AddrOf - Find Address of a Variable
                   ===================================
                       File: AddrOf - Update: 1.00
                 Author: J.G.Harston - Date: 02-Dec-2009


BBC BASIC for Windows implements the ^identifier function (as opposed to
the num ^ num operation) to return the address of a variable. For example,
^fred% returns the address where the data for the variable fred% is stored.

This can be implemented on other versions of BBC BASIC with the AddrOf
library.

Requirements and Dependancies
=============================
None. The appropriate AddrOf library routine must be used for the platform
running on. Routines are available for 6502, Z80, PDP-11, DOS and ARM,
called with FNAddrOf_65, FNAddrOf_Z80, FNAddrOf_PDP, FNAddrOf_DOS and
FNAddrOf_ARM.

Usage
=====
Once FNAddrOf is called to set up the machine code with, for example:

    addrof%=FNAddrOf_65

then the address of a variable can be found with

    CALL addrof%,result%,variable

leaving the address in result%. result% must be a pre-existing integer
variable, and CALL addrof% must have at least two parameters or undefined
problems will occur.

BBC BASIC for Windows allows ^PROCname and ^FNname. This code only supports
those variables that can be passed to the CALL command.

After calling, result% holds the address of the variable's data block, as
with parameters passed to CALL. This will be the following:

    CALL addrof%,result%,num     result% -> 5-byte floating point value
    CALL addrof%,result%,num%    result% -> 4-byte integer value
    CALL addrof%,result%,str$    result% -> string information block

The string information block is different on different platforms.

6502:    result%+0 2-byte string address
                +2 1-byte allocated string length
                +3 1-byte actual string length

Z80:     result%+0 2-byte string address
                +2 1-byte allocated string length
                +3 1-byte actual string length

DOS:     result%+0 allocated string length
                +1 1-byte actual string length
                +2 2-byte string address

PDP-11:  result%+0 2-byte string address
                +2 1-byte allocated string length
                +3 1-byte actual string length

ARM:     result%+0 4-byte string address
                +4 1-byte actual string length

Windows: result%+0 4-byte string address
                +4 2-byte allocated length
                +6 2-byte string length

Examples
========
    REM Print exponent of PI
    num=PI
    CALL addrof%,A%,num
    PRINT "Exponent: ";?A%


    REM Load 4-byte file into a variable
    fred%=0:CALL addrof%,addr%,fred%
    OSCLI "LOAD data "+STR$~addr%


    REM Swap byte order
    big%=&12345678
    little%=big%
    CALL addrof%,A%,little%
    tmp%=A%?0:A%?0=A%?3:A%?3=tmp%
    tmp%=A%?1:A%?1=A%?2:A%?2=tmp%

Version History
===============
1.00 02-Dec-2009 First version written.