Try this
(I took out the Sub and End Sub statements)

'GetBLASTER goes directly after the SUB keyword
GetBLASTER (DMA%, BasePort%, IRQ%, errcd%)
' This subroutine parses the BLASTER environment string and returns settings.
IF LEN(ENVIRON$("BLASTER")) = 0 THEN errcd% = 1: EXIT SUB'PRINT "BLASTER environment variable not set.": EXIT SUB
FOR Length% = 1 TO LEN(ENVIRON$("BLASTER"))
   SELECT CASE MID$(ENVIRON$("BLASTER"), Length%, 1)
      CASE "A"
        BasePort% = VAL("&H" + MID$(ENVIRON$("BLASTER"), Length% + 1, 3))
      CASE "I"
        IRQ% = VAL(MID$(ENVIRON$("BLASTER"), Length% + 1, 1))
      CASE "D"
        DMA% = VAL(MID$(ENVIRON$("BLASTER"), Length% + 1, 1))
   END SELECT
NEXT
'end sub goes here

thank you for
sub getblaster (dma%, baseport%, irq, errcd%)
.........
..........
end sub