谁能解释这段386为何出错吗？马上给分

%macro Descriptor 3     dw %2 & 0FFFFh       dw %1 & 0FFFFh       db (%1 >> 16) & 0FFh       dw ((%2 >> 8) & 0F00h) | (%3 & 0F0FFh)      db (%1 >> 24) & 0FFh  %endmacro      org 07C00h    jmp start8086gdt_desc:        Descriptor        0,                    0,                        0code_desc:        Descriptor        0,                    SegLen -1 ,        98h+4000h ; 修改段界限data_desc:        Descriptor        0B8000h,            0FFFFh,                    92h gdt_len            equ        $ - gdt_descgdt_ptr                dw        gdt_len - 1                            dd        0                    sel_code        equ        code_desc - gdt_descsel_data        equ        data_desc     - gdt_desc;[SECTION .s16][BITS 16]        start8086:    mov ax,cs     mov ds,ax     mov es,ax     mov ss,ax     mov sp,100h     ;*** initialize code descriptor ****    xor    eax, eax    mov ax, cs    shl    eax, 4    add eax, start386                        mov word [code_desc + 2], ax            shr eax, 16                                mov byte [code_desc + 4], al            mov byte [code_desc + 7], ah                ;*** load GDTR register ****    xor    eax, eax    mov ax, ds    shl eax, 4    add eax, gdt_desc    mov dword [gdt_ptr + 2 ], eax    lgdt [gdt_ptr]        cli     in al, 92h    or al, 00000010b    out 92h, al    mov eax, cr0    or eax, 1    mov cr0, eax    jmp dword sel_code:0        ;[SECTION .s32][BITS 32]        start386:        mov ax, sel_data    mov es,ax    ; 设置显示的属性    mov ah, 0Ch    mov al,'X'    mov [es:((80*0+0)*2)],ax ;个人感觉这样写行列容易看出    jmp $SegLen    equ        $ - start386 ; 添加这一句; 补齐剩余的字节和扇区结束标识times 510 - ($ - $$) db 0    dw 0xaa55