protect mode 1 switch to protect mode

he Global Descriptor Table (GDT) is a binary data structure specific to the IA-32 and x86-64 architectures. It contains entries telling the CPU about memory segments.

Segment Descriptor

63 56	55 52	51 48	47 40	39 32
Base 31 24	Flags 3 0	Limit 19 16	Access Byte 7 0	Base 23 16
31 16	15 0
Base 15 0	Limit 15 0

1. Entry 0 in a descriptor table, or the Null Descriptor
2. A DPL 0 Code Segment descriptor
3. A Data Segment descriptor
   增加gdt，jmp to protected mode，使用命令target remote | qemu-system-x86_64 -hda ./boot.bin -S -gdb stdio
   输入c继续，ctrl+c停止，输入layout asm查看执行的汇编代码。

enable a20 line

    in al, 0x92
    or al, 2
    out 0x92, al

cross compile

https://wiki.osdev.org/GCC_Cross-Compiler
gdb进入后
add-symbol-file ./build/kernelfull.o 0x100000添加符号，break _start设置断点，目标连接后停在了0x100000 _start 位置。