NetWare Loadable Module

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. 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 • EPC C/C++ • Novell NLMLINK.EXE NLM development is also possible with GNU cc and binutils. More details are available in NetWare Loadable Module Programming HOWTO by Martin Hinner. ==Client-side NLMs with NIOS==

Announcing it in March 1995, in 1996, replacing the former 16-bit client based on ODI/VLM. Its NIOS component (NetWare I/O Subsystem) 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. ==See also==