mem_FH = HFN
field0$ = "field0"
field1$ = "field1"
field2$ = "field2"
field3 = 123

key$ = field0$ + field1$
! write to the file
write(mem_FH, key=key$)key$,field0$,field1$,field2$,field3

! read to the file
read(mem_FH, key=field0$ + field1$, err=*next)key$,field0$,field1$,field2$,field3

!these values are now populated and you can work with them the same as you would with a regular data file.