中国DOS联盟

我最近买了一张无忧启动盘。在研究它的启动时有一些问题。希望各位大师给我解答一下。在autoexec.bat文件里有一些语句不懂是什么意思。
@ECHO OFF
PROMPT $P$G
set temp=c:\
set tmp=c:\

SET EXPAND=YES
SET DIRCMD=/O:N
set LglDrv=27 * 26 Z 25 Y 24 X 23 W 22 V 21 U 20 T 19 S 18 R 17 Q 16 P 15
set LglDrv=%LglDrv% O 14 N 13 M 12 L 11 K 10 J 9 I 8 H 7 G 6 F 5 E 4 D 3 C
call setramd.bat %LglDrv%
path=%RAMD%:\;%RAMD%:\BAT;%CDROM%:\;%CDROM%:\UCDOS;%CDROM%:\BOOT;A:\;C:\;
copy get.exe %RAMD%:\ > NUL
copy attrib.exe %RAMD%:\ > NUL
copy command.com %RAMD%:\ > NUL
copy edit.com %RAMD%:\ > NUL
copy format.com %RAMD%:\ > NUL
copy fdisk.exe %RAMD%:\ > NUL
copy deltree.exe %RAMD%:\ > NUL
copy restart.com %RAMD%:\ > NUL
copy readme.exe %RAMD%:\ > NUL
copy mouse.* %RAMD%:\ > NUL
copy xxcopy.exe %RAMD%:\ > NUL
rem copy extract.exe %RAMD%:\ > NUL
set comspec=%RAMD%:\command.com
LH MSCDEX.EXE /D:MSCD000 /L:%CDROM%

CDA %CDROM% > NUL
IF errorlevel 1 GOTO NEXTCD
IF NOT EXIST %CDROM%:\BOOT\TW.EXE GOTO NEXTCD
GOTO BOOT

:NEXTCD
CALL CHCDROM.BAT C D E F G H I J K L M N O P Q R S T U V W X Y Z
GOTO BOOT

:BOOT
LH SMARTDRV.EXE
LH DOSKEY.COM > NUL
Set LglDrv=

MD %RAMD%:\TEMP
XXCOPY /E /H %CDROM%:\BOOT\EBD %RAMD%:\ > NUL
%RAMD%:
CD \
LH MOUSE.COM > NUL
MENU.BAT

其中不懂的语句有如下：
1、 SET EXPAND=YES
2、 SET DIRCMD=/O:N
3、 set LglDrv=27 * 26 Z 25 Y 24 X 23 W 22 V 21 U 20 T 19 S 18 R 17 Q 16 P 15
4、 set LglDrv=%LglDrv% O 14 N 13 M 12 L 11 K 10 J 9 I 8 H 7 G 6 F 5 E 4 D 3 C

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