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 start8086


gdt_desc:        Descriptor        0,                    0,                        0
code_desc:        Descriptor        0,                    SegLen -1 ,        98h+4000h ; 修改段界限
data_desc:        Descriptor        0B8000h,            0FFFFh,                    92h 


gdt_len            equ        $ - gdt_desc


gdt_ptr                dw        gdt_len - 1        
                    dd        0                    

sel_code        equ        code_desc - gdt_desc
sel_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