78. Reading *FX calls
    ~~~~~~~~~~~~~~~~~
*FX, (Osbyte), calls 156 and 165 upwards, are special read/write ones. (See
the Advanced User Guide for full details.) You may write to them from BASIC
with *FXa,x  where a is the call number, and x is the new value to be
written, eg *FX229,3 . You can also read the existing value, without altering
it, using FNfx(a). Eg value=FNfx(229) . Note that strictly speaking, you
should add X%=0 to line 2000, but defining LOCAL variables automatically sets
them to zero, so you need not bother.

 2000 DEFFNfx(A%):LOCALX%,Y%:Y%=&FF
 2010 =(USR(&FFF4)AND&FF00)DIV&100