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MDFS::Info.Comp.Spectrum.ProgTips.Interface1.drive/htm

When a program starts it is often useful to be able to know what device the program has loaded from so that it can load more files from the same place. Quite often, the usual method is to not bother, and hard-code the source into separate loader programs. For instance:

Tape loader:
  10 CLEAR 32767:LOAD "code"CODE:RANDOMIZE USR 32768

Microdrive loader:
  10 CLEAR 32767:LOAD *"m";1;"code"CODE:RANDOMIZE USR 32768

However, it is more useful to be able to have just one loader program and have it load its resources from where it has just loaded from so you don't have to have different programs for different devices.

This can be done comprehensively as part of generalised device selection code. However, if all the program needs to do is load some more files and that's it, for example, loading and entering some machine code, it can be done a lot simpler.

  10 CLEAR 32767:GOSUB 100:RANDOMIZE USR 32768
 100 LET d=PEEK 23649+256*PEEK 23650-2
 110 LET d=d+1:IF PEEK d<>128 THEN GOTO 110
 120 IF PEEK (d+19) THEN GOTO 170
 130 LET d=PEEK 23769
 140 IF d=CODE"M" THEN LOAD *"m";PEEK 23766;"code"CODE:RETURN
 150 IF d=CODE"N" THEN LOAD *"n";PEEK 23766 CODE:RETURN
 160 IF d=CODE"B" THEN LOAD *"b"CODE:RETURN
 170 LOAD "code"CODE:RETURN

Authored by J.G.Harston
Last update: 17-Oct-2004