Re: Kernel Syscall from Ada?

["Dan'l Miller" <optikos@verizon.net>]

On Thursday, June 23, 2016 at 5:18:14 PM UTC-5, Randy Brukardt wrote:
> "Diogenes"  wrote in message 
> news:2048d6d6-04e2-4e2c-9483-e3769da59781@googlegroups.com...
> >Is there a simple way to make a direct (Linux)Kernel syscall from Ada 
> >without
> >using the system C library? i.e. Make a direct call as in Assembler?
> 
> On almost all modern systems, a system call IS a call to some C subprogram. 
> (On Windows, they're calls into a DLL; Linux appears similar although I've 
> never studied the details.) There's nothing like the set-registers and trap 
> like there was in the old days on MS-DOS, XENIX, and other systems.
> 
> So the only way to make a system call is via aspect Import to the needed C 
> routine. (On Windows, I''d expect that a program that didn't do any system 
> calls would be much smaller than a "normal" program; Windows needs some 
> stuff added to every program; it doesn't have anything to do with Ada 
> per-se.)
> 
>                                   Randy.

One of the more succinct summaries of the legacy 8086, 32-bit x86, and 64-bit x86-64:
https://stackoverflow.com/questions/12806584/what-is-better-int-0x80-or-syscall

Randy is mostly accurate:  interrupt 80₁₆ has faded into antiquity and currently everything about system calls boils down to C subprograms.

Conversely, the syscall mechanism is best thought of as an •assembly• subprogram (and calling convention) that crosses a ring boundary whose syscall-table array of assembly subprograms does have some vague resemblance to the old interrupt 80₁₆ vector table.  (No processor manufacturer would hardcode C-language conventions into their silicon for context switches between protection rings.  It is all C out of habit, not hardware-imposed necessity.)

Conceivably Ada could present its own array of Ada subprograms in its own syscall-table, so that Ada would not come down to invoking C subprograms in userspace.  An Ada-centric new distribution of Linux (or BSD Unix or GNU Hurd or any other operating system, for that matter) would be needed to add an Ada-only syscall vector/lookup table (LUT) in addition to the current C-only syscall LUT.  But such a new distro of Ada-Linux (or Ada-BSD or Ada-Hurd) could showcase 100%-Ada software in userspace, encourage Ada programming in kernelspace, and use that Ada-based package manager announced recently on c.l.a.

lucid explanation of 32-bit sysenter:
http://articles.manugarg.com/systemcallinlinux2_6.html

a taste of VDSO to avoid a userspace-to-kernelspace context switch for some would-be/has-been syscalls:
https://en.wikipedia.org/wiki/VDSO

the complexity of on which processors AMD syscall versus Intel sysenter instructions are supported or not: 
https://ReverseEngineering.stackexchange.com/questions/16454/struggling-between-syscall-or-sysenter-windows