BLib.Context - Read and Write Filing System Context
	    ===================================================
		       File: Context - Update: 1.01
		 Author: J.G.Harston - Date: 12-Aug-2011


  The Context library provides functions for reading and writing the filing
system context independantly of the filing system being used. It allows
programs to read the current context, change the current directory and/or
filing system, and then restore back to the previous state.

Requirements and Dependancies
=============================
The library requires a global control block of at least 16 bytes in length
to be accessible with X% holding the address of this block, and Y% holding
X% DIV 256, and a data block long enough to hold the longest filename likely
to be encountered pointed to by name%.

This can easily be set up with DIM ctrl% 15, name% 15 near the start of the
program and X%=ctrl%:Y%=X%DIV256 at the begining of the major program code
and at the start of any main program loop.

The library requires the Net and the FileIO libraries.

Routine Summary
===============
  FNfs_RdCSD         - Read current filing system context
PROCfs_WrCSD(csd%)   - Restore filing system context

FNfs_RdCSD - Read current filing system context
-----------------------------------------------
Returns 0 or a value to pass back to PROCfs_WrCSD(). The returned value
encodes the current filing system and the currently selected directory. It
is an opaque value and, other than the value zero, should not be interpreted
in any way.

Zero is returned if the function could not determine the current context. If
zero is returned, the CSD path can be found as a string with FNPath_Name in
the Pathname library.

PROCfs_WrCSD(csd%) - Restore filing system context
--------------------------------------------------
On entry, csd%=0 or a previous context from FNfs_RdCSD. The only valid entry
values are zero, which is a null operation, or a value previously returned
from FNfs_RdCSD.

Note, each call to FNfs_RdCSD must be balanced by a corresponding
PROCfs_WrCSD, as otherwise open handles may be left hanging around.
PROCfs_WrCSD(0) is a valid null operation

Examples
========
  csd%=FNfs_RdCSD:IF csd%=0:fs%=FNfs:csd$=FNPath_Name
  *NET
  *EX
  *DISK
  *CAT
  IF csd%:PROCfs_WrCSD(csd%) ELSE OSCLI "FX143,18,"+STR$fs%:OSCLI "DIR "+csd$

Implementation
==============
The Context library currently understands DFS, NFS, ADFS and HADFS. It
implicitly understands TAPE and ROMFS as they have no context.