为什么我们在计算字符串长度时要切换段？

我提供了两种不同的计算字符串长度时切换段的实现。我不明白为什么我们要切换额外段和数据段，或者更准确地说，将数据段（寄存器 DS）的内容复制到额外段（寄存器 ES）。如果我删除这些行push ds，pop es结果不会改变，但是当我les di, [bp+4]用替换时mov di, [bp+4] ，屏幕上会显示消息，但带有一堆其他不同的字符。

为什么会发生这种情况？我是装配方面的新手。

org 0x0100
jmp start

message: db 'Hello world!', 0      ; First string to be concatenated


; Subroutine to calculate the length of a string
; Takes the segment and offset of a string as parameters
strlen:
    push bp
    mov bp, sp
    push es
    push cx
    push di
    les di, [bp+4]        ; Point es:di to string
    mov cx, 0xffff        ; Load maximum number in cx
    xor al, al            ; Load a zero in al
    repne scasb           ; Find zero in the string
    mov ax, 0xffff        ; Load maximum number in ax
    sub ax, cx            ; Find change in cx
    dec ax                ; Exclude null from length
    pop di
    pop cx
    pop es
    pop bp
    ret 4

; Subroutine to print a string
; Takes the x position, y position, attribute, and address of a null-terminated string as parameters
printstr:
    push bp
    mov bp, sp
    pusha
    push di
    push ds               ; Push segment of string
    mov ax, [bp+4]
    push ax               ; Push offset of string
    call strlen           ; Calculate string length

    cmp ax, 0             ; Is the string empty?
    jz exit               ; No printing if string is empty

    mov cx, ax            ; Save length in cx
    mov ax, 0xb800        ; Video memory base address
    mov es, ax            ; Point es to video base
    mov al, 80            ; Load al with columns per row
    mul byte [bp+8]       ; Multiply with y position
    add ax, [bp+10]       ; Add x position
    shl ax, 1             ; Turn into byte offset
    mov di, ax            ; Point di to required location
    mov si, [bp+4]        ; Point si to string
    mov ah, [bp+6]        ; Load attribute in ah
    cld                   ; Clear direction flag for auto-increment mode

 nextchar:
    lodsb                 ; Load next char in al
    stosw                 ; Print char/attribute pair
    loop nextchar         ; Repeat for the whole string

 exit:
    pop ds
    pop di
    popa
    pop bp
    ret 8

start:
    mov ax, 30
    push ax               ; Push x position
    mov ax, 20
    push ax               ; Push y position
    mov ax, 0x7           ; Blue on white attribute
    push ax               ; Push attribute
    mov ax, message
    push ax               ; Push address of message
    call printstr         ; Call the printstr subroutine

    mov ax, 0x4c00        ; Terminate program
    int 0x21

替代版本：

    push bp
    mov bp, sp
    pusha

    push ds
    pop es         ; load ds in es
    mov di, [bp+4] ; point di to string
    mov cx, 0xffff ; load maximum number in cx
    xor al, al 
    repne scasb 
    mov ax, 0xffff 
    sub ax, cx 
    dec ax 
    jz done 

    mov cx, ax
    ...