Low-level programming in Ada?

[Paul Rubin <no.email@nospam.invalid>]

I'm wondering if anyone can suggest a reference (preferably online)
about low-level programming (e.g. for operating system implementation)
in Ada.  Not about the language itself, but examples of dealing with
machine addresses, device registers, page tables, memory management,
etc., preferably without dropping to assembler more than a tiny bit.

This isn't for a specific project or anything like that.  It's just
general interest in how to do this stuff that's traditionally the domain
of C.

Thanks.