DECLARE FUNCTION GetBootDriveLetter! ()
DECLARE SUB SwapDisk ()
    'Main Program Code
Start:
    CLS
    PRINT "This is a sample."
    DO: LOOP UNTIL INKEY$ <> ""
    'Program Complete, Clear Then Run Program In A: Drive
    COLOR 7, 0
    CLS
    SwapDisk
    'Boot Drive Empty, Ask For A Disk
    PRINT "Please insert a System or Program Disk in the disk drive you booted from."
    PRINT "Press Enter to continue, or Esc to cancel."
Exitloop:
    DO
        X$ = INKEY$
    LOOP UNTIL X$ <> ""
    SELECT CASE X$
        CASE CHR$(13)
            SwapDisk
        CASE CHR$(27)
            GOTO Start
        CASE ELSE
    END SELECT
    GOTO Exitloop

FUNCTION GetBootDriveLetter
DEF SEG = 0
IF PEEK(&H4F0) = 21 THEN
  GetBootDriveLetter = PEEK(&H4F1)
END IF
END FUNCTION

SUB SwapDisk
ON LOCAL ERROR GOTO NoSwapDisk
D$ = CHR$(GetBootDriveLetter)
IF D$ = CHR$(0) THEN D$ = "A"
F = FREEFILE
OPEN D$ + ":\config.sys" FOR INPUT AS #F
DO
    LINE INPUT #F, CMD$
    IF UCASE$(LEFT$(CMD$, 6)) = "SHELL=" THEN
        CMD$ = RIGHT$(CMD$, LEN(CMD$) - 6)
        IF UCASE$(CMD$) <> "SAMPLE.EXE" THEN
            'Use CHAIN if your version of BASIC does not support RUN
            'DO NOT USE SHELL!
            RUN D$ + ":\" + CMD$
        ELSE
            BEEP
        END IF
    END IF
LOOP UNTIL EOF(F)
CLOSE #F
NoSwapDisk:
RESUME ExitSwapDisk
ExitSwapDisk:
END SUB