Why does this old DOS program work?

This is a strange question since I have solved my problem but can't imagine why it works.

I have been trying to get an old DOS database to work on an XP machine. I discovered that it would work if run from a CD-ROM instead of from the hard drive. I'm no systems expert but I assume that it can't run on a drive as big as newer computers have. I created a CD with the program and the data. The batch file to run the program is on the C: drive and has an early command
CD D:\progname (D: is the CD-DVD drive)
The program and data are on both the CD and the C: drive.

This worked. Then I took the CD to another XP machine to check that it would work there. When I came back to the first one I was surprised that the program still worked without the CD in it. I turned the computer off and on to make sure it was not in memory. It still worked. Why is that? The same occurred on the second computer. I got btrieve errors when I tried to run from the hard drive before creating the CD, but now with the batch file saying cd D: it works even if D: is empty.

Stumped

 

I'd need to see the entire batch file to be entirely sure, but it would seem your program is running off the hard drive if the batch file is run.

The CD D:\progname command changes the current directory for drive D, but without a command to change the current drive, that will remain as C, and ITS current directory is of course, unchanged.

 

Of course. Shows how long it is since I've used DOS. Now I'm totally stumped as to why I got error messages before but not since I thought I was running from the CD. I suppose that I could recover the old directories from the recycle bin and play with them, but is it worth the bother since everything works now?

Many thanks for your explanation.