Selecting any filing system by number
                  =====================================

A filing system can be selected by its filing system number returned with
OSARGS 0,0. However, TAPE and ROM do not respond to the normal sideways ROM
service call and have to be selected with their own OSBYTE calls. The
following code allows you to select any filing system by its filing system
number.

  REM Select filing system Y%, must not be zero
  REM -----------------------------------------
  DEFPROCfs(Y%):LOCAL X%,A%
  IF Y%<4:A%=139.5+Y%/2:X%=Y%+1 ELSE A%=143:X%=18
  CALL &FFF4:ENDPROC
  
  ; Select filing system in Y, must not be zero
  ; -------------------------------------------
  .SelectFS
  LDA #143:LDX #18              ; 143 for service call
  CPY #4:BCS SelectFS2          ; Select via sideways ROM
  TYA:TAX:INX                   ; X=TAPE speed
  LSR A:ADC #139                ; 140/141 for TAPE/ROM
  .SelectFS2
  JMP OSBYTE