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Newsgroups: comp.lang.ada
Date: Thu, 4 Jul 2019 02:04:15 -0700 (PDT)
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Subject: FreeDOS, Janus/Ada and GNAT
From: joakimds@kth.se
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Xref: reader01.eternal-september.org comp.lang.ada:56812
Date: 2019-07-04T02:04:15-07:00
List-Id: <comp.lang.ada>

FreeDOS is an open source implementation of MS-DOS and in June 2019 celebra=
ted 25 years in existence. In it there is an application called DIMPLES tha=
t is the package manager for FreeDOS. DIMPLES is like pip for Python, apt (=
aptitude) for Debian systems, and so on. To compile Ada applications in Fre=
eDOS one can use the DJGPP compiler which is a port of gcc (GNAT) to FreeDO=
S (and one can use DIMPLES to install it if it is not already installed). I=
t works well except that it cannot create an executable from Ada code that =
uses tasks. So if one wants to make Ada applications for FreeDOS there can =
be no tasking. This has to do with how tasking in GNAT is implemented. On W=
indows, Mac OS X and Linux the tasks are
mapped in a one-to-one correspondance to OS-threads (at least my understand=
ing).
FreeDOS does not support or provide threads and is a single thread/task ope=
rating system. It has therefore not been a trivial exercise to port the sta=
ndard GNAT-Runtime support for tasks to FreeDOS.

There is another Ada compiler called Janus/Ada where tasks are not mapped b=
y the Ada-Runtime to OS-threads and an Ada application built with Janus/Ada=
 is effectively a single thread application no matter how many tasks are us=
ed in the Ada code. Considering the task implementation by Janus/Ada, it so=
unds like a perfect fit for FreeDOS. Of course Janus/Ada produces win32 app=
lications and not DOS application but there is a DOS Extender called HX DOS=
 which can execute many win32 applicatins on FreeDOS. Trying it out reveals=
 that there are three win32 calls which HX DOS could not emulate:

import not found: InitializeSListHead
import not found: GetModuleHandleExW
import not found: FindFirstFileExW

If HX DOS could be extended to emulate these calls then Janus/Ada can *prob=
ably* be used to build multi-task executables for FreeDOS. Jim Hall (creato=
r of the FreeDOS project) wrote in April 2009 on the subject of multi-taski=
ng in DOS:
"People sometimes ask me what I'd love to see in a future version of DOS.
I would like to see some level of running multiple applications at once.
...
True multitasking would be ideal, but I'd be happy if we supported
the simpler "task switching" - this would be a huge leap forward for FreeDO=
S.
..."

One area where FreeDOS can be used today is in embedded systems (and it has=
 been used there). The implementation of FreeDOS requires the CPU to be x86=
 and many embedded systems are nowadays ARM based (which rules out FreeDOS)=
. Any known hard-ware vendors where it is known one can run FreeDOS?

To summarize: It is possible today to use the DJGPP (GNAT) compiler to deve=
lop single task applications on FreeDOS. It is *almost* possible to run mul=
ti-task Ada applications on FreeDOS using the Janus/Ada compiler. AdaCore m=
ay have an Ada-Runtime that can support tasking on FreeDOS, but outside the=
 scope of what I know.

Best regards,
Joakim