
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mov %eax, %cr0
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ret
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print: xor %bx, %bx
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2025-07-04 04:02:05 +02:00
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1: lodsb
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or %al, %al
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jz 2f
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2025-07-04 03:10:55 +02:00
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mov $0x0E, %ah
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int $0x10
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jmp 1b
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2025-07-04 04:02:05 +02:00
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2025-07-04 15:29:41 +02:00
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2: ret
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gdt: // entry 0: null descriptor
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.word 0
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.word 0
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.byte 0
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.byte 0
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.byte 0
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.byte 0
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// entry 1: code segment
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.word 0xFFFF
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.word 0
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.byte 0
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.byte 0b10011010
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.byte 0x8F
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.byte 0
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// entry 2: data segment
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.word 0xFFFF
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.word 0
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.byte 0
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.byte 0b10010010
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.byte 0x8F
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.byte 0
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.set gdt_size, .-gdt
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gdt_ptr: .word gdt_size-1
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.long gdt
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2025-07-04 04:02:05 +02:00
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2025-07-04 15:29:41 +02:00
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message: .asciz "Hello, World!\r\n"
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