汇编-显示我放到AL中的数值

按书上练习，有一些我看不懂。

.
.model flat,stdcall
option casemap:none
include \masmplus\include\windows.inc
include \masmplus\include\kernel32.inc
includelib \masmplus\lib\kernel32.lib
include \masmplus\include\user32.inc
includelib \masmplus\lib\user32.lib

.data
MsgBoxCaption db
MsgBoxText db ,,,,,,,,,,,,,,,,,
.code
start:
mov al,9Eh
call ShowAL

invoke MessageBox,NULL,addr MsgBoxText,addr MsgBoxCaption,MB_YESNOCANCEL
invoke ExitProcess,NULL

ShowAL proc
    mov dl,al

    shr al,cl
    mov cx,02h
    xor ebx,ebx
    mov esi,offset MsgBoxText

Low4bit:

    jbe larger

larger:
    add al,30h
    mov [esi],al
    inc esi
    mov al,dl
    and al,0FH
    loop Low4bit
    ret
ShowAL endp
end start