IBM 5150 Manual To Operations page 80

If Your Computer Is On
1
Insert the DOS Diskette in drive A: and close the drive
door.
2
Press and hold Ctrl and Alt, and then press Del. Then
release them all:
OBO
+
[It]]]
andthen
(IIJ]J
3
You will see the diskette drive "in use" light on while
~
DOS is being read, and you may hear some clicks and
whirs.
Note that you will see these three keys used again in the
section "To Start DOS Again." This is the same-that is,
use Ctrl
+
Alt
+
Del to start DOS if your computer was
already on and you were running something else or to
restart DOS. This is called System Reset.
DOS 3-6