This column has introduced 'system variables' several times and also mentioned that the SWI "OS_ReadVarVal" is used to read their values.

This SWI is a little complicated to describe fully in this column (translation: "I haven't yet managed to understand it all!") but for use when the specified system variable definitely exists, the following will help:

BlockSize%=256
DIM Block% BlockSize%
PRINT FNreadsysvariable("Obey$Dir")
END

DEF FNreadsysvariable(SysVar$)
LOCAL Len%
SYS"OS_ReadVarVal",SysVar$,Block%,BlockSize%,,3 TO ,,Len%
Block%?Len%=13
=$Block%

Note that the value returned is always a string - although it might sometimes look like a number, in which case you may then have to use VAL to convert it for use afterwards. In the above example, the result is obviously going to be a directory specification string.

You can also use this SWI to check if a particular system variable exists. Further, a wildcarded name can be used for SysVar$, and all matches can then be found in sequence. But that is another story...