There are a couple of straight foreword ways to emulate an A: drive.

If you have a memory card of some type you may be able to set it as drive A: in Disk Management (under Computer management). Some of these can cards be made bootable.

You could also use the subst command. This substitutes a directory for a drive name. With subst you’d make a directory, say c:\FakeADrive and by use of the subst command it would that appear as a:\

I don’t think you can make this kind of drive bootable.

For details, open a command window and type subst /?