NetWare Loadable Module

NetWare Loadable Module
Filename extension	.nlm
Magic number	NetWare Loadable Module
Developed by	Novell
Type of format	Executable, Shared library

Last updated March 13, 2023

A NetWare Loadable Module^[1]^[2]^[3] (NLM) is a loadable kernel module (a binary code module) that can be loaded into Novell's NetWare operating system. NLMs can implement hardware drivers, server functions (e.g. clustering), applications (e.g. GroupWise), system libraries or utilities.

Functionality

Upon loading, a NLM requests resources, such as memory and process threads, from the NetWare kernel. The NetWare kernel tracks such requests, and can identify memory and other resources assigned to a specific NLM. NLMs may auto-load other NLMs upon which they themselves depend.

NLMs may register commands with the NetWare kernel, extending the command vocabulary available at the NetWare console prompt.

When properly coded, NLMs can be re-entrant, allowing multiple instances of the same code to be loaded and run.

Programming issues

Initially, Novell published a development toolkit for NLM programming including kernel API documentation and a C compiler (Watcom), but third-party support for the NLM executable function was very limited.

In early NetWare versions (prior to v4.x), all processes were executed in the kernel address space, without specific memory protection. It was therefore possible for bugs in NLMs to overwrite the kernel's or other NLM's address space and ultimately crash the server — in the mainframe-derived Novell terminology, this was known as an ABEND or ABnormal END.

Moreover, NetWare used a non-preemptive, or cooperative, multitasking model, meaning that an NLM was required to yield to the kernel regularly.^[4] An NLM executing an infinite loop, for example, could therefore not be stopped.

In NetWare v4.x, Novell introduced a limited form of memory protection, which was extended to Protected Address Spaces in NetWare v5.x. Through these mechanisms, properly-coded NLMs can be loaded into their own address space, and mis-behaving NLMs can be prevented from compromising the kernel integrity.

Compilers

Official compilers that produce NLMs are:

Watcom C/C++ compiler / OpenWatcom
Metrowerks CodeWarrior for NetWare^[5]
EPC C/C++
Novell NLMLINK.EXE

NLM development is also possible with GNU cc and binutils.^[6] More details are available in NetWare Loadable Module Programming HOWTO by Martin Hinner.^[7]

Client-side NLMs with NIOS

Announcing it in March 1995,^[8]^[9] Novell introduced a new 32-bit DOS/Windows NetWare client (Client 32 based on ODI32/NIOS)^[10] in 1996, replacing the former 16-bit client based on ODI/VLM. Its NIOS component (NetWare I/O Subsystem ^[8]^[11]^[12]) used techniques similar to DPMS to relocate and run NLMs in protected mode and extended memory in order to reduce the conventional memory footprint of the client down to 2 to 5 KB.^[13]^[11]

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References

↑ Dave Kearns (October 3, 2006). "Revelation: A NetWare Loadable Module that's still around". Computerworld .
↑ "Database server choices for NetWare". Computerworld .
↑ "Hands on: Getting Macs and PCs to play well together". Computerworld .
↑ Mark Foust (2001). NetWare Administration: NetWare 4.0-6.0.
↑ "Metrowerks website". Archived from the original on January 6, 2009.
↑ "NLM development under Linux".
↑ "NetWare Loadable Module Programming HOWTO".
1 2 "Novell to dress up Tuxedo and deliver 32-bit client". Network World : 141. March 27, 1995. Retrieved August 18, 2018.
↑ "Network Edition - Cover Story". PC Magazine : NE27. October 24, 1995. Retrieved August 18, 2018.
↑ "Netware Input/Output Subsystem".
1 2 Liebing, Edward A. (May 1, 1996). "NetWare Client 32 for DOS/Windows: Overview of Architecture and Features". Novell AppNotes. Novell. Archived from the original on August 18, 2018. Retrieved August 18, 2018.
↑ Hendrick, Ben (November 1, 1996). "Overview of NetWare Client 32 for Windows 95". Novell AppNotes. Novell. Archived from the original on August 18, 2018. Retrieved August 18, 2018.
↑ Paul, Matthias R. (July 30, 1997). NWDOS-TIPs — Tips & Tricks rund um Novell DOS 7, mit Blick auf undokumentierte Details, Bugs und Workarounds. MPDOSTIP. Release 157 (in German) (3 ed.). Archived from the original on May 22, 2016. Retrieved January 11, 2012. (NB. NWDOSTIP.TXT is a comprehensive work on Novell DOS 7 and OpenDOS 7.01, including the description of many undocumented features and internals. It is part of the author's yet larger MPDOSTIP.ZIP collection maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of the NWDOSTIP.TXT file.)