王爽汇编试验 11-------编写子程序（比较容易，主要学习跳转指令）

assume cs:code
data segment
    db "Beginner's All-purpose Symbolic Instruction Code.",0
data ends

code segment
start:
    mov ax,data
    mov ds,ax
    mov si,0
    call letterc
   
    mov ax,4c00h
    int 21h

letterc:
    push si
    push ax
begin:   
    mov al,[si]
    cmp al,'a'
    jna next
    cmp al,'z'
    jnb next
    and al,0dfh
    mov [si],al
next:
    cmp al,0
    je over
    inc si
    jmp begin
over:   
    pop ax
    pop si
    ret
   
code ends
end start