汇编问题，这几个问怎么做

汇编问题，这几个问怎么做

题目程序
mov cx,400h
mov ax,2000h
mov ds,ax
mov si,0
xor ax,ax
again:
add al,[si]
adc ah,0
inc si
loop again

每个单元内容加2
mov cx,400h
mov ax,2000h
mov ds,ax
mov si,0
mov al,2
again:
add [si],al
inc si
loop again

每个单元内容加1
mov cx,400h
mov ax,2000h
mov ds,ax
mov si,0
mov al,1
again:
add [si],al
inc si
loop again

统计'a'的个数存AL
mov cx,400h
mov ax,2000h
mov ds,ax
mov si,0
mov ah,'a'
xor al,al
again:
cmp ah,[si]
jne next
inc al
next:
inc si
loop again

就是这样做喽，太多问题 ，就不一一写了，有哪个不懂再问追问变量SUM开始是什么意思追答就是
mov cx,400h
mov ax,2000h
mov ds,ax
mov si,offset sum
xor ax,ax
again:
add al,[si]
adc ah,0
inc si
loop again