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ld: warning: cannot find entry symbol start; defaulting to 00100000
o\main.o:main.c:(.text+0x7): undefined reference to `_main'

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gcc -c main.c -o o\main.o -nostdlib -nostdinc -fno-builtin -fno-stack-protector -fno-leading-underscore
nasm boot.asm -o o\boot.o -fcoff
ld -o bin\kernel.bin o\boot.o o\main.o -Tlink.ld

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ENTRY(start)
SECTIONS
{
    .text 0x100000 :
    {
        code = .;
        _code = .;
        __code = .;
        *(.text)
        . = ALIGN(4096);
    }

    .data :
    {
        data = .;
        _data = .;
        __data = .;
        *(.data)
        *(.rodata)
        . = ALIGN(4096);
    }

    .bss :
    {
        bss = .;
        _bss = .;
        __bss = .;
        *(.bss)
        . = ALIGN(4096);
    }

    end = .;
    _end = .;
    __end = .;
}

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int main()
{
    return 0xDEADBABA;
}

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MBOOT_PAGE_ALIGN    equ 1<<0
MBOOT_MEM_INFO      equ 1<<1
MBOOT_HEADER_MAGIC  equ 0x1BADB002
MBOOT_HEADER_FLAGS  equ MBOOT_PAGE_ALIGN | MBOOT_MEM_INFO
MBOOT_CHECKSUM      equ -(MBOOT_HEADER_MAGIC + MBOOT_HEADER_FLAGS)
[bits 32]
[global mboot]
[extern code]
[extern bss]
[extern end]

mboot:
    dd  MBOOT_HEADER_MAGIC
    dd  MBOOT_HEADER_FLAGS
    dd  MBOOT_CHECKSUM
    dd  mboot
    dd  code
    dd  bss
    dd  end
    dd  start

[extern main]
[global start]

start:
    push ebx
    cli
    call main
    jmp $

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nasm -f elf -o start.o start.S
gcc -Wall -O -fstrength-reduce -fomit-frame-pointer -finline-functions -nostdinc -fno-builtin -I./include -c -o main.o main.c
ld -T link.ld -o kernel.bin start.o main.o

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start.o: In function 'stublet':
start.S:(.text+0x29): undefined reference to '_main'