OS specific support in ADA

[Rich Maggio <maggior@world2u.com>]

I am an experienced 'C' programmer.  For a course that I will be taking in the fall, I will need to learn ADA. 
 I have downloaded GNAT for DOS and I have poked around some tutorials to get familiar with the language 
philosophies, purpose, and over-all general structure.

I noticed that GNAT is available for a multitude of OS's as well as hardware platforms.  What kind of OS calls 
are supported in GNAT?  For example, can you call a DOS interrupt directly from an ADA program?  Can you 
utilize OS/2's or Win32 semaphore API's?  Or do you have to rely upon the ADA implementation of these OS 
features, hoping that the compiler's RTL didn't leave anything out.

Also, is it possible to do ISR's in DOS using the GNAT compiler?  I understand that GNAT supports calling 'C' 
functions from and ADA application.  I was wondering if functions that call the OS directly would need to be 
written in 'C' and these functions would need to be called from the ADA program.

My understanding is that ADA was originally developed as an embedded system language, so I was surprised when 
I saw no mention of ISR's or OS calls in any of the docs that I have read so far.

Any info would be appreciated.

Thanks,
Rich Maggio