Editing DNIX (section)

=== Compatibility ===
In addition to the native <code>dnix(2)</code> call, a complete set of 'standard' [[libc]] interface calls was available.
<code>open(2)</code>, <code>close(2)</code>, <code>read(2)</code>, <code>write(2)</code>, etc. Besides being useful for backwards compatibility, these were implemented in a binary-compatible manner with the [[NCR Corporation|NCR Tower]] computer, so that binaries compiled for it would run unchanged under DNIX. The DNIX kernel had two trap dispatchers internally, one for the DNIX method and one for the Unix method. Choice of dispatcher was up to the programmer, and using both interchangeably was acceptable. Semantically they were identical wherever functionality overlapped. (In these machines the [[68000]] <code>trap #0</code> instruction was used for the <code>unix(2)</code> calls, and the <code>trap #4</code> instruction for <code>dnix(2)</code>. The two trap handlers were very similar, though the [usually hidden] <code>unix(2)</code> call held the function code in the processor's D0 register, whereas <code>dnix(2)</code> held it on the stack with the rest of the parameters.)

DNIX 5.2 had no networking protocol stacks internally (except for the thin [[X.25]]-based [[Ethernet]] [[protocol stack]] added by ISC for use by its diskless workstation support package), all networking was conducted by reading and writing to Handlers. Thus, there was no [[Berkeley sockets|socket]] mechanism, but a <code>libsocket(3)</code> existed that used asynchronous I/O to talk to the TCP/IP handler. The typical Berkeley-derived networking program could be compiled and run unchanged (modulo the usual Unix [[porting]] problems), though it might not be as efficient as an equivalent program that used native asynchronous I/O.